TypeScript

For the typed instance analogous to a handwritten document, see manuscript.

TypeScript

Paradigm	Multi-paradigm: functional, generic, imperative, object-oriented
Designed by	Microsoft
Developer	Microsoft
First appeared	1 October 2012 (2012-10-01)
Stable release	5.4.2  / 6 March 2024 (6 March 2024)
Typing discipline	Duck, gradual, structural
License	Apache License 2.0
Filename extensions	.ts, .tsx, .mts, .cts
Website	www.typescriptlang.org
Influenced by
C#, F#, Java, JavaScript, ActionScript
Influenced
AtScript, AssemblyScript, ArkTS

TypeScript is a free and open-source high-level programming language developed by Microsoft that adds static typing with optional type annotations to JavaScript. It is designed for the development of large applications and transpiles to JavaScript. Because TypeScript is a superset of JavaScript, all JavaScript programs are syntactically valid TypeScript, but they can fail to type-check for safety reasons.

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 transpilation. 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 itself 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 the development of 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 itself, but criticized the lack of mature IDE support apart from Microsoft Visual Studio, which was not available on Linux and OS X at that time. 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 provides 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.

Design

TypeScript originated from the shortcomings of JavaScript for the development of 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.

TypeScript developers sought a solution that would not break compatibility with the standard and its cross-platform support. Knowing that the current ECMAScript standard proposal promised future support for class-based programming, TypeScript was based on that proposal. That led to a JavaScript compiler with a set of syntactical language extensions, a superset based on the proposal, that transforms the extensions into regular JavaScript. In this sense, the class feature of TypeScript was a preview of what to expect from ECMAScript 2015. A unique aspect not in the proposal, but added to TypeScript, is optional static typing (also known as gradual typing) that enables static language analysis to facilitate tooling and IDE support.

ECMAScript 2015 support

Main article: ECMAScript § 6th Edition - ECMAScript 2015

TypeScript adds support for features such as classes, modules, and an arrow function syntax as defined in the ECMAScript 2015 standard.

Features

TypeScript is a language extension that adds features to ECMAScript 6. Additional features include:

• Type annotations and compile-time type checking
• Type inference
• Type erasure
• Interfaces
• Enumerated types
• Generics
• Namespaces
• Tuples
• Async/await
• Explicit Resource Management

The following features are backported from ECMAScript 2015:

• Classes
• Modules
• Abbreviated "arrow" syntax for anonymous functions
• Optional parameters and default parameters

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.

Compatibility with JavaScript

Main article: JavaScript

TypeScript is a strict superset of ECMAScript 2015, which is itself a superset of ECMAScript 5, commonly referred to as JavaScript. As such, a JavaScript program is also a valid TypeScript program and a TypeScript program can seamlessly consume JavaScript. By default the compiler targets ECMAScript 5, the current prevailing standard, but is also able to generate constructs used in ECMAScript 3 or 2015.

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 provided with the source code.

Type annotations

TypeScript provides static typing through type annotations to enable type checking at compile time. This is optional and can be ignored to use the regular dynamic typing of JavaScript.

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

Primitive types are annotated using the types number, boolean, and string. These types are distinct from their class counterparts (Number, Boolean, etc), which cannot have operations performed from values directly. For instance, a Number and a number cannot be added. There is additionally undefined and null types for their respective values.

All other types are annotated using their class name rather than primitives, 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, although this is generally discouraged practice.

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 to infer types 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. However, explicitly declaring the return type allows the compiler to verify correctness.

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

Main article: Generic programming

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 {
    private 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})`;
    }
}

Union types

This section is an excerpt from Union type § TypeScript.[edit]

Union types are supported in TypeScript. The values are implicitly "tagged" with a type by the language, and may be retrieved using a typeof call for primitive values and an instanceof comparison for complex data types. Types with overlapping usage (e.g. a slice method exists on both strings and arrays, the plus operator works on both strings and numbers) don't need additional narrowing to use these features.

function successor(n: number | bigint): number | bigint {
    // types that support the same operations don't need narrowing
    return ++n;
}

function dependsOnParameter(v: string | Array<string> | number) {
    // distinct types need narrowing
    if (v instanceof Array) {
        // do something
    } else if (typeof(v) === "string") {
        // do something else
    } else {
        // has to be a number
    }
}

Enumerated types

This section is an excerpt from Enumerated type § TypeScript.[edit]

TypeScript adds an 'enum' data type to JavaScript.

enum Cardsuit {Clubs, Diamonds, Hearts, Spades};
var c: Cardsuit = Cardsuit.Diamonds;

By default, enums number members starting at 0; this can be overridden by setting the value of the first:

enum Cardsuit {Clubs = 1, Diamonds, Hearts, Spades};
var c: Cardsuit = Cardsuit.Diamonds;

All the values can be set:

enum Cardsuit {Clubs = 1, Diamonds = 2, Hearts = 4, Spades = 8};
var c: Cardsuit = Cardsuit.Diamonds;

TypeScript supports mapping the numeric value to its name. For example, this finds the name of the value 2:

enum Cardsuit {Clubs = 1, Diamonds, Hearts, Spades};
var suitName: string = Cardsuit[2];

alert(suitName);

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) utilize JavaScript immediately-invoked function expressions to encapsulate code, whereas modules (formerly external modules) leverage JavaScript library patterns to do so (AMD or CommonJS).

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 particular edition of ECMAScript (such as ES5 for legacy browser compatibility), but by default compiles to the latest version.

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.
• Tide – TypeScript Interactive Development Environment for Emacs.

Integration with build automation tools

Main article: Build automation

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.

CodeDOM Provider

CodeDOM provides types that represent common types of source code elements, which will be transformed to data types, classes and statements etc. of a programming language through a CodeDOMProvider. Programmers use CodeDOM and a CodeDOM provider to construct a code generator that generates codes for an application domain. TypeScript CodeDOM Provider generates TypeScript codes according to a CodeDOM.

Release history

Version number	Release date	Significant changes
0.8	1 October 2012 (2012-10-01)
0.9	18 June 2013 (2013-06-18)
1.0	12 April 2014 (2014-04-12)
1.1	6 October 2014 (2014-10-06)	performance improvements
1.3	12 November 2014 (2014-11-12)	protected modifier, tuple types
1.4	20 January 2015 (2015-01-20)	union types, let and const declarations, template strings, type guards, type aliases
1.5	20 July 2015 (2015-07-20)	ES6 modules, namespace keyword, for..of support, decorators
1.6	16 September 2015 (2015-09-16)	JSX support, intersection types, local type declarations, abstract classes and methods, user-defined type guard functions
1.7	30 November 2015 (2015-11-30)	async and await support,
1.8	22 February 2016 (2016-02-22)	constraints generics, control flow analysis errors, string literal types, allowJs
2.0	22 September 2016 (2016-09-22)	null- and undefined-aware types, control flow based type analysis, discriminated union types, never type, readonly keyword, type of this for functions
2.1	8 November 2016 (2016-11-08)	keyof and lookup types, mapped types, object spread and rest,
2.2	22 February 2017 (2017-02-22)	mix-in classes, object type,
2.3	27 April 2017 (2017-04-27)	async iteration, generic parameter defaults, strict option
2.4	27 June 2017 (2017-06-27)	dynamic import expressions, string enums, improved inference for generics, strict contravariance for callback parameters
2.5	31 August 2017 (2017-08-31)	optional catch clause variables
2.6	31 October 2017 (2017-10-31)	strict function types
2.7	31 January 2018 (2018-01-31)	constant-named properties, fixed-length tuples
2.8	27 March 2018 (2018-03-27)	conditional types, improved keyof with intersection types
2.9	14 May 2018 (2018-05-14)	support for symbols and numeric literals in keyof and mapped object types
3.0	30 July 2018 (2018-07-30)	project references, extracting and spreading parameter lists with tuples
3.1	27 September 2018 (2018-09-27)	mappable tuple and array types
3.2	30 November 2018 (2018-11-30)	stricter checking for bind, call, and apply
3.3	31 January 2019 (2019-01-31)	relaxed rules on methods of union types, incremental builds for composite projects
3.4	29 March 2019 (2019-03-29)	faster incremental builds, type inference from generic functions, readonly modifier for arrays, const assertions, type-checking global this
3.5	29 May 2019 (2019-05-29)	faster incremental builds, omit helper type, improved excess property checks in union types, smarter union type checking
3.6	28 August 2019 (2019-08-28)	Stricter generators, more accurate array spread, better Unicode support for identifiers
3.7	5 November 2019 (2019-11-05)	Optional chaining, nullish coalescing
3.8	20 February 2020 (2020-02-20)	Type-only imports and exports, ECMAScript private fields, top-level await
3.9	12 May 2020 (2020-05-12)	Improvements in inference, speed improvements
4.0	20 August 2020 (2020-08-20)	Variadic tuple types, labeled tuple elements
4.1	19 November 2020 (2020-11-19)	Template literal types, key remapping in mapped types, recursive conditional types
4.2	25 February 2021 (2021-02-25)	Smarter type alias preservation, leading/middle rest elements in tuple types, stricter checks for the in operator, abstract construct signatures
4.3	26 May 2021 (2021-05-26)	Separate write types on properties, override and the --noImplicitOverride flag, template string type improvements
4.4	26 August 2021 (2021-08-26)	Control flow analysis of aliased conditions and discriminants, symbol and template string pattern index signatures
4.5	17 November 2021 (2021-11-17)	Type and promise improvements, supporting lib from node_modules, template string types as discriminants, and es2022 module
4.6	28 February 2022 (2022-02-28)	Type inference and checks improvements, support for ES2022 target, better ECMAScript handling
4.7	24 May 2022 (2022-05-24)	Support for ES modules, instantiation expressions, variance annotations for type parameters, better control-flow checks and type check improvements
4.8	25 August 2022 (2022-08-25)	Intersection and union types improvements, better type inference
4.9	15 November 2022 (2022-11-15)	satisfies operator, auto-accessors in classes (proposal), improvements in type narrowing and checks
5.0	16 March 2023 (2023-03-16)	ES decorators (proposal), type inference improvements, bundler module resolution mode, speed and size optimizations
5.1	1 June 2023 (2023-06-01)	Easier implicit returns for undefined and unrelated types for getters and setters
5.2	24 August 2023 (2023-08-24)	using declarations and explicit resource management, decorator metadata and named and anonymous tuple elements
5.3	20 November 2023 (2023-11-20)	Improved type narrowing, correctness checks and performance optimizations
5.4	6 March 2024	Object.groupBy and Map.groupBy support
5.5 (Beta)	25 April 2024	Inferred Type Predicates, Regular Expression Syntax Checking, and Type Imports in JSDoc

See also

• Dart
• Kotlin
• JS++
• PureScript

References

Sources

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

External links

• TypeScript project at GitHub
• TypeScript Language Specification
• CATS Cross Platform TypeScript Editor
• The Concise TypeScript Book
• TypeScript Origins: The Documentary on YouTube