TypeScript

TypeScript (abbreviated as TS) is a high-level programming language that adds static typing with optional type annotations to JavaScript. It is designed for developing large applications and transpiles to JavaScript. It is developed by Microsoft as free and open-source software released under an Apache License 2.0.

TypeScript may be used to develop JavaScript applications for both client-side and server-side execution (as with Node.js, Deno or Bun). Multiple options are available for transpiling. The default TypeScript Compiler can be used, or the Babel compiler can be invoked to convert TypeScript to JavaScript.

TypeScript supports definition files that can contain type information of existing JavaScript libraries, much like C++ header files can describe the structure of existing object files. This enables other programs to use the values defined in the files as if they were statically typed TypeScript entities. There are third-party header files for popular libraries such as jQuery, MongoDB, and D3.js. TypeScript headers for the Node.js library modules are also available, allowing development of Node.js programs within TypeScript.

The TypeScript compiler is written in TypeScript and compiled to JavaScript. It is licensed under the Apache License 2.0. Anders Hejlsberg, lead architect of C# and creator of Delphi and Turbo Pascal, has worked on developing TypeScript.

History

TypeScript was released to the public in October 2012, with version 0.8, after two years of internal development at Microsoft. Soon after the initial public release, Miguel de Icaza praised the language, but criticized the lack of mature integrated development environment (IDE) support apart from Microsoft Visual Studio, which was unavailable then on Linux and macOS. As of April 2021 there is support in other IDEs and text editors, including Emacs, Vim, WebStorm, Atom and Microsoft's own Visual Studio Code. TypeScript 0.9, released in 2013, added support for generics.

TypeScript 1.0 was released at Microsoft's Build developer conference in 2014. Visual Studio 2013 Update 2 provided built-in support for TypeScript. Further improvement were made in July 2014, when the development team announced a new TypeScript compiler, asserted to have a five-fold performance increase. Simultaneously, the source code, which was initially hosted on CodePlex, was moved to GitHub.

On 22 September 2016, TypeScript 2.0 was released, introducing several features, including the ability for programmers to optionally enforce null safety, to mitigate what's sometimes referred to as the billion-dollar mistake.

TypeScript 3.0 was released on 30 July 2018, bringing many language additions like tuples in rest parameters and spread expressions, rest parameters with tuple types, generic rest parameters and so on.

TypeScript 4.0 was released on 20 August 2020. While 4.0 did not introduce any breaking changes, it added language features such as Custom JSX Factories and Variadic Tuple Types.

TypeScript 5.0 was released on 16 March 2023 and included support for decorators.

On March 11, 2025 Anders Hejlsberg announced on the TypeScript blog that the team is working on a Go port of the TypeScript compiler to be released as TypeScript version 7.0 later this year. It is expected to feature a 10x speedup.

Design

TypeScript originated from the shortcomings of JavaScript for developing large-scale applications both at Microsoft and among their external customers. Challenges with dealing with complex JavaScript code led to demand for custom tooling to ease developing of components in the language.

Developers sought a solution that would not break compatibility with the ECMAScript (ES) standard and its ecosystem, so a compiler was developed to transform a superset of JavaScript with type annotations and classes (TypeScript files) back into vanilla ECMAScript 5 code. TypeScript classes were based on the then-proposed ECMAScript 6 class specification to make writing prototypal inheritance less verbose and error-prone, and type annotations enabled IntelliSense and improved tooling.

Features

TypeScript adds the following syntax extensions to JavaScript:

• Type signatures (annotations) and compile-time type checking
• Type inference
• Interfaces
• Enumerated types
• Generics
• Namespaces
• Tuples
• Explicit resource management

Syntactically, TypeScript is very similar to JScript .NET, another Microsoft implementation of the ECMA-262 language standard that added support for static typing and classical object-oriented language features such as classes, inheritance, interfaces, and namespaces. Other inspirations include Java and C#.

Type annotations

TypeScript provides static typing through type annotations to enable type checking at compile time.

function add(left: number, right: number): number {
 return left + right;
}

Primitive types are annotated using all-lowercase types, such as number, boolean, bigint, and string. These types are distinct from their boxed counterparts (Number, Boolean, etc), which cannot have operations performed from values directly (a Number and number cannot be added). There are additionally undefined and null types for their respective values.

All other non-primitive types are annotated using their class name, such as Error. Arrays can be written in two different ways which are both syntactically the same: the generic-based syntax Array<T> and a shorthand with T[].

Additional built-in data types are tuples, unions, never and any:

• An array with predefined data types at each index is a tuple, represented as [type1, type2, ..., typeN].
• A variable that can hold more than one type of data is a union, represented using the logical OR | symbol (string | number).
• The never type is used when a given type should be impossible to create, which is useful for filtering mapped types.
• A value of type any supports the same operations as a value in JavaScript and minimal static type checking is performed, which makes it suitable for weakly or dynamically-typed structures. This is generally discouraged practice and should be avoided when possible.

Type annotations can be exported to a separate declarations file to make type information available for TypeScript scripts using types already compiled into JavaScript. Annotations can be declared for an existing JavaScript library, as has been done for Node.js and jQuery.

The TypeScript compiler makes use of type inference when types are not given. For example, the add method in the code above would be inferred as returning a number even if no return type annotation had been provided. This is based on the static types of left and right being numbers, and the compiler's knowledge that the result of adding two numbers is always a number.

If no type can be inferred because of lack of declarations (such as in a JavaScript module without types), then it defaults to the dynamic any type. Additional module types can be provided using a .d.ts declaration file using the declare module "moduleName" syntax.

Declaration files

When a TypeScript script gets compiled, there is an option to generate a declaration file (with the extension .d.ts) that functions as an interface to the components in the compiled JavaScript. In the process, the compiler strips away all function and method bodies and preserves only the signatures of the types that are exported. The resulting declaration file can then be used to describe the exported virtual TypeScript types of a JavaScript library or module when a third-party developer consumes it from TypeScript.

The concept of declaration files is analogous to the concept of header files found in C/C++.

declare namespace Arithmetics {
    add(left: number, right: number): number;
    subtract(left: number, right: number): number;
    multiply(left: number, right: number): number;
    divide(left: number, right: number): number;
}

Type declaration files can be written by hand for existing JavaScript libraries, as has been done for jQuery and Node.js.

Large collections of declaration files for popular JavaScript libraries are hosted on GitHub in DefinitelyTyped.

Generics

TypeScript supports generic programming using a syntax similar to Java. The following is an example of the identity function.

function id<T>(x: T): T {
    return x;
}

Classes

TypeScript uses the same annotation style for class methods and fields as for functions and variables respectively. Compared with vanilla JavaScript classes, a TypeScript class can also implement an interface through the implements keyword, use generic parameters similarly to Java, and specify public and private fields.

class Person {
    public name: string;
    private age: number;
    private salary: number;

    constructor(name: string, age: number, salary: number) {
        this.name = name;
        this.age = age;
        this.salary = salary;
    }

    toString(): string {
        return `${this.name} (${this.age}) (${this.salary})`;
    }
}

Modules and namespaces

TypeScript distinguishes between modules and namespaces. Both features in TypeScript support encapsulation of classes, interfaces, functions and variables into containers. Namespaces (formerly internal modules) use JavaScript immediately-invoked function expressions to encapsulate code, whereas modules (formerly external modules) use existing JavaScript library patterns (CommonJS or ES Modules).

Compatibility with JavaScript

As TypeScript is simply a superset of JavaScript, existing JavaScript can be quickly adapted to TypeScript and TypeScript program can seamlessly consume JavaScript. The compiler can target all ECMAScript versions 5 and above, transpiling modern features like classes and arrow functions to their older counterparts.

With TypeScript, it is possible to use existing JavaScript code, incorporate popular JavaScript libraries, and call TypeScript-generated code from other JavaScript. Type declarations for these libraries are usually provided with the source code but can be declared or installed separately if needed.

Development tools

Compiler

The TypeScript compiler, named tsc, is written in TypeScript. As a result, it can be compiled into regular JavaScript and can then be executed in any JavaScript engine (e.g. a browser). The compiler package comes bundled with a script host that can execute the compiler. It is also available as a Node.js package that uses Node.js as a host.

The compiler can target a given edition of ECMAScript (such as ECMAScript 5 for legacy browser compatibility), but by default compiles for the latest standards.

IDE and editor support

• Microsoft provides a plug-in for Visual Studio 2012 and WebMatrix, full integrated support in Visual Studio 2013, Visual Studio 2015, and basic text editor support for Emacs and Vim.
• Visual Studio Code supports TypeScript in addition to several other languages, and offers features like debugging and intelligent code completion.
• alm.tools is an open source cloud IDE for TypeScript built using TypeScript, ReactJS and TypeStyle.
• JetBrains supports TypeScript with code completion, refactoring and debugging in its IDEs built on IntelliJ platform, such as PhpStorm 6, WebStorm 6, and IntelliJ IDEA, as well as their Visual Studio Add-in and extension, ReSharper 8.1.
• Atom has a TypeScript plugin with support for code completion, navigation, formatting, and fast compilation.
• The online Cloud9 IDE and Codenvy support TypeScript.
• A plugin is available for the NetBeans IDE.
• A plugin is available for the Eclipse IDE (version Kepler)
• TypEcs is available for the Eclipse IDE.
• The Cross Platform Cloud IDE Codeanywhere supports TypeScript.
• Webclipse An Eclipse plugin designed to develop TypeScript and Angular 2.
• Angular IDE A standalone IDE available via npm to develop TypeScript and Angular 2 applications, with integrated terminal support.
• TideTemplate:Snd TypeScript Interactive Development Environment for Emacs.

Integration with build automation tools

Using plug-ins, TypeScript can be integrated with build automation tools, including Grunt (grunt-ts), Apache Maven (TypeScript Maven Plugin), Gulp (gulp-typescript) and Gradle (TypeScript Gradle Plugin).

Linting tools

TSLint scans TypeScript code for conformance to a set of standards and guidelines. ESLint, a standard JavaScript linter, also provided some support for TypeScript via community plugins. However, ESLint's inability to leverage TypeScript's language services precluded certain forms of semantic linting and program-wide analysis. In early 2019, the TSLint team announced the linter's deprecation in favor of typescript-eslint, a joint effort of the TSLint, ESLint and TypeScript teams to consolidate linting under the ESLint umbrella for improved performance, community unity and developer accessibility.

Release history

Version number	Release date	Significant changes
Template:Version
Template:Version
Template:Version
Template:Version		performance improvements
Template:Version		protected modifier, tuple types
Template:Version		union types, let and const declarations, template strings, type guards, type aliases
Template:Version		ES6 modules, namespace keyword, for..of support, decorators
Template:Version		JSX support, intersection types, local type declarations, abstract classes and methods, user-defined type guard functions
Template:Version		async and await support,
Template:Version		constraints generics, control flow analysis errors, string literal types, allowJs
Template:Version		null- and undefined-aware types, control flow based type analysis, discriminated union types, never type, readonly keyword, type of this for functions
Template:Version		keyof and lookup types, mapped types, object spread and rest,
Template:Version		mix-in classes, object type,
Template:Version		async iteration, generic parameter defaults, strict option
Template:Version		dynamic import expressions, string enums, improved inference for generics, strict contravariance for callback parameters
Template:Version		optional catch clause variables
Template:Version		strict function types
Template:Version		constant-named properties, fixed-length tuples
Template:Version		conditional types, improved keyof with intersection types
Template:Version		support for symbols and numeric literals in keyof and mapped object types
Template:Version		project references, extracting and spreading parameter lists with tuples
Template:Version		mappable tuple and array types
Template:Version		stricter checking for bind, call, and apply
Template:Version		relaxed rules on methods of union types, incremental builds for composite projects
Template:Version		faster incremental builds, type inference from generic functions, readonly modifier for arrays, const assertions, type-checking global this
Template:Version		faster incremental builds, omit helper type, improved excess property checks in union types, smarter union type checking
Template:Version		Stricter generators, more accurate array spread, better Unicode support for identifiers
Template:Version		Optional chaining, nullish coalescing
Template:Version		Type-only imports and exports, ECMAScript private fields, top-level await
Template:Version		Improvements in inference, speed improvements
Template:Version		Variadic tuple types, labeled tuple elements
Template:Version		Template literal types, key remapping in mapped types, recursive conditional types
Template:Version		Smarter type alias preservation, leading/middle rest elements in tuple types, stricter checks for the in operator, abstract construct signatures
Template:Version		Separate write types on properties, override and the --noImplicitOverride flag, template string type improvements
Template:Version		Control flow analysis of aliased conditions and discriminants, symbol and template string pattern index signatures
Template:Version		Type and promise improvements, supporting lib from node_modules, template string types as discriminants, and es2022 module
Template:Version		Type inference and checks improvements, support for ES2022 target, better ECMAScript handling
Template:Version		Support for ES modules, instantiation expressions, variance annotations for type parameters, better control-flow checks and type check improvements
Template:Version		Intersection and union types improvements, better type inference
Template:Version		satisfies operator, auto-accessors in classes (proposal), improvements in type narrowing and checks
Template:Version		ES decorators (proposal), type inference improvements, bundler module resolution mode, speed and size optimizations
Template:Version		Easier implicit returns for undefined and unrelated types for getters and setters
Template:Version		using declarations and explicit resource management, decorator metadata and named and anonymous tuple elements
Template:Version		Improved type narrowing, correctness checks and performance optimizations
Template:Version	6 March 2024	Object.groupBy and Map.groupBy support
Template:Version	20 June 2024	Inferred Type Predicates, Regular Expression Syntax Checking, and Type Imports in JSDoc
Template:Version	9 September 2024	Advanced type inference, variadic tuple enhancements, partial module declarations.
Template:Version	22 November 2024
Template:Version	28 February 2025
Template:Version	2025
Template:Version		Introduce some deprecations and breaking changes to align with the upcoming native codebase.
Template:Version	2025	Rewrite in Go with faster performance.

See also

• Dart
• Kotlin
• JS++
• PureScript

References

Sources

• "Webclipse: Eclipse Plugin" Template:Webarchive Genuitec. Retrieved 9 November 2016.
• "Angular IDE by Webclipse: Standalone IDE" Template:Webarchive Genuitec. Retrieved 9 November 2016.

External links

• Template:GitHub