View source for JavaScript syntax

{{Short description|Set of rules defining correctly structured programs}}
{{Update|date=November 2020|reason=New features/versions now in JavaScript}}
{{Use dmy dates|date=April 2022}}
{{Use American English|date=April 2025}}

[[File:Source code in Javascript.png|thumb|A snippet of [[JavaScript]] code with keywords [[Syntax highlighting|highlighted]] in different colors]]

The '''[[Syntax (programming languages)|syntax]] of [[JavaScript]]''' is the set of rules that define a correctly structured JavaScript program.

The examples below make use of the log function of the console object present in most browsers for [[Standard streams#Standard output .28stdout.29|standard text output]].

The JavaScript [[standard library]] lacks an official standard text output function (with the exception of <code>document.write</code>). Given that JavaScript is mainly used for [[client-side scripting]] within modern [[web browser]]s, and that almost all Web browsers provide the alert function, <code>alert</code> can also be used, but is not commonly used.

==Origins==
[[Brendan Eich]] summarized the ancestry of the syntax in the first paragraph of the JavaScript 1.1 specification<ref>{{Cite web |url=http://hepunx.rl.ac.uk/~adye/jsspec11/intro.htm#1006028 |title=JavaScript 1.1 specification |access-date=19 April 2006 |archive-date=26 February 2017 |archive-url=https://web.archive.org/web/20170226200426/http://hepunx.rl.ac.uk/~adye/jsspec11/intro.htm#1006028 |url-status=live }}</ref><ref>{{cite web|title=Chapter 1. Basic JavaScript|url=http://speakingjs.com/es5/ch01.html|access-date=22 September 2020|website=speakingjs.com|archive-date=10 February 2022|archive-url=https://web.archive.org/web/20220210041253/http://speakingjs.com/es5/ch01.html|url-status=dead}}</ref> as follows:
{{Quote|JavaScript borrows most of its syntax from [[Java (programming language)|Java]], but also inherits from [[Awk]] and [[Perl]], with some indirect influence from [[Self (programming language)|Self]] in its object prototype system.}}

==Basics==

===Case sensitivity===
JavaScript is [[Case sensitivity|case sensitive]]. It is common to start the name of a [[#Constructors|constructor]] with a [[CamelCase|capitalized]] letter, and the name of a function or variable with a lower-case letter.

Example:

<syntaxhighlight lang="javascript">
var a = 5;
console.log(a); // 5
console.log(A); // throws a ReferenceError: A is not defined
</syntaxhighlight>

===Whitespace and semicolons===
Unlike in [[C (programming language)|C]], whitespace in JavaScript source can directly impact [[Semantics (computer science)|semantics]]. [[Semicolon]]s end statements in JavaScript. Because of [[Lexical analysis#Semicolon insertion|automatic semicolon insertion]] (ASI), some statements that are well formed when a newline is [[Parsing|parsed]] will be considered complete, as if a semicolon were inserted just prior to the newline. Some authorities advise supplying statement-terminating semicolons explicitly, because it may lessen unintended effects of the automatic semicolon insertion.<ref>{{cite book
|title=JavaScript: The definitive Guide
|url=https://archive.org/details/javascript00libg_297
|url-access=registration
|last=Flanagan
|first=David
|page=[https://archive.org/details/javascript00libg_297/page/n14 16]
|quote=Omitting semicolons is not a good programming practice; you should get into the habit of inserting them.
|isbn=978-0-596-10199-2
|year=2006|publisher="O'Reilly Media, Inc."
}}</ref>

There are two issues: five tokens can either begin a statement or be the extension of a complete statement; and five restricted productions, where line breaks are not allowed in certain positions, potentially yielding incorrect parsing.

The five problematic tokens are the open parenthesis "<code>(</code>", open bracket "<code>[</code>", slash "<code>/</code>", plus "<code>+</code>", and minus "<code>-</code>". Of these, the open parenthesis is common in the [[immediately invoked function expression]] pattern, and open bracket occurs sometimes, while others are quite rare. An example:

<syntaxhighlight lang="javascript">
a = b + c
(d + e).foo()

// Treated as:
//  a = b + c(d + e).foo();
</syntaxhighlight>
with the suggestion that the preceding statement be terminated with a semicolon.

Some suggest instead the use of ''leading'' semicolons on lines starting with '<code>(</code>' or '<code><nowiki>[</nowiki></code>', so the line is not accidentally joined with the previous one. This is known as a '''defensive semicolon''', and is particularly recommended, because code may otherwise become ambiguous when it is rearranged. For example:

<syntaxhighlight lang="javascript">
a = b + c
;(d + e).foo()

// Treated as:
//  a = b + c;
//  (d + e).foo();
</syntaxhighlight>
Initial semicolons are also sometimes used at the start of JavaScript libraries, in case they are appended to another library that omits a trailing semicolon, as this can result in ambiguity of the initial statement.

The five restricted productions are <code>return</code>, <code>throw</code>, <code>break</code>, <code>continue</code>, and post-increment/decrement. In all cases, inserting semicolons does not fix the problem, but makes the parsed syntax clear, making the error easier to detect. <code>return</code> and <code>throw</code> take an optional value, while <code>break</code> and <code>continue</code> take an optional label. In all cases, the advice is to keep the value or label on the same line as the statement. This most often shows up in the return statement, where one might return a large object literal, which might be accidentally placed starting on a new line. For post-increment/decrement, there is potential ambiguity with pre-increment/decrement, and again it is recommended to simply keep these on the same line.

<syntaxhighlight lang="javascript">
return
a + b;

// Returns undefined. Treated as:
//   return;
//   a + b;
// Should be written as:
//   return a + b;
</syntaxhighlight>

===Comments===
{{Main|Comment (computer programming)}}
[[Comment (computer programming)|Comment]] syntax is the same as in [[C++]], [[Swift (programming language)|Swift]] and many other languages.

<syntaxhighlight lang="javascript">
// a short, one-line comment

/* this is a long, multi-line comment
  about my script. May it one day
  be great. */

/* Comments /* may not be nested */ Syntax error */
</syntaxhighlight>

==Variables==
{{Main|Variable (programming)}}

[[Variable (programming)|Variable]]s in standard JavaScript have no [[Type system|type]] attached, so any value (each ''value'' has a type) can be stored in any variable. Starting with [[ECMAScript#6th Edition – ECMAScript 2015|ES6]], the 6th version of the language, variables could be declared with <code>var</code> for function scoped variables, and <code>let</code> or <code>const</code> which are for [[block scope|block level]] variables. Before ES6, variables could only be declared with a <code>var</code> statement.  Values assigned to variables declared with <code>const</code> cannot be changed, but their properties can. <code>var</code> should no longer be used since <code>let</code> and <code>const</code> are supported by modern browsers.<ref>{{Cite web |date=9 May 2023 |title=Storing the information you need — Variables - Learn web development {{!}} MDN |url=https://developer.mozilla.org/en-US/docs/Learn/JavaScript/First_steps/Variables |access-date=23 June 2023 |website=developer.mozilla.org |language=en-US}}</ref> A variable's [[Identifier (computer languages)|identifier]] must start with a letter, underscore (<code>_</code>), or dollar sign (<code>$</code>), while subsequent characters can also be digits (<code>0-9</code>). JavaScript is case sensitive, so the uppercase characters "A" through "Z" are different from the lowercase characters "a" through "z".

Starting with JavaScript 1.5, [[ISO 8859-1]] or [[Unicode]] letters (or <code>\uXXXX</code> Unicode escape sequences) can be used in identifiers.<ref>{{cite web | url=https://developer.mozilla.org/en/JavaScript/Guide/Values,_Variables,_and_Literals&revision=22#Variables | title=Values, Variables, and Literals - MDC | date=16 September 2010 | publisher=Mozilla Developer Network | access-date=1 February 2020 | archive-url=https://web.archive.org/web/20110629131728/https://developer.mozilla.org/en/JavaScript/Guide/Values%2C_Variables%2C_and_Literals%26revision%3D22#Variables | archive-date=29 June 2011 | url-status=dead }}</ref> In certain JavaScript implementations, the at sign (@) can be used in an identifier, but this is contrary to the specifications and not supported in newer implementations. {{Citation needed|date=January 2021}}

===Scoping and hoisting===

Variables declared with <code>var</code> are [[lexical scoping|lexically scoped]] at a [[function scope|function level]], while ones with <code>let</code> or <code>const</code> have a [[block scope|block level]] scope. Since declarations are processed before any code is executed, a variable can be assigned to and used prior to being declared in the code.<ref>{{cite web |title=JavaScript Hoisting |publisher=[[W3Schools]] |url=https://www.w3schools.com/js/js_hoisting.asp |quote=In JavaScript, a variable can be declared after it has been used. In other words; a variable can be used before it has been declared. |access-date=17 December 2021 |archive-date=31 March 2022 |archive-url=https://web.archive.org/web/20220331225545/https://www.w3schools.com/js/js_hoisting.asp |url-status=live }}</ref> This is referred to as ''{{visible anchor|hoisting}}'', and it is equivalent to variables being [[Forward declaration|forward declared]] at the top of the function or block.<ref>"[http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html JavaScript Scoping and Hoisting] {{Webarchive|url=https://web.archive.org/web/20210508121632/http://www.adequatelygood.com/JavaScript-Scoping-and-Hoisting.html |date=8 May 2021 }}", [http://www.adequatelygood.com/about.html Ben Cherry] {{Webarchive|url=https://web.archive.org/web/20210228091155/http://www.adequatelygood.com/about.html |date=28 February 2021 }}, ''[http://www.adequatelygood.com/ Adequately Good] {{Webarchive|url=https://web.archive.org/web/20220308081510/http://www.adequatelygood.com/ |date=8 March 2022 }},'' 8 February 2010</ref><!-- Might not explain scoping very well -->

With <code>var</code>, <code>let</code>, and <code>const</code> statements, only the declaration is hoisted; assignments are not hoisted. Thus a {{code|lang=javascript|code=var x = 1}} statement in the middle of the function is equivalent to a {{code|lang=javascript|code=var x}} declaration statement at the top of the function, and an {{code|lang=javascript|code=x = 1}} assignment statement at that point in the middle of the function. This means that values cannot be accessed before they are declared; [[forward reference]] is not possible. With <code>var</code> a variable's value is <code>undefined</code> until it is initialized. Variables declared with <code>let</code> or <code>const</code> cannot be accessed until they have been initialized, so referencing such variables before will cause an error.<!-- Some sources: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/let and https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/var#var_hoisting. Referencing a let or const will not throw if there's a type of. An exception to the typeof exception is "const x = x;", which you can try in the console. -->

Function declarations, which declare a variable and assign a function to it<!-- Function technically isn't actually a type. They're objects which can be executed in a subroutine. -->, are similar to variable statements, but in addition to hoisting the declaration, they also hoist the assignment – as if the entire statement appeared at the top of the containing function – and thus forward reference is also possible: the location of a function statement within an enclosing function is irrelevant. This is different from a function expression being assigned to a variable in a  <code>var</code>, <code>let</code>, or <code>const</code> statement.

So, for example, 
<syntaxhighlight lang="javascript">
var func = function() { .. } // declaration is hoisted only
function func() { .. } // declaration and assignment are hoisted
</syntaxhighlight>

Block scoping can be produced by wrapping the entire block in a function and then executing it – this is known as the [[immediately-invoked function expression]] pattern – or by declaring the variable using the <code>let</code> keyword. <!-- Two totally slightly different things, wrapping cope in a block executes the whole code in a block, while let-or-const variables can be seen in the same block. So block-scope code, where code is executed, block-scope variables, where variables can be accessed.  -->

===Declaration and assignment===

Variables declared outside a scope are [[global variable|global]]. If a variable is declared in a higher scope, it can be accessed by child scopes.

When JavaScript tries to '''resolve''' an identifier, it looks in the local scope. If this identifier is not found, it looks in the next outer scope, and so on along the ''scope chain'' until it reaches the ''global scope'' where global variables reside. If it is still not found, JavaScript will raise a <code>ReferenceError</code> exception.

When '''assigning''' an identifier, JavaScript goes through exactly the same process to retrieve this identifier, except that if it is not found in the ''global scope'', it will create the "variable" in the scope where it was created.<ref>ECMA-262 5e edition clarified this behavior with the ''Declarative Environment Record'' and ''Object Environment Record''. With this formalism, the ''global object'' is the ''Object Environment Record'' of the global ''Lexical Environment'' (the ''global scope'').</ref> As a consequence, a variable never declared will be global, if assigned. Declaring a variable (with the keyword <code>var</code>) in the ''global scope'' (i.e. outside of any function body (or block in the case of let/const)), assigning a never declared identifier or adding a property to the ''global object'' (usually ''window'') will also create a new global variable.

Note that JavaScript's ''strict mode'' forbids the assignment of an undeclared variable, which avoids global namespace pollution. <!-- Removed, unrelated to the first statement and sentence was already stated. Delete this comment if you edit this, this is just an explanation for my big edit. -->

=== Examples ===

Here are some examples of variable declarations and scope:
<!-- Maybe in this example, function f should also have a local shadowing variable name x2 -->
<syntaxhighlight lang="javascript">
var x1 = 0; // A global variable, because it is not in any function
let x2 = 0; // Also global, this time because it is not in any block

function f() {
  var z = 'foxes', r = 'birds'; // 2 local variables
  m = 'fish'; // global, because it was not declared anywhere before

  function child() {
    var r = 'monkeys'; // This variable is local and does not affect the "birds" r of the parent function.
    z = 'penguins'; // Closure: Child function is able to access the variables of the parent function.
  }

  twenty = 20; // This variable is declared on the next line, but usable anywhere in the function, even before, as here
  var twenty;

  child();
  return x1 + x2; // We can use x1 and x2 here, because they are global
}

f();

console.log(z); // This line will raise a ReferenceError exception, because the value of z is no longer available
</syntaxhighlight>

<syntaxhighlight lang="javascript">
for (let i = 0; i < 10; i++) console.log(i);
console.log(i); // throws a ReferenceError: i is not defined
</syntaxhighlight>

<syntaxhighlight lang="javascript">
for (const i = 0; i < 10; i++) console.log(i); // throws a TypeError: Assignment to constant variable

for (const i of [1,2,3]) console.log(i); //will not raise an exception. i is not reassigned but recreated in every iteration

const pi; // throws a SyntaxError: Missing initializer in const declaration
</syntaxhighlight>

==Primitive data types==
{{Main|Primitive data type}}
The JavaScript language provides six [[primitive data type]]s:

* Undefined
* Number
* BigInt
* String
* Boolean
* Symbol

Some of the primitive data types also provide a set of named values that represent the extents of the type boundaries. These named values are described within the appropriate sections below.

===Undefined===
{{Main|Undefined value}}

The [[undefined value|value of "undefined"]] is assigned to all [[uninitialized variable]]s, and is also returned when checking for object properties that do not exist. In a Boolean context, the undefined value is considered a false value.

Note: undefined is considered a genuine primitive type. Unless explicitly converted, the undefined value may behave unexpectedly in comparison to other types that evaluate to false in a logical context.

<syntaxhighlight lang="javascript">
let test;                         // variable declared, but not defined, ...
                                  // ... set to value of undefined
const testObj = {};
console.log(test);                // test variable exists, but value not ...
                                  // ... defined, displays undefined
console.log(testObj.myProp);      // testObj exists, property does not, ...
                                  // ... displays undefined
console.log(undefined == null);   // unenforced type during check, displays true
console.log(undefined === null);  // enforce type during check, displays false
</syntaxhighlight>

Note: There is no built-in language literal for undefined. Thus {{code|lang=javascript|code=(x === undefined)}} is not a foolproof way to check whether a variable is undefined, because in versions before ECMAScript 5, it is legal for someone to write {{code|lang=javascript|code=var undefined = "I'm defined now";}}. A more robust approach is to compare using {{code|lang=javascript|code=(typeof x === 'undefined')}}.

Functions like this will not work as expected:

<syntaxhighlight lang="javascript">
function isUndefined(x) { let u; return x === u; }             // like this...
function isUndefined(x) { return x === void 0; }               // ... or that second one
function isUndefined(x) { return (typeof x) === "undefined"; } // ... or that third one
</syntaxhighlight>

Here, calling <code>isUndefined(my_var)</code> raises a {{mono|ReferenceError}} if {{mono|my_var}} is an unknown identifier, whereas {{code|lang=javascript|code=typeof my_var === 'undefined'}} does not.

===Number===
Numbers are represented in binary as [[IEEE 754]] [[floating point]] doubles. Although this format provides an accuracy of nearly 16 [[significant digits]], it cannot always exactly represent real numbers, including fractions.

This becomes an issue when comparing or formatting numbers. For example:

<syntaxhighlight lang="javascript">
console.log(0.2 + 0.1 === 0.3); // displays false
console.log(0.94 - 0.01);       // displays 0.9299999999999999
</syntaxhighlight>

As a result, a routine such as the {{mono|toFixed()}} method should be used to round numbers whenever they are [http://www.jibbering.com/faq/#formatNumber formatted for output].

Numbers may be specified in any of these notations:

<syntaxhighlight lang="javascript">
345;    // an "integer", although there is only one numeric type in JavaScript
34.5;   // a floating-point number
3.45e2; // another floating-point, equivalent to 345
0b1011; // a binary integer equal to 11
0o377;   // an octal integer equal to 255
0xFF;   // a hexadecimal integer equal to 255, digits represented by the ...
        // ... letters A-F may be upper or lowercase
</syntaxhighlight>

There is also a numeric separator, {{mono|_}} (the underscore), introduced in ES2021:

<syntaxhighlight lang="javascript">
// Note: Wikipedia syntax does not support numeric separators yet
1_000_000_000;    // Used with big numbers
1_000_000.5;      // Support with decimals
1_000e1_000;      // Support with exponents

// Support with binary, octals and hex
0b0000_0000_0101_1011;
0o0001_3520_0237_1327;
0xFFFF_FFFF_FFFF_FFFE;

// But users cannot use them next to a non-digit number part, or at the start or end
_12; // Variable is not defined (the underscore makes it a variable identifier)
12_; // Syntax error (cannot be at the end of numbers)
12_.0; // Syntax error (does not make sense to put a separator next to the decimal point)
12._0; // Syntax error
12e_6; // Syntax error (next to "e", a non-digit. Does not make sense to put a separator at the start)
1000____0000; // Syntax error (next to "_", a non-digit. Only 1 separator at a time is allowed
</syntaxhighlight>

The extents [[extended real number line|'''+∞''', '''&minus;∞''']] and '''[[NaN]]''' (Not a Number) of the number type may be obtained by two program expressions:

<syntaxhighlight lang="javascript">
Infinity; // positive infinity (negative obtained with -Infinity for instance)
NaN;      // The Not-A-Number value, also returned as a failure in ...
          // ... string-to-number conversions
</syntaxhighlight>

Infinity and NaN are numbers:

<syntaxhighlight lang="javascript">
typeof Infinity;   // returns "number"
typeof NaN;        // returns "number"
</syntaxhighlight>

These three special values correspond and behave as the [[IEEE-754]] describes them.

The Number constructor (used as a function), or a unary + or -, may be used to perform explicit numeric conversion:

<syntaxhighlight lang="javascript">
const myString = "123.456";
const myNumber1 = Number(myString);
const myNumber2 = +myString;
</syntaxhighlight>

When used as a constructor, a numeric ''wrapper'' object is created (though it is of little use):

<syntaxhighlight lang="javascript">
const myNumericWrapper = new Number(123.456);
</syntaxhighlight>

However, NaN is not equal to itself:

<syntaxhighlight lang="javascript">
const nan = NaN;
console.log(NaN == NaN);        // false
console.log(NaN === NaN);       // false
console.log(NaN !== NaN);       // true
console.log(nan !== nan);       // true

// Users can use the isNaN methods to check for NaN
console.log(isNaN("converted to NaN"));     // true
console.log(isNaN(NaN));                    // true
console.log(Number.isNaN("not converted")); // false
console.log(Number.isNaN(NaN));             // true
</syntaxhighlight>

===BigInt===
BigInts can be used for arbitrarily large [[integer]]s. Especially whole numbers larger than 2<sup>53</sup> - 1, which is the largest number JavaScript can reliably represent with the Number primitive and represented by the Number.MAX_SAFE_INTEGER constant.

When dividing BigInts, the results are [[Truncation|truncated]].

=== String ===
A [[String (computer science)|string]] in JavaScript is a sequence of characters. In JavaScript, strings can be created directly (as literals) by placing the series of characters between double (") or single (') quotes. Such strings must be written on a single line, but may include escaped newline characters (such as \n). The JavaScript standard allows the [[Grave accent#Use in programming|backquote]] character (`, a.k.a. grave accent or backtick) to quote multiline literal strings, as well as embedded expressions using the syntax ${''expression''}.<ref>{{cite web|url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals|title=Template literals|website=MDN Web Docs|language=en-US|access-date=4 November 2023|archive-date=31 March 2022|archive-url=https://web.archive.org/web/20220331204510/https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Template_literals|url-status=live}}</ref>

<syntaxhighlight lang="javascript">
const greeting = "Hello, World!";
const anotherGreeting = 'Greetings, people of Earth.';
const aMultilineGreeting = `Warm regards,
John Doe.`

// Template literals type-coerce evaluated expressions and interpolate them into the string.
const templateLiteral = `This is what is stored in anotherGreeting: ${anotherGreeting}.`;
console.log(templateLiteral); // 'This is what is stored in anotherGreeting: 'Greetings, people of Earth.''
console.log(`You are ${Math.floor(age)=>18 ? "allowed" : "not allowed"} to view this web page`); 
</syntaxhighlight>

Individual characters within a string can be accessed using the {{mono|charAt}} method (provided by {{mono|String.prototype}}). This is the preferred way when accessing individual characters within a string, because it also works in non-modern browsers:

<syntaxhighlight lang="javascript">
const h = greeting.charAt(0);
</syntaxhighlight>

In modern browsers, individual characters within a string can be accessed (as strings with only a single character) through the same notation as arrays:

<syntaxhighlight lang="javascript">
const h = greeting[0];
</syntaxhighlight>

However, JavaScript strings are [[immutable object|immutable]]:

<syntaxhighlight lang="javascript">
greeting[0] = "H"; // Fails.
</syntaxhighlight>

Applying the <!-- loose -->equality operator ("==") to two strings returns true, if the strings have the same contents, which means: of the same length and containing the same sequence of characters (case is significant for alphabets). Thus:

<syntaxhighlight lang="javascript">
const x = "World";
const compare1 = ("Hello, " + x == "Hello, World"); // Here compare1 contains true.
const compare2 = ("Hello, " + x == "hello, World"); // Here compare2 contains  ...
                                                    // ... false since the ...
                                                    // ... first characters ...
                                                    // ... of both operands ...
                                                    // ... are not of the same case.
</syntaxhighlight>

Quotes of the same type cannot be nested unless they are [[String literal#Escape character|escaped]].

<syntaxhighlight lang="javascript">
let x = '"Hello, World!" he said.'; // Just fine.
x = ""Hello, World!" he said."; //  Not good.
x = "\"Hello, World!\" he said."; // Works by escaping " with \"
</syntaxhighlight>

The {{mono|String}} constructor creates a string object (an object wrapping a string):

<syntaxhighlight lang="javascript">
const greeting = new String("Hello, World!");
</syntaxhighlight>

These objects have a {{mono|valueOf}} method returning the primitive string wrapped within them<!-- also works with regular strings -->:

<syntaxhighlight lang="javascript">
const s = new String("Hello !");
typeof s; // Is 'object'.
typeof s.valueOf(); // Is 'string'.
</syntaxhighlight>

Equality between two {{mono|String}} objects does not behave as with string primitives:

<syntaxhighlight lang="javascript">
const s1 = new String("Hello !");
const s2 = new String("Hello !");
s1 == s2; // Is false, because they are two distinct objects.
s1.valueOf() == s2.valueOf(); // Is true.
</syntaxhighlight>

===Boolean===

[[JavaScript]] provides a [[Boolean data type]] with {{mono|true}} and {{mono|false}} literals. The {{mono|[[typeof]]}} operator returns the string {{mono|"boolean"}} for these [[primitive types]]. When used in a logical context, {{mono|0}}, {{mono|-0}}, {{mono|null}}, {{mono|NaN}}, {{mono|undefined}}, and the empty string ({{mono|""}}) evaluate as {{mono|false}} due to automatic [[type conversion]]. All other values (the [[complement (set theory)|complement]] of the previous list) evaluate as {{mono|true}}, including the strings {{mono|"0"}}, {{mono|"false"}} and any object.

=== Type conversion ===
Automatic type coercion by the equality comparison operators (<code>==</code> and <code>!=</code>) can be avoided by using the type checked comparison operators (<code>===</code> and <code>!==</code>).

When type conversion is required, JavaScript converts {{mono|Boolean}}, {{mono|Number}}, {{mono|String}}, or {{mono|Object}} operands as follows:<ref>{{cite web | url=https://developer.mozilla.org/en/JavaScript/Reference/Operators/Comparison_Operators | title=Comparison Operators - MDC Doc Center | publisher=Mozilla | date=5 August 2010 | access-date=5 March 2011 | archive-date=4 May 2012 | archive-url=https://web.archive.org/web/20120504005400/https://developer.mozilla.org/en/JavaScript/Reference/Operators/Comparison_Operators | url-status=live }}</ref>
;{{small|Number and String}}: The string is converted to a number value. JavaScript attempts to convert the string numeric literal to a Number type value. First, a mathematical value is derived from the string numeric literal. Next, this value is rounded to nearest Number type value.
;{{small|Boolean}}: If one of the operands is a Boolean, the Boolean operand is converted to 1 if it is {{mono|true}}, or to 0 if it is {{mono|false}}.
;{{small|Object}}: If an object is compared with a number or string, JavaScript attempts to return the default value for the object. An object is converted to a primitive String or Number value, using the {{mono|.valueOf()}} or {{mono|.toString()}} methods of the object. If this fails, a runtime error is generated.

==== Boolean type conversion ====
{{anchor|truthy and falsy}}
{{further|Truthy (computing)}}
[[Douglas Crockford]] advocates the terms "truthy" and "falsy" to describe how values of various types behave when evaluated in a logical context, especially in regard to edge cases.<ref>{{cite web | url=http://javascript.crockford.com/style2.html | title=The Elements of JavaScript Style | publisher=Douglas Crockford | access-date=5 March 2011 | archive-date=17 March 2011 | archive-url=https://web.archive.org/web/20110317074944/http://javascript.crockford.com/style2.html | url-status=live }}</ref>
The binary logical operators returned a Boolean value in early versions of JavaScript, but now they return one of the operands instead. The left–operand is returned, if it can be evaluated as : {{mono|false}}, in the case of [[logical conjunction|conjunction]]: (<code>a && b</code>), or {{mono|true}}, in the case of [[logical disjunction|disjunction]]: (<code>a || b</code>); otherwise the right–operand is returned. Automatic type coercion by the comparison operators may differ for cases of mixed Boolean and number-compatible operands (including strings that can be evaluated as a number, or objects that can be evaluated as such a string), because the Boolean operand will be compared as a numeric value. This may be unexpected. An expression can be explicitly cast to a Boolean primitive by doubling the logical [[negation|negation operator]]: ({{mono|!!}}), using the {{mono|Boolean()}} function, or using the [[Conditional (programming)|conditional operator]]: (<code>c ? t : f</code>).

<syntaxhighlight lang="javascript">
// Automatic type coercion
console.log(true  ==   2 ); // false... true  → 1 !== 2 ←  2
console.log(false ==   2 ); // false... false → 0 !== 2 ←  2
console.log(true  ==   1 ); // true.... true  → 1 === 1 ←  1
console.log(false ==   0 ); // true.... false → 0 === 0 ←  0
console.log(true  ==  "2"); // false... true  → 1 !== 2 ← "2"
console.log(false ==  "2"); // false... false → 0 !== 2 ← "2"
console.log(true  ==  "1"); // true.... true  → 1 === 1 ← "1"
console.log(false ==  "0"); // true.... false → 0 === 0 ← "0"
console.log(false ==  "" ); // true.... false → 0 === 0 ← ""
console.log(false ==  NaN); // false... false → 0 !== NaN

console.log(NaN == NaN); // false...... NaN is not equivalent to anything, including NaN.

// Type checked comparison (no conversion of types and values)
console.log(true === 1); // false...... data types do not match

// Explicit type coercion
console.log(true === !!2);   // true.... data types and values match
console.log(true === !!0);   // false... data types match, but values differ
console.log( 1  ? true : false); // true.... only ±0 and NaN are "falsy" numbers
console.log("0" ? true : false); // true.... only the empty string is "falsy"
console.log(Boolean({}));    // true.... all objects are "truthy"
</syntaxhighlight>

The new operator can be used to create an object wrapper for a Boolean primitive. However, the {{mono|typeof}} operator does not return {{mono|boolean}} for the object wrapper, it returns {{mono|object}}. Because all objects evaluate as {{mono|true}}, a method such as {{mono|.valueOf()}}, or {{mono|.toString()}}, must be used to retrieve the wrapped value. For explicit coercion to the Boolean type, Mozilla recommends that the {{mono|Boolean()}} function (without {{mono|new}}) be used in preference to the Boolean object.

<syntaxhighlight lang="javascript">
const b = new Boolean(false);   // Object  false {}
const t = Boolean(b);           // Boolean true
const f = Boolean(b.valueOf()); // Boolean false
let n = new Boolean(b);       // Not recommended
n = new Boolean(b.valueOf()); // Preferred

if (0 || -0 || "" || null || undefined || b.valueOf() || !new Boolean() || !t) {
  console.log("Never this");
} else if ([] && {} && b && typeof b === "object" && b.toString() === "false") {
  console.log("Always this");
}
</syntaxhighlight>

===Symbol===
New in ECMAScript6. A ''Symbol'' is a unique and immutable identifier.

Example:

<syntaxhighlight lang="javascript">
let x = Symbol(1);
const y = Symbol(1);
x === y; // => false

const symbolObject = {};
const normalObject = {};

// since x and y are unique,
// they can be used as unique keys in an object
symbolObject[x] = 1;
symbolObject[y] = 2;

symbolObject[x]; // => 1
symbolObject[y]; // => 2

// as compared to normal numeric keys
normalObject[1] = 1;
normalObject[1] = 2; // overrides the value of 1

normalObject[1]; // => 2

// changing the value of x does not change the key stored in the object
x = Symbol(3);
symbolObject[x]; // => undefined

// changing x back just creates another unique Symbol
x = Symbol(1);
symbolObject[x]; // => undefined
</syntaxhighlight>

There are also ''well known symbols''. <!-- TODO: Add table or something -->

One of which is <code>Symbol.iterator</code>; if something implements <code>Symbol.iterator</code>, it is iterable:

<syntaxhighlight lang="javascript">
const x = [1, 2, 3, 4]; // x is an Array
x[Symbol.iterator] === Array.prototype[Symbol.iterator]; // and Arrays are iterable

const xIterator = x[Symbol.iterator](); // The [Symbol.iterator] function should provide an iterator for x
xIterator.next(); // { value: 1, done: false }
xIterator.next(); // { value: 2, done: false }
xIterator.next(); // { value: 3, done: false }
xIterator.next(); // { value: 4, done: false }
xIterator.next(); // { value: undefined, done: true }
xIterator.next(); // { value: undefined, done: true }

// for..of loops automatically iterate values
for (const value of x) {
   console.log(value); // 1 2 3 4
}

// Sets are also iterable:
[Symbol.iterator] in Set.prototype; // true

for (const value of new Set(['apple', 'orange'])) {
   console.log(value); // "apple" "orange"
}
</syntaxhighlight>

==Native objects==

The JavaScript language provides a handful of native [[Object (computer science)|objects]]. JavaScript native objects are considered part of the JavaScript specification. JavaScript environment notwithstanding, this set of objects should always be available.

===Array===
{{Main|Array data type}}
An [[Array data type|Array]] is a JavaScript object prototyped from the <code>Array</code> constructor specifically designed to store data values indexed by integer keys. Arrays, unlike the basic Object type, are prototyped with methods and properties to aid the programmer in routine tasks (for example, <code>join</code>, <code>slice</code>, and <code>push</code>).

As in the [[:Category:C programming language family|C family]], arrays use a zero-based indexing scheme: A value that is inserted into an empty array by means of the <code>push</code> method occupies the 0th index of the array.

<syntaxhighlight lang="javascript">
const myArray = [];          // Point the variable myArray to a newly ...
                             // ... created, empty Array
myArray.push("hello World"); // Fill the next empty index, in this case 0
console.log(myArray[0]);     // Equivalent to console.log("hello World");
</syntaxhighlight>

Arrays have a <code>length</code> property that is guaranteed to always be larger than the largest integer index used in the array. It is automatically updated, if one creates a property with an even larger index. Writing a smaller number to the <code>length</code> property will remove larger indices.

Elements of <code>Array</code>s may be accessed using normal object property access notation:

<syntaxhighlight lang="javascript">
myArray[1];  // the 2nd item in myArray
myArray["1"];
</syntaxhighlight>

The above two are equivalent. It is not possible to use the "dot"-notation or strings with alternative representations of the number:

<syntaxhighlight lang="javascript">
myArray.1;     // syntax error
myArray["01"]; // not the same as myArray[1]
</syntaxhighlight>

Declaration of an array can use either an <code>Array</code> literal or the <code>Array</code> constructor:

<syntaxhighlight lang="javascript">
let myArray;

// Array literals
myArray = [1, 2]; // length of 2
myArray = [1, 2,]; // same array - Users can also have an extra comma at the end

// It is also possible to not fill in parts of the array
myArray = [0, 1, /* hole */, /* hole */, 4, 5]; // length of 6
myArray = [0, 1, /* hole */, /* hole */, 4, 5,]; // same array
myArray = [0, 1, /* hole */, /* hole */, 4, 5, /* hole */,]; // length of 7

// With the constructor
myArray = new Array(0, 1, 2, 3, 4, 5); // length of 6
myArray = new Array(365);              // an empty array with length 365
</syntaxhighlight>

[[Array data structure|Arrays]] are implemented so that only the defined elements use memory; they are "[[sparse array]]s". Setting {{code|lang=javascript|code=myArray[10] = 'someThing'}} and {{code|lang=javascript|code=myArray[57] = 'somethingOther'}} only uses space for these two elements, just like any other object. The <code>length</code> of the array will still be reported as 58. The maximum length of an array is 4,294,967,295 which corresponds to 32-bit binary number (11111111111111111111111111111111)<sub>2</sub>.

One can use the object declaration literal to create objects that behave much like associative arrays in other languages:

<syntaxhighlight lang="javascript">
const dog = {color: "brown", size: "large"};
dog["color"]; // results in "brown"
dog.color;    // also results in "brown"
</syntaxhighlight>

One can use the object and array declaration literals to quickly create arrays that are associative, multidimensional, or both. (Technically, JavaScript does not support multidimensional arrays, but one can mimic them with arrays-of-arrays.)

<syntaxhighlight lang="javascript">
const cats = [{color: "brown", size: "large"},
    {color: "black", size: "small"}];
cats[0]["size"];      // results in "large"

const dogs = {rover: {color: "brown", size: "large"},
    spot: {color: "black", size: "small"}};
dogs["spot"]["size"]; // results in "small"
dogs.rover.color;     // results in "brown"
</syntaxhighlight>

===Date===

A <code>Date</code> object stores a signed millisecond count with zero representing 1970-01-01 00:00:00 UT and a range of ±10<sup>8</sup> days. There are several ways of providing arguments to the <code>Date</code> constructor. Note that months are zero-based.

<syntaxhighlight lang="javascript">
new Date();                       // create a new Date instance representing the current time/date.
new Date(2010, 2, 1);             // create a new Date instance representing 2010-Mar-01 00:00:00
new Date(2010, 2, 1, 14, 25, 30); // create a new Date instance representing 2010-Mar-01 14:25:30
new Date("2010-3-1 14:25:30");    // create a new Date instance from a String.
</syntaxhighlight>

Methods to extract fields are provided, as well as a useful <code>toString</code>:

<syntaxhighlight lang="javascript">
const d = new Date(2010, 2, 1, 14, 25, 30); // 2010-Mar-01 14:25:30;

// Displays '2010-3-1 14:25:30':
console.log(d.getFullYear() + '-' + (d.getMonth() + 1) + '-' + d.getDate() + ' '
    + d.getHours() + ':' + d.getMinutes() + ':' + d.getSeconds());

// Built-in toString returns something like 'Mon 1 March, 2010 14:25:30 GMT-0500 (EST)':
console.log(d);
</syntaxhighlight>

===Error===

Custom error messages can be created using the <code>Error</code> class:

<syntaxhighlight lang="javascript">
throw new Error("Something went wrong.");
</syntaxhighlight>

These can be caught by try...catch...finally blocks as described in the section on [[#Exception_handling|exception handling]].

===Math===
The {{mono|Math}} object contains various math-related constants (for example, {{pi}}) and functions (for example, cosine). (Note that the {{mono|Math}} object has no constructor, unlike {{mono|Array}} or {{mono|Date}}. All its methods are "static", that is "class" methods.) All the trigonometric functions use angles expressed in [[radian]]s, not [[Degree (angle)|degrees]] or [[Grad (angle)|grads]].

{| class="wikitable" border="1"
|+ Some of the constants contained in the Math object
|-
!Property!!Returned value<br />rounded to 5 digits!!Description
|-
| {{mono|Math.E}} || 2.7183 || {{mvar|[[e (mathematical constant)|e]]}}: Natural logarithm base
|-
| {{mono|Math.LN2}} || 0.69315 || [[Natural logarithm]] of 2
|-
| {{mono|Math.LN10}} || 2.3026 || Natural logarithm of 10
|-
| {{mono|Math.LOG2E}} || 1.4427 || [[Logarithm]] to the base 2 of {{mvar|e}}
|-
| {{mono|Math.LOG10E}} || 0.43429 || Logarithm to the base 10 of {{mvar|e}}
|-
| {{mono|Math.PI}} || 3.14159 || [[Pi|{{pi}}]]: circumference/diameter of a circle
|-
| {{mono|Math.SQRT1_2}} || 0.70711 || [[Square root]] of ½
|-
| {{mono|Math.SQRT2}} || 1.4142 || [[Square root of 2]]
|}

{| class="wikitable" border="1"
|+ Methods of the Math object
|-
!Example!!Returned value<br />rounded to 5 digits!!Description
|-
| {{mono|Math.abs(-2.3)}} || 2.3 || [[Absolute value]]
|-
| {{mono|Math.acos(Math.SQRT1_2)}} || {{val|0.78540|ul=rad|fmt=none}} = 45° || [[Arccosine]]
|-
| {{mono|Math.asin(Math.SQRT1_2)}} || {{val|0.78540|u=rad|fmt=none}} = 45° || [[Arcsine]]
|-
| {{mono|Math.atan(1)}} || {{val|0.78540|u=rad|fmt=none}} = 45° || Half circle [[arctangent]] ({{tmath|-\pi/2}} to {{tmath|+\pi/2}})
|-
| {{mono|Math.atan2(-3.7, -3.7)}} || {{val|-2.3562|u=rad}} = {{val|-135|u=deg}} || Whole circle arctangent ({{tmath|-\pi}} to {{tmath|+\pi}})
|-
| {{mono|Math.ceil(1.1)}} || 2 || Ceiling: [[rounding|round]] up to smallest integer ≥ argument
|-
| {{mono|Math.cos(Math.PI/4)}} || 0.70711 || [[Trigonometric functions|Cosine]]
|-
| {{mono|Math.exp(1)}}|| 2.7183 || [[Exponential function]]: {{mvar|e}} raised to this power
|-
| {{mono|Math.floor(1.9)}} || 1 || Floor: round down to largest integer ≤ argument
|-
| {{mono|Math.log(Math.E)}} || 1 || Natural logarithm, base {{mvar|e}}
|-
| {{mono|Math.max(1, -2)}} || 1 || Maximum: {{mono|(x > y) ? x : y}}
|-
| {{mono|Math.min(1, -2)}} || {{val|-2}} || Minimum: {{mono|(x < y) ? x : y}}
|-
| {{mono|Math.pow(-3, 2)}} || 9 || [[Exponentiation]] (raised to the power of): {{mono|Math.pow(x, y)}} gives x<sup>y</sup>
|-
| {{mono|Math.random()}} || e.g. 0.17068 || [[Pseudorandom]] number between 0 (inclusive) and 1 (exclusive)
|-
| {{mono|Math.round(1.5)}} || 2 || Round to the nearest integer; half fractions are rounded up (e.g. 1.5 rounds to 2)
|-
| {{mono|Math.sin(Math.PI/4)}} || 0.70711 || [[Sine]]
|-
| {{mono|Math.sqrt(49)}} || 7 || Square root
|-
| {{mono|Math.tan(Math.PI/4)}} || 1 || [[Trigonometric functions|Tangent]]
|}

===Regular expression===
{{main|Regular expression}}
<syntaxhighlight lang="javascript">
/expression/.test(string);     // returns Boolean
"string".search(/expression/); // returns position Number
"string".replace(/expression/, replacement);

// Here are some examples
if (/Tom/.test("My name is Tom")) console.log("Hello Tom!");
console.log("My name is Tom".search(/Tom/));          // == 11 (letters before Tom)
console.log("My name is Tom".replace(/Tom/, "John")); // == "My name is John"
</syntaxhighlight>

====Character classes====
<syntaxhighlight lang="javascript">
// \d  - digit
// \D  - non digit
// \s  - space
// \S  - non space
// \w  - word char
// \W  - non word
// [ ] - one of
// [^] - one not of
//  -  - range

if (/\d/.test('0'))                   console.log('Digit');
if (/[0-9]/.test('6'))                console.log('Digit');
if (/[13579]/.test('1'))              console.log('Odd number');
if (/\S\S\s\S\S\S\S/.test('My name')) console.log('Format OK');
if (/\w\w\w/.test('Tom'))             console.log('Hello Tom');
if (/[a-zA-Z]/.test('B'))             console.log('Letter');
</syntaxhighlight>

====Character matching====
<syntaxhighlight lang="javascript">
// A...Z a...z 0...9  - alphanumeric
// \u0000...\uFFFF    - Unicode hexadecimal
// \x00...\xFF        - ASCII hexadecimal
// \t                 - tab
// \n                 - new line
// \r                 - CR
// .                  - any character
// |                  - OR

if (/T.m/.test('Tom')) console.log ('Hi Tom, Tam or Tim');
if (/A|B/.test("A"))  console.log ('A or B');
</syntaxhighlight>

====Repeaters====
<syntaxhighlight lang="javascript">
// ?   - 0 or 1 match
// *   - 0 or more
// +   - 1 or more
// {n}   - exactly n
// {n,}  - n or more
// {0,n} - n or less
// {n,m} - range n to m

if (/ab?c/.test("ac"))       console.log("OK"); // match: "ac", "abc"
if (/ab*c/.test("ac"))       console.log("OK"); // match: "ac", "abc", "abbc", "abbbc" etc.
if (/ab+c/.test("abc"))      console.log("OK"); // match: "abc", "abbc", "abbbc" etc.
if (/ab{3}c/.test("abbbc"))  console.log("OK"); // match: "abbbc"
if (/ab{3,}c/.test("abbbc")) console.log("OK"); // match: "abbbc", "abbbbc", "abbbbbc" etc.
if (/ab{1,3}c/.test("abc"))  console.log("OK"); // match: "abc", "abbc", "abbbc"
</syntaxhighlight>

====Anchors====
<syntaxhighlight lang="javascript">
// ^  - string starts with
// $  - string ends with

if (/^My/.test("My name is Tom"))   console.log ("Hi!");
if (/Tom$/.test("My name is Tom"))  console.log ("Hi Tom!");
</syntaxhighlight>

====Subexpression====
<syntaxhighlight lang="javascript">
// ( )  - groups characters

if (/water(mark)?/.test("watermark")) console.log("Here is water!"); // match: "water", "watermark",
if (/(Tom)|(John)/.test("John"))      console.log("Hi Tom or John!");
</syntaxhighlight>

====Flags====
<syntaxhighlight lang="javascript">
// /g  - global
// /i  - ignore upper/lower case
// /m  - allow matches to span multiple lines

console.log("hi tom!".replace(/Tom/i, "John"));  // == "hi John!"
console.log("ratatam".replace(/ta/, "tu"));      // == "ratutam"
console.log("ratatam".replace(/ta/g, "tu"));     // == "ratutum"
</syntaxhighlight>

====Advanced methods====
<syntaxhighlight lang="javascript">
my_array = my_string.split(my_delimiter);
// example
my_array = "dog,cat,cow".split(",");      // my_array==["dog","cat","cow"];

my_array = my_string.match(my_expression);
// example
my_array = "We start at 11:30, 12:15 and 16:45".match(/\d\d:\d\d/g); // my_array==["11:30","12:15","16:45"];
</syntaxhighlight>

====Capturing groups====
<syntaxhighlight lang="javascript">
const myRe = /(\d{4}-\d{2}-\d{2}) (\d{2}:\d{2}:\d{2})/;
const results = myRe.exec("The date and time are 2009-09-08 09:37:08.");
if (results) {
  console.log("Matched: " + results[0]); // Entire match
  const my_date = results[1]; // First group == "2009-09-08"
  const my_time = results[2]; // Second group == "09:37:08"
  console.log(`It is ${my_time} on ${my_date}`);
} else console.log("Did not find a valid date!");
</syntaxhighlight>

===Function===
Every function in JavaScript is an instance of the <code>Function</code> constructor<!-- There's also constructor for async / generator functions: (async()=>{}).constructor gets AsyncFunction, (async function*(){}).constructor gets AsyncGeneratorFunction, (function*(){}).constructor gets GeneratorFunction -->:

<syntaxhighlight lang="javascript">
// x, y is the argument. 'return x + y' is the function body, which is the last in the argument list.
const add = new Function('x', 'y', 'return x + y');
add(1, 2); // => 3
</syntaxhighlight>

The add function above may also be defined using a function expression:

<syntaxhighlight lang="javascript">
const add = function(x, y) {
  return x + y;
};
add(1, 2); // => 3
</syntaxhighlight>

In ES6, arrow function syntax was added, allowing functions that return a value to be more concise. They also retain the <code>this</code> of the global object instead of inheriting it from where it was called / what it was called on, unlike the <code>function() {}</code> expression.

<syntaxhighlight lang="javascript">
const add = (x, y) => {return x + y;};
// values can also be implicitly returned (i.e. no return statement is needed)
const addImplicit = (x, y) => x + y;

add(1, 2); // => 3
addImplicit(1, 2) // => 3
</syntaxhighlight>

For functions that need to be hoisted, there is a separate expression:

<syntaxhighlight lang="javascript">
function add(x, y) {
  return x + y;
}
add(1, 2); // => 3
</syntaxhighlight>

Hoisting allows users to use the function before it is "declared":

<syntaxhighlight lang="javascript">
add(1, 2); // => 3, not a ReferenceError
function add(x, y) {
  return x + y;
}
</syntaxhighlight>

A function instance has properties and methods.

<syntaxhighlight lang="javascript">
function subtract(x, y) {
  return x - y;
}

console.log(subtract.length); // => 2, arity of the function (number of arguments)
console.log(subtract.toString());

/*
"function subtract(x, y) {
  return x - y;
}"
*/
</syntaxhighlight>

==Operators==
The '+' operator is [[Operator overloading|overloaded]]: it is used for string concatenation and arithmetic addition. This may cause problems when inadvertently mixing strings and numbers. As a unary operator, it can convert a numeric string to a number.

<syntaxhighlight lang="javascript">
// Concatenate 2 strings
console.log('He' + 'llo'); // displays Hello

// Add two numbers
console.log(2 + 6);  // displays 8

// Adding a number and a string results in concatenation (from left to right)
console.log(2 + '2');    // displays 22
console.log('$' + 3 + 4);  // displays $34, but $7 may have been expected
console.log('$' + (3 + 4)); // displays $7
console.log(3 + 4 + '7'); // displays 77, numbers stay numbers until a string is added

// Convert a string to a number using the unary plus
console.log(+'2' === 2); // displays true
console.log(+'Hello'); // displays NaN
</syntaxhighlight>

Similarly, the '*' operator is overloaded: it can convert a string into a number.

<syntaxhighlight lang="javascript">
console.log(2 + '6'*1);  // displays 8
console.log(3*'7'); // 21
console.log('3'*'7'); // 21
console.log('hello'*'world'); // displays NaN
</syntaxhighlight>

===Arithmetic===
JavaScript supports the following '''binary arithmetic operators''':

{| class="wikitable"
|-
| align="center" | <code>+</code> || addition
|-
| align="center" | <code>-</code> || subtraction
|-
| align="center" | <code>*</code> || multiplication
|-
| align="center" | <code>/</code> || division (returns a floating-point value)
|-
| align="center" | <code>%</code> || modulo (returns the remainder)
|-
| align="center" | <code>**</code> || exponentiation
|}

JavaScript supports the following '''unary arithmetic operators''':

{| class="wikitable"
|-
| align="center" | <code>+</code> || unary conversion of string to number
|-
| align="center" | <code>-</code> || unary negation (reverses the sign)
|-
| align="center" | <code>++</code> || increment (can be prefix or postfix)
|-
| align="center" | <code>--</code> || decrement (can be prefix or postfix)
|}

<syntaxhighlight lang="javascript">
let x = 1;
console.log(++x); // x becomes 2; displays 2
console.log(x++); // displays 2; x becomes 3
console.log(x);  // x is 3; displays 3
console.log(x--); //  displays 3; x becomes 2
console.log(x);  //  displays 2; x is 2
console.log(--x); //  x becomes 1; displays 1
</syntaxhighlight>

The modulo operator displays the remainder after division by the modulus. If negative numbers are involved, the returned value depends on the operand.

<syntaxhighlight lang="javascript">
const x = 17;
console.log(x%5); // displays 2
console.log(x%6); // displays 5
console.log(-x%5); // displays -2
console.log(-x%-5); // displays -2
console.log(x%-5); // displays 2
</syntaxhighlight>

To always return a non-negative number, users can re-add the modulus and apply the modulo operator again:

<syntaxhighlight lang="javascript">
const x = 17;
console.log((-x%5+5)%5); // displays 3
</syntaxhighlight>
Users could also do:
<syntaxhighlight lang="javascript">
const x = 17;
console.log(Math.abs(-x%5)); // also 3
</syntaxhighlight>

===Assignment===
{| class="wikitable"
|-
| align="center" | <code>=</code> || assign
|-
| align="center" | <code>+=</code> || add and assign
|-
| align="center" | <code>-=</code> || subtract and assign
|-
| align="center" | <code>*=</code> || multiply and assign
|-
| align="center" | <code>/=</code> || divide and assign
|-
| align="center" | <code>%=</code> || modulo and assign
|-
| align="center" | <code>**=</code> || exponentiation and assign
|}

[[Assignment (computer science)|Assignment]] of [[primitive type]]s

<syntaxhighlight lang="javascript">
let x = 9;
x += 1; 
console.log(x); // displays: 10
x *= 30;
console.log(x); // displays: 300
x /= 6;
console.log(x); // displays: 50
x -= 3;
console.log(x); // displays: 47
x %= 7;
console.log(x); // displays: 5
</syntaxhighlight>

Assignment of object types

<syntaxhighlight lang="javascript">
/**
 * To learn JavaScript objects...
 */
const object_1 = {a: 1};		// assign reference of newly created object to object_1
let object_2 = {a: 0};
let object_3 = object_2;	    // object_3 references the same object as object_2 does
	 
object_3.a = 2;
message();	        	// displays 1 2 2
	 
object_2 = object_1;		// object_2 now references the same object as object_1
	        	        // object_3 still references what object_2 referenced before
message();		        // displays 1 1 2
	 
object_2.a = 7;  	        // modifies object_1
message();		        // displays 7 7 2

object_3.a = 5;                 // object_3 does not change object_2
message();	                // displays 7 7 5

object_3 = object_2;	
object_3.a=4;                  // object_3 changes object_1 and object_2
message();                     // displays 4 4 4

/**
 * Prints the console.log message
 */
function message() {
	console.log(object_1.a + " " + object_2.a + " " + object_3.a);
}
</syntaxhighlight>

==== Destructuring assignment ====
In Mozilla's JavaScript, since version 1.7, destructuring assignment allows the assignment of parts of data structures to several variables at once. The left hand side of an assignment is a pattern that resembles an arbitrarily nested object/array literal containing l-lvalues at its leaves that are to receive the substructures of the assigned value.

<syntaxhighlight lang="javascript">
let a, b, c, d, e;
[a, b, c] = [3, 4, 5];
console.log(`${a},${b},${c}`); // displays: 3,4,5
e = {foo: 5, bar: 6, baz: ['Baz', 'Content']};
const arr = [];
({baz: [arr[0], arr[3]], foo: a, bar: b} = e);
console.log(`${a},${b},${arr}`);	// displays: 5,6,Baz,,,Content
[a, b] = [b, a];		// swap contents of a and b
console.log(a + ',' + b);		// displays: 6,5

[a, b, c] = [3, 4, 5]; // permutations
[a, b, c] = [b, c, a];
console.log(`${a},${b},${c}`); // displays: 4,5,3
</syntaxhighlight>

==== Spread/rest operator ====

The ECMAScript 2015 standard introduced the "<code>...</code>" array operator, for the related concepts of "spread syntax"<ref>{{Cite web|url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_syntax|title=Spread syntax (...) - JavaScript &#124; MDN|date=25 September 2023|website=developer.mozilla.org}}</ref> and "rest parameters".<ref>{{cite web| url = https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters| title = rest parameters| date = 9 September 2024| access-date = 29 September 2016| archive-date = 30 May 2018| archive-url = https://web.archive.org/web/20180530204951/https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/rest_parameters| url-status = live}}</ref> Object spreading was added in ECMAScript 2018.

'''Spread syntax''' provides another way to destructure arrays and objects. For arrays, it indicates that the elements should be used as the parameters in a function call or the items in an array literal. For objects, it can be used for merging objects together or overriding properties.

In other words, "<code>...</code>" transforms "<code>[...foo]</code>" into "<code>[foo[0], foo[1], foo[2]]</code>", and "<code>this.bar(...foo);</code>" into "<code>this.bar(foo[0], foo[1], foo[2]);</code>", and "<code>{ ...bar }</code>" into <code>{ prop: bar.prop, prop2: bar.prop2 }</code>.

<syntaxhighlight lang="javascript" line="1">
const a = [1, 2, 3, 4];

// It can be used multiple times in the same expression
const b = [...a, ...a]; // b = [1, 2, 3, 4, 1, 2, 3, 4];

// It can be combined with non-spread items.
const c = [5, 6, ...a, 7, 9]; // c = [5, 6, 1, 2, 3, 4, 7, 9];

// For comparison, doing this without the spread operator 
// creates a nested array.
const d = [a, a]; // d = [[1, 2, 3, 4], [1, 2, 3, 4]]

// It works the same with function calls
function foo(arg1, arg2, arg3) {
    console.log(`${arg1}:${arg2}:${arg3}`);
}

// Users can use it even if it passes more parameters than the function will use
foo(...a); // "1:2:3" → foo(a[0], a[1], a[2], a[3]);

// Users can mix it with non-spread parameters
foo(5, ...a, 6); // "5:1:2" → foo(5, a[0], a[1], a[2], a[3], 6);

// For comparison, doing this without the spread operator
// assigns the array to arg1, and nothing to the other parameters.
foo(a); // "1,2,3,4:undefined:undefined"

const bar = { a: 1, b: 2, c: 3 };

// This would copy the object
const copy = { ...bar }; // copy = { a: 1, b: 2, c: 3 };

// "b" would be overridden here
const override = { ...bar, b: 4 }; // override = { a: 1, c: 3, b: 4 }
</syntaxhighlight>

When <code>...</code> is used in a function ''declaration'', it indicates a '''rest parameter'''. The rest parameter must be the final named parameter in the function's parameter list. It will be assigned an <code>Array</code> containing any arguments passed to the function in excess of the other named parameters. In other words, it gets "the rest" of the arguments passed to the function (hence the name).

<syntaxhighlight lang="javascript">
function foo(a, b, ...c) {
    console.log(c.length);
}

foo(1, 2, 3, 4, 5); // "3" → c = [3, 4, 5]
foo('a', 'b'); // "0" → c = []
</syntaxhighlight>

Rest parameters are similar to Javascript's <code>arguments</code> object, which is an array-like object that contains all of the parameters (named and unnamed) in the current function call. Unlike <code>arguments</code>, however, rest parameters are true <code>Array</code> objects, so methods such as <code>.slice()</code> and <code>.sort()</code> can be used on them directly.

===Comparison===
{| class=wikitable
|-
| align="center" | <code>==</code> || equal
|-
| align="center" | <code>!=</code> || not equal
|-
| align="center" | <code>&gt;</code> || greater than
|-
| align="center" | <code>&gt;=</code> || greater than or equal to
|-
| align="center" | <code>&lt;</code> || less than
|-
| align="center" | <code>&lt;=</code> || less than or equal to
|-
| align="center" | <code>===</code> || identical (equal and of same type)
|-
| align="center" | <code>!==</code> || not identical
|}

Variables referencing objects are equal or identical only if they reference the same object:

<syntaxhighlight lang="javascript">
const obj1 = {a: 1};
const obj2 = {a: 1};
const obj3 = obj1;
console.log(obj1 == obj2);  //false
console.log(obj3 == obj1);  //true
console.log(obj3 === obj1); //true
</syntaxhighlight>

See also [[#String|String]].

===Logical===
JavaScript provides four logical operators:
* unary [[negation]] (<code>NOT = !a</code>)
* binary [[logical disjunction|disjunction]] (<code>OR = a || b</code>) and [[logical conjunction|conjunction]] (<code>AND = a && b</code>)
* ternary [[Conditional (programming)|conditional]] (<code>c ? t : f</code>)

In the context of a logical operation, any expression evaluates to true except the following''':'''
* Strings: <code>""</code>, <code><nowiki>''</nowiki></code>,
* Numbers: <code>0</code>, <code>-0</code>, <code>NaN</code>,
* Special: <code>null</code>, <code>undefined</code>,
* Boolean: <code>false</code>.

The Boolean function can be used to explicitly convert to a primitive of type <code>Boolean</code>:

<syntaxhighlight lang="javascript">
// Only empty strings return false
console.log(Boolean("")      === false);
console.log(Boolean("false") === true);
console.log(Boolean("0")     === true);

// Only zero and NaN return false
console.log(Boolean(NaN) === false);
console.log(Boolean(0)   === false);
console.log(Boolean(-0)  === false); // equivalent to -1*0
console.log(Boolean(-2)  === true);

// All objects return true
console.log(Boolean(this) === true);
console.log(Boolean({})   === true);
console.log(Boolean([])   === true);

// These types return false
console.log(Boolean(null)      === false);
console.log(Boolean(undefined) === false); // equivalent to Boolean()
</syntaxhighlight>

The NOT operator evaluates its operand as a Boolean and returns the negation. Using the operator twice in a row, as a [[double negative]], explicitly converts an expression to a primitive of type Boolean:

<syntaxhighlight lang="javascript">
console.log( !0 === Boolean(!0));
console.log(Boolean(!0) === !!1);
console.log(!!1 === Boolean(1));
console.log(!!0 === Boolean(0));
console.log(Boolean(0) === !1);
console.log(!1 === Boolean(!1));
console.log(!"" === Boolean(!""));
console.log(Boolean(!"") === !!"s");
console.log(!!"s" === Boolean("s"));
console.log(!!"" === Boolean(""));
console.log(Boolean("") === !"s");	
console.log(!"s" === Boolean(!"s"));
</syntaxhighlight>

The ternary operator can also be used for explicit conversion:

<syntaxhighlight lang="javascript">
console.log([] == false); console.log([] ? true : false); // “truthy”, but the comparison uses [].toString()
console.log([0] == false); console.log([0]? true : false); // [0].toString() == "0"
console.log("0" == false); console.log("0"? true : false); // "0" → 0 ... (0 == 0) ... 0 ← false
console.log([1] == true); console.log([1]? true : false); // [1].toString() == "1"
console.log("1" == true); console.log("1"? true : false); // "1" → 1 ... (1 == 1) ... 1 ← true
console.log([2] != true); console.log([2]? true : false); // [2].toString() == "2"
console.log("2" != true); console.log("2"? true : false); // "2" → 2 ... (2 != 1) ... 1 ← true
</syntaxhighlight>

Expressions that use features such as post–incrementation (<code>i++</code>) have an anticipated [[Side effect (computer science)|side effect]]. JavaScript provides [[short-circuit evaluation]] of expressions; the right operand is only executed if the left operand does not suffice to determine the value of the expression.

<syntaxhighlight lang="javascript">
console.log(a || b);  // When a is true, there is no reason to evaluate b.
console.log(a && b);  // When a is false, there is no reason to evaluate b.
console.log(c ? t : f); // When c is true, there is no reason to evaluate f.
</syntaxhighlight>

In early versions of JavaScript and [[JScript]], the binary logical operators returned a Boolean value (like most C-derived programming languages). However, all contemporary implementations return one of their operands instead:

<syntaxhighlight lang="javascript">
console.log(a || b); // if a is true, return a, otherwise return b
console.log(a && b); // if a is false, return a, otherwise return b
</syntaxhighlight>

Programmers who are more familiar with the behavior in C might find this feature surprising, but it allows for a more concise expression of patterns like [[null coalescing operator|null coalescing]]:

<syntaxhighlight lang="javascript">
const s = t || "(default)"; // assigns t, or the default value, if t is null, empty, etc.
</syntaxhighlight>

=== Logical assignment ===
{| class="wikitable"
|-
| align="center" | <code>??=</code>
| Nullish assignment
|-
| align="center" | <code><nowiki>||=</nowiki></code>
| Logical Or assignment
|-
| align="center" | <code>&&=</code>
| Logical And assignment
|}

=== Bitwise ===
JavaScript supports the following '''binary [[Bitwise operation|bitwise operators]]''':

{| class="wikitable"
|-
| align="center" | <code>&</code> || AND
|-
| align="center" | <code>|</code> || OR
|-
| align="center" | <code>^</code> || XOR
|-
| align="center" | <code>!</code> || NOT
|-
| align="center" | <code>&lt;&lt;</code> || shift left (zero fill at right)
|-
| align="center" | <code>&gt;&gt;</code> || shift right (sign-propagating); copies of the<br />leftmost bit (sign bit) are shifted in from the left
|-
| align="center" | <code>&gt;&gt;&gt;</code> || shift right (zero fill at left). For positive numbers,<br /><code>&gt;&gt;</code> and <code>&gt;&gt;&gt;</code> yield the same result.
|}

Examples:

<syntaxhighlight lang="javascript">
const x = 11 & 6;
console.log(x); // 2
</syntaxhighlight>

JavaScript supports the following '''unary bitwise operator''':

{| class="wikitable"
|-
| align="center" | <code>~</code> || NOT (inverts the bits)
|}

===Bitwise Assignment===

JavaScript supports the following '''binary assignment operators:'''

{| class="wikitable"
|-
| align="center" | <code>&=</code> || and
|-
| align="center" | <code>|=</code> || or
|-
| align="center" | <code>^=</code> || xor
|-
| align="center" | <code>&lt;&lt;=</code> || shift left (zero fill at right)
|-
| align="center" | <code>&gt;&gt;=</code> || shift right (sign-propagating); copies of the<br />leftmost bit (sign bit) are shifted in from the left
|-
| align="center" | <code>&gt;&gt;&gt;=</code> || shift right (zero fill at left). For positive numbers,<br /><code>&gt;&gt;=</code> and <code>&gt;&gt;&gt;=</code> yield the same result.
|}

Examples:

<syntaxhighlight lang="javascript">
let x=7;
console.log(x); // 7
x<<=3;
console.log(x); // 7->14->28->56
</syntaxhighlight>

===String===

{| class="wikitable"
|-
| align="center" | <code>=</code> || assignment
|-
| align="center" | <code>+</code> || concatenation
|-
| align="center" | <code>+=</code> || concatenate and assign
|}

Examples:

<syntaxhighlight lang="javascript">
let str = "ab" + "cd";  // "abcd"
str += "e";      // "abcde"

const str2 = "2" + 2;     // "22", not "4" or 4.
</syntaxhighlight>

===??===
{{excerpt|Null coalescing operator|JavaScript}}

==Control structures==

===Compound statements===

A pair of curly brackets <code>{&nbsp;}</code> and an enclosed sequence of statements constitute a compound statement, which can be used wherever a statement can be used.

===If ... else===
<syntaxhighlight lang="javascript">
if (expr) {
  //statements;
} else if (expr2) {
  //statements;
} else {
  //statements;
}
</syntaxhighlight>

=== Conditional (ternary) operator ===

The conditional operator creates an expression that evaluates as one of two expressions depending on a condition. This is similar to the ''if'' statement that selects one of two statements to execute depending on a condition. I.e., the conditional operator is to expressions what ''if'' is to statements.

<syntaxhighlight lang="javascript">
const result = condition ? expression : alternative;
</syntaxhighlight>

is the same as:

<syntaxhighlight lang="javascript">
if (condition) {
    const result = expression;
} else {
    const result = alternative;
}
</syntaxhighlight>

Unlike the ''if'' statement, the conditional operator cannot omit its "else-branch".

===Switch statement===
{{Main|Switch statement}}
The syntax of the JavaScript [[Control flow#Choice|switch statement]] is as follows:

<syntaxhighlight lang="javascript">
 switch (expr) {
  case SOMEVALUE:
   // statements;
   break;
  case ANOTHERVALUE:
    // statements for when ANOTHERVALUE || ORNAOTHERONE
    // no break statement, falling through to the following case
  case ORANOTHERONE:
   // statements specific to ORANOTHERONE (i.e. !ANOTHERVALUE && ORANOTHER);
   break; //The buck stops here.
  case YETANOTHER:
   // statements;
   break;
  default:
   // statements;
   break;
 }
</syntaxhighlight>

* <code>break;</code> is optional; however, it is usually needed, since otherwise code execution will continue to the body of the next case block. This ''fall through'' behavior can be used when the same set of statements apply in several cases, effectively creating a [[disjunction]] between those cases.
* Add a break statement to the end of the last case as a precautionary measure, in case additional cases are added later.
* String literal values can also be used for the case values.
* Expressions can be used instead of values.
* The default case (optional) is executed when the expression does not match any other specified cases.
* Braces are required.

===For loop===
The syntax of the JavaScript [[for loop]] is as follows:

<syntaxhighlight lang="javascript">
 for (initial; condition; loop statement) {
  /*
   statements will be executed every time
   the for{} loop cycles, while the
   condition is satisfied
  */
 }
</syntaxhighlight>

or

<syntaxhighlight lang="javascript">
 for (initial; condition; loop statement(iteration)) // one statement
</syntaxhighlight>

===For ... in loop===
The syntax of the JavaScript <code>[[Foreach loop|for ... in loop]]</code> is as follows:

<syntaxhighlight lang="javascript">
for (var property_name in some_object) {
  // statements using some_object[property_name];
}
</syntaxhighlight>

* Iterates through all enumerable properties of an object.
* Iterates through all used indices of array including all user-defined properties of array object, if any. Thus it may be better to use a traditional for loop with a numeric index when iterating over arrays.
* There are differences between the various Web browsers with regard to which properties will be reflected with the for...in loop statement. In theory, this is controlled by an internal state property defined by the ECMAscript standard called "DontEnum", but in practice, each browser returns a slightly different set of properties during introspection. It is useful to test for a given property using {{nowrap|{{code|lang=javascript|code=if (some_object.hasOwnProperty(property_name)) { ... } }}}}. Thus, adding a method to the array prototype with {{nowrap|{{code|lang=javascript|code=Array.prototype.newMethod = function() {...} }}}} may cause <code>for ... in</code> loops to loop over the method's name.

===While loop===
The syntax of the JavaScript [[while loop]] is as follows:

<syntaxhighlight lang="javascript">
while (condition) {
  statement1;
  statement2;
  statement3;
  ...
}
</syntaxhighlight>

===Do ... while loop===
The syntax of the JavaScript <code>[[do while loop|do ... while loop]]</code> is as follows:

<syntaxhighlight lang="javascript">
do {
  statement1;
  statement2;
  statement3;
  ...
} while (condition);
</syntaxhighlight>

===With===
The with statement adds all of the given object's properties and methods into the following block's scope, letting them be referenced as if they were local variables.

<syntaxhighlight lang="javascript">
with (document) {
  const a = getElementById('a');
  const b = getElementById('b');
  const c = getElementById('c');
};
</syntaxhighlight>
* Note the absence of {{mono|document.}} before each {{mono|getElementById()}} invocation.

The semantics are similar to the with statement of [[Pascal (programming language)|Pascal]].

Because the availability of with statements hinders program performance and is believed to reduce code clarity (since any given variable could actually be a property from an enclosing {{mono|with}}), this statement is not allowed in ''strict mode''.

===Labels===
JavaScript supports nested labels in most implementations. Loops or blocks can be labeled for the break statement, and loops for <code>continue</code>. Although <code>[[goto]]</code> is a reserved word,<ref>ECMA-262, Edition 3, 7.5.3 Future Reserved Words</ref> <code>goto</code> is not implemented in JavaScript.

<syntaxhighlight lang="javascript">
loop1: for (let a = 0; a < 10; ++a) {
  if (a === 4) break loop1; // Stops after the 4th attempt
  console.log('a = ' + a);
  loop2: for (let b = 0; b < 10; ++b) {
    if (b === 3) continue loop2; // Number 3 is skipped
    if (b === 6) continue loop1; // Continues the first loop, 'finished' is not shown
    console.log('b = ' + b);
  } //end of loop2
  console.log('finished');
} //end of loop1
block1: {
  console.log('Hello'); // Displays 'Hello'
  break block1;
  console.log('World'); // Will never get here
}
goto block1; // Parse error.
</syntaxhighlight>

==Functions==
{{Main|Function (computer programming)}}

A [[Function (computer programming)|function]] is a block with a (possibly empty) parameter list that is normally given a name. A function may use local variables. If a user exits the function without a return statement, the value {{mono|undefined}} is returned.

<syntaxhighlight lang="javascript+genshitext">
function gcd(number1, number2) {
  if (isNaN(number1*number2)) throw TypeError("Non-Numeric arguments not allowed.");
  number1 = Math.round(number1);
  number2 = Math.round(number2);
  let difference = number1 - number2;
  if (difference === 0) return number1;
  return difference > 0 ? gcd(number2, difference) : gcd(number1, -difference);
}
console.log(gcd(60, 40)); // 20

//In the absence of parentheses following the identifier 'gcd' on the RHS of the assignment below,
//'gcd' returns a reference to the function itself without invoking it.
let mygcd = gcd; // mygcd and gcd reference the same function.
console.log(mygcd(60, 40)); // 20
</syntaxhighlight>

Functions are [[first class object]]s and may be assigned to other variables.

The number of arguments given when calling a function may not necessarily correspond to the number of arguments in the function definition; a named argument in the definition that does not have a matching argument in the call will have the value {{mono|undefined}} (that can be implicitly cast to false). Within the function, the arguments may also be accessed through the {{mono|arguments}} object; this provides access to all arguments using indices (e.g. {{code|lang=javascript|code=arguments[0], arguments[1], ... arguments[n]}}), including those beyond the number of named arguments. (While the arguments list has a <code>.length</code> property, it is ''not'' an instance of {{mono|Array}}; it does not have methods such as {{mono|.slice()}}, {{mono|.sort()}}, etc.)

<syntaxhighlight lang="javascript">
function add7(x, y) {
  if (!y) {
    y = 7;
  }
  console.log(x + y + arguments.length);
};
add7(3); // 11
add7(3, 4); // 9
</syntaxhighlight>

Primitive values (number, boolean, string) are passed by value. For objects, it is the reference to the object that is passed.

<syntaxhighlight lang="javascript">
const obj1 = {a : 1};
const obj2 = {b : 2};
function foo(p) {
  p = obj2; // Ignores actual parameter
  p.b = arguments[1];
}
foo(obj1, 3); // Does not affect obj1 at all. 3 is additional parameter
console.log(`${obj1.a} ${obj2.b}`); // writes 1 3
</syntaxhighlight>

Functions can be declared inside other functions, and access the outer function's local variables. Furthermore, they implement full [[closure (computer science)|closure]]s by remembering the outer function's local variables even after the outer function has exited.

<syntaxhighlight lang="javascript">
let t = "Top";
let bar, baz;
function foo() {
  let f = "foo var";
  bar = function() { console.log(f) };
  baz = function(x) { f = x; };
}
foo();
baz("baz arg");
bar(); // "baz arg" (not "foo var") even though foo() has exited.
console.log(t); // Top
</syntaxhighlight>

An ''anonymous function'' is simply a function without a name and can be written either using function or arrow notation.  In these equivalent examples an anonymous function is passed to the  '''map''' function and is applied to each of the elements of the array.<ref>{{cite web |last=Yadav |first=Amitya |date=4 October 2024 |title=Named function vs Anonymous Function Impacts |url=https://aditya003-ay.medium.com/named-function-vs-anonymous-function-impacts-94e2472ed7bb |website=medium.com |access-date=19 February 2025}}</ref>

<syntaxhighlight lang="javascript">
 [1,2,3].map(function(x) { return x*2;);  //returns [2,4,6]
 [1,2,3].map((x) => { return x*2;});      //same result
</syntaxhighlight>

A ''generator function'' is signified placing an * after the keyword ''function'' and contains one or more ''yield'' statements. The effect is to return a value and pause execution at the current state. Declaring an generator function returns an iterator.  Subsequent calls to ''iterator.next()'' resumes execution until the next ''yield''.  When the iterator returns without using a yield statement there are no more values and the '''done''' property of the iterator is set to '''true'''.<ref> {{cite web |author=<!-- not stated --> |title=function* |url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/function*|website=mdm web docs |access-date=23 February 2025}}</ref>

With the exception of [[iOS]] devices from Apple, generators are not implemented for browsers on mobile devices. <ref> {{cite web |author=<!-- not stated --> |title=ES6 Generators |url=https://caniuse.com/?search=generator|website=medium.com |access-date=23 February 2025}}</ref> 

<syntaxhighlight lang="javascript">
   function* generator() {
                 yield "red";
                 yield "green";
                 yield "blue";
            } 

    let iterator=generator(); 
    let current;

    while(current=iterator.next().value)
          console.log(current);         //displays red, green then blue
    console.log(iterator.next().done)  //displays true         
</syntaxhighlight> 


===Async/await===
{{excerpt|Async/await|In JavaScript}}

==Objects==
For convenience, types are normally subdivided into ''primitives'' and ''objects''. Objects are entities that have an identity (they are only equal to themselves) and that map property names to values ("slots" in [[prototype-based programming]] terminology). Objects may be thought of as [[associative arrays]] or hashes, and are often implemented using these data structures. However, objects have additional features, such as a [[Prototype-based programming|prototype chain]], which ordinary associative arrays do not have.

JavaScript has several kinds of built-in objects, namely <code>Array</code>, <code>Boolean</code>, <code>Date</code>, <code>Function</code>, <code>Math</code>, <code>Number</code>, <code>Object</code>, <code>RegExp</code> and <code>String</code>. Other objects are "host objects", defined not by the language, but by the runtime environment. For example, in a browser, typical host objects belong to the DOM (window, form, links, etc.).

===Creating objects===
Objects can be created using a constructor or an object literal. The constructor can use either a built-in Object function or a custom function. It is a convention that constructor functions are given a name that starts with a capital letter:

<syntaxhighlight lang="javascript">
// Constructor
const anObject = new Object();

// Object literal
const objectA = {};
const objectA2 = {};  // A != A2, {}s create new objects as copies.
const objectB = {index1: 'value 1', index2: 'value 2'};

// Custom constructor (see below)
</syntaxhighlight>

Object literals and array literals allow one to easily create flexible data structures:

<syntaxhighlight lang="javascript">
const myStructure = {
  name: {
    first: "Mel",
    last: "Smith"
  },
  age: 33,
  hobbies: ["chess", "jogging"]
};
</syntaxhighlight>
This is the basis for [[JSON]], which is a simple notation that uses JavaScript-like syntax for data exchange.

===Methods===

A [[Method (computer programming)|method]] is simply a function that has been assigned to a property name of an object. Unlike many object-oriented languages, there is no distinction between a function definition and a method definition in object-related JavaScript. Rather, the distinction occurs during function calling; a function can be called as a method.

When called as a method, the standard local variable ''{{mono|this}}'' is just automatically set to the object instance to the left of the "{{mono|.}}". (There are also ''{{mono|call}}'' and ''{{mono|apply}}'' methods that can set ''{{mono|this}}'' explicitly—some packages such as [[jQuery]] do unusual things with ''{{mono|this}}''.)

In the example below, Foo is being used as a constructor. There is nothing special about a constructor - it is just a plain function that initializes an object. When used with the ''{{mono|new}}'' keyword, as is the norm, ''{{mono|this}}'' is set to a newly created blank object.

Note that in the example below, Foo is simply assigning values to slots, some of which are functions. Thus it can assign different functions to different instances. There is no prototyping in this example.

<syntaxhighlight lang="javascript">
function px() { return this.prefix + "X"; }

function Foo(yz) {
  this.prefix = "a-";
  if (yz > 0) {
    this.pyz = function() { return this.prefix + "Y"; };
  } else {
    this.pyz = function() { return this.prefix + "Z"; };
  }
  this.m1 = px;
  return this;
}

const foo1 = new Foo(1);
const foo2 = new Foo(0);
foo2.prefix = "b-";

console.log("foo1/2 " + foo1.pyz() + foo2.pyz());
// foo1/2 a-Y b-Z

foo1.m3 = px; // Assigns the function itself, not its evaluated result, i.e. not px()
const baz = {"prefix": "c-"};
baz.m4 = px; // No need for a constructor to make an object.

console.log("m1/m3/m4 " + foo1.m1() + foo1.m3() + baz.m4());
// m1/m3/m4 a-X a-X c-X

foo1.m2(); // Throws an exception, because foo1.m2 does not exist.
</syntaxhighlight>

===Constructors===

[[Constructor function]]s simply assign values to slots of a newly created object. The values may be data or other functions.

Example: Manipulating an object:

<syntaxhighlight lang="javascript">
function MyObject(attributeA, attributeB) {
  this.attributeA = attributeA;
  this.attributeB = attributeB;
}

MyObject.staticC = "blue"; // On MyObject Function, not object
console.log(MyObject.staticC); // blue

const object = new MyObject('red', 1000);

console.log(object.attributeA); // red
console.log(object.attributeB); // 1000

console.log(object.staticC); // undefined
object.attributeC = new Date(); // add a new property

delete object.attributeB; // remove a property of object
console.log(object.attributeB); // undefined

</syntaxhighlight>

The constructor itself is referenced in the object's prototype's ''constructor'' slot. So,

<syntaxhighlight lang="javascript">
function Foo() {}
// Use of 'new' sets prototype slots (for example, 
// x = new Foo() would set x's prototype to Foo.prototype,
// and Foo.prototype has a constructor slot pointing back to Foo).
const x = new Foo();
// The above is almost equivalent to
const y = {};
y.constructor = Foo;
y.constructor();
// Except
x.constructor == y.constructor; // true
x instanceof Foo; // true
y instanceof Foo; // false
// y's prototype is Object.prototype, not
// Foo.prototype, since it was initialized with
// {} instead of new Foo.
// Even though Foo is set to y's constructor slot,
// this is ignored by instanceof - only y's prototype's
// constructor slot is considered.
</syntaxhighlight>

Functions are objects themselves, which can be used to produce an effect similar to "static properties" (using C++/Java terminology) as shown below. (The function object also has a special <code>prototype</code> property, as discussed in the "Inheritance" section below.)

Object deletion is rarely used as the scripting engine will [[Garbage collection (computer science)|garbage collect]] objects that are no longer being referenced.

===Inheritance===
JavaScript supports inheritance hierarchies through prototyping in the manner of [[Self (programming language)|Self]].

In the following example, the {{mono|Derived}} class inherits from the {{mono|Base}} class.
When {{mono|d}} is created as {{mono|Derived}}, the reference to the base instance of {{mono|Base}} is copied to {{mono|d.base}}.

Derive does not contain a value for {{mono|aBaseFunction}}, so it is retrieved from {{mono|aBaseFunction}} ''when {{mono|aBaseFunction}} is accessed''. This is made clear by changing the value of {{mono|base.aBaseFunction}}, which is reflected in the value of {{mono|d.aBaseFunction}}.

Some implementations allow the prototype to be accessed or set explicitly using the {{mono|__proto__}} slot as shown below.

<syntaxhighlight lang="javascript">
function Base() {
  this.anOverride = function() { console.log("Base::anOverride()"); };

  this.aBaseFunction = function() { console.log("Base::aBaseFunction()"); };
}

function Derived() {
  this.anOverride = function() { console.log("Derived::anOverride()"); };
}

const base = new Base();
Derived.prototype = base; // Must be before new Derived()
Derived.prototype.constructor = Derived; // Required to make `instanceof` work

const d = new Derived();    // Copies Derived.prototype to d instance's hidden prototype slot.
d instanceof Derived; // true
d instanceof Base;    // true

base.aBaseFunction = function() { console.log("Base::aNEWBaseFunction()"); };

d.anOverride();    // Derived::anOverride()
d.aBaseFunction(); // Base::aNEWBaseFunction()
console.log(d.aBaseFunction == Derived.prototype.aBaseFunction); // true

console.log(d.__proto__ == base); // true in Mozilla-based implementations and false in many others.
</syntaxhighlight>

The following shows clearly how references to prototypes are ''copied'' on instance creation, but that changes to a prototype can affect all instances that refer to it.

<syntaxhighlight lang="javascript">
function m1() { return "One"; }
function m2() { return "Two"; }
function m3() { return "Three"; }

function Base() {}

Base.prototype.m = m2;
const bar = new Base();
console.log("bar.m " + bar.m()); // bar.m Two

function Top() { this.m = m3; }
const t = new Top();

const foo = new Base();
Base.prototype = t;
// No effect on foo, the *reference* to t is copied.
console.log("foo.m " + foo.m()); // foo.m Two

const baz = new Base();
console.log("baz.m " + baz.m()); // baz.m Three

t.m = m1; // Does affect baz, and any other derived classes.
console.log("baz.m1 " + baz.m()); // baz.m1 One
</syntaxhighlight>

In practice many variations of these themes are used, and it can be both powerful and confusing.

==Exception handling==
JavaScript includes a <code>try ... catch ... finally</code> [[exception handling]] statement to handle run-time errors.

The <code>try ... catch ... finally</code> statement catches [[exception handling|exceptions]] resulting from an error or a throw statement. Its syntax is as follows:

<syntaxhighlight lang="javascript">
try {
  // Statements in which exceptions might be thrown
} catch(errorValue) {
  // Statements that execute in the event of an exception
} finally {
  // Statements that execute afterward either way
}
</syntaxhighlight>

Initially, the statements within the try block execute. If an exception is thrown, the script's control flow immediately transfers to the statements in the catch block, with the exception available as the error argument. Otherwise the catch block is skipped. The catch block can {{mono|throw(errorValue)}}, if it does not want to handle a specific error.

In any case the statements in the finally block are always executed. This can be used to free resources, although memory is automatically garbage collected.

Either the catch or the finally clause may be omitted. The catch argument is required.

The Mozilla implementation allows for multiple catch statements, as an extension to the ECMAScript standard. They follow a syntax similar to that used in [[Java (programming language)|Java]]:

<syntaxhighlight lang="javascript">
try { statement; }
catch (e if e == "InvalidNameException")  { statement; }
catch (e if e == "InvalidIdException")    { statement; }
catch (e if e == "InvalidEmailException") { statement; }
catch (e)                                 { statement; }
</syntaxhighlight>

In a browser, the {{mono|onerror}} event is more commonly used to trap exceptions.
<!-- This needs verification -->

<syntaxhighlight lang="javascript">
onerror = function (errorValue, url, lineNr) {...; return true;};
</syntaxhighlight>

==Native functions and methods==
{{Inline hatnote|(Not related to Web browsers.)}}

===eval (expression) ===
Evaluates the first parameter as an expression, which can include assignment statements. Variables local to functions can be referenced by the expression. However, {{code|eval}} represents a major security risk, as it allows a bad actor to execute arbitrary code, so its use is discouraged.<ref name="deve_eval">{{cite web |title=eval() |work=MDN Web Docs |access-date=29 January 2020 |url=https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#Never_use_eval! |archive-date=1 April 2022 |archive-url=https://web.archive.org/web/20220401220204/https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/eval#Never_use_eval! |url-status=live }}</ref>
<syntaxhighlight lang="nodejsrepl">
> (function foo() {
...   var x = 7;
...   console.log("val " + eval("x + 2"));
... })();
val 9
undefined
</syntaxhighlight>

==See also==
* [[Comparison of JavaScript-based source code editors]]
* [[JavaScript]]

==References==
{{Reflist}}

==Further reading==
* Danny Goodman: ''JavaScript Bible'', Wiley, John & Sons, {{ISBN|0-7645-3342-8}}.
* David Flanagan, Paula Ferguson: ''JavaScript: The Definitive Guide'', O'Reilly & Associates, {{ISBN|0-596-10199-6}}.
* Thomas A. Powell, Fritz Schneider: ''JavaScript: The Complete Reference'', McGraw-Hill Companies, {{ISBN|0-07-219127-9}}.
* Axel Rauschmayer: ''Speaking JavaScript: An In-Depth Guide for Programmers'', 460 pages, O'Reilly Media, 25 February 2014, {{ISBN|978-1449365035}}. ([http://speakingjs.com/ free online edition])
* Emily Vander Veer: ''JavaScript For Dummies, 4th Edition'', Wiley, {{ISBN|0-7645-7659-3}}.

==External links==
{{Wikibooks|JavaScript}}
* [https://developer.mozilla.org/en/docs/A_re-introduction_to_JavaScript A re-introduction to JavaScript - Mozilla Developer Center]
* [https://codetopology.com/category/scripts/javascript-tutorials/ JavaScript Loops]
* ECMAScript standard references: [http://www.ecma-international.org/publications/standards/Ecma-262.htm ECMA-262]
* [https://web.archive.org/web/20120527095306/http://javalessons.com/cgi-bin/fun/java-tutorials-main.cgi?sub=javascript&code=script Interactive JavaScript Lessons - example-based]
* [http://javascript.about.com/ JavaScript on About.com: lessons and explanation] {{Webarchive|url=https://web.archive.org/web/20170225135310/http://javascript.about.com/ |date=25 February 2017 }}
* [https://web.archive.org/web/20150110202627/http://wisentechnologies.com/it-courses/JavaScript-Training-in-Chennai.aspx JavaScript Training]
* Mozilla Developer Center Core References for JavaScript versions [https://developer.mozilla.org/en/docs/Core_JavaScript_1.5_Reference 1.5], [https://web.archive.org/web/20070210000908/http://research.nihonsoft.org/javascript/CoreReferenceJS14/index.html 1.4], [https://web.archive.org/web/20070210000504/http://research.nihonsoft.org/javascript/ClientReferenceJS13/index.html 1.3] and [https://web.archive.org/web/20070210000545/http://research.nihonsoft.org/javascript/jsref/index.htm 1.2]
* [https://developer.mozilla.org/en/docs/JavaScript Mozilla JavaScript Language Documentation]

{{JavaScript}}

{{DEFAULTSORT:Javascript Syntax}}
[[Category:JavaScript|syntax]]
[[Category:Articles with example JavaScript code]]
[[Category:Programming language syntax]]