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.[6] 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,[7] 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.[8]
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.[9][10][11][12]
TypeScript was released to the public in October 2012, with version 0.8, after two years of internal development at Microsoft.[13][14] 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 macOS at the time.[15][16] As of April 2021 there is support in other IDEs and text editors, including Emacs, Vim, WebStorm, Atom[17] and Microsoft's own Visual Studio Code.[18] TypeScript 0.9, released in 2013, added support for generics.[19]
TypeScript 1.0 was released at Microsoft's Build developer conference in 2014.[20] Visual Studio 2013 Update 2 provided built-in support for TypeScript.[21] 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.[22]
On 22 September 2016, TypeScript 2.0 was released, introducing several features, including the ability for programmers to optionally enforce null safety,[23] to mitigate what's sometimes referred to as the billion-dollar mistake.
TypeScript 3.0 was released on 30 July 2018,[24] bringing many language additions like tuples in rest parameters and spread expressions, rest parameters with tuple types, generic rest parameters and so on.[25]
TypeScript 4.0 was released on 20 August 2020.[26] While 4.0 did not introduce any breaking changes, it added language features such as Custom JSX Factories and Variadic Tuple Types.[26]
TypeScript 5.0 was released on 16 March 2023 and included support for decorators.[27]
TypeScript originated from the shortcomings of JavaScript for the development of large-scale applications both at Microsoft and among their external customers.[28] Challenges with dealing with complex JavaScript code led to demand for custom tooling to ease developing of components in the language.[29]
Developers sought a solution that would not break compatibility with the ECMAScript 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.
TypeScript adds the following syntax extensions to JavaScript:
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#.
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:
[type1, type2, ..., typeN].| symbol (string | number).never type is used when a given type should be impossible to create, which is useful for filtering mapped types.any supports the same operations as a value in JavaScript and minimal static type checking is performed,[31] which makes it suitable for weakly or dynamically-typed structures. This is generally discouraged practice and should be avoided when possible.[32]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.
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.
TypeScript supports generic programming using a syntax similar to Java.[33] The following is an example of the identity function.[34]
function id<T>(x: T): T { return x;}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})`; }}Union types are supported in TypeScript.[35] 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 }}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);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) use existing JavaScript library patterns (CommonJS or ES Modules).[36]
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 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.[37] Type declarations for these libraries are usually provided with the source code but can be declared or installed separately if needed.
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 for the latest standards.
Using plug-ins, TypeScript can be integrated with build automation tools, including Grunt (grunt-ts[43]), Apache Maven (TypeScript Maven Plugin[44]), Gulp (gulp-typescript[45]) and Gradle (TypeScript Gradle Plugin[46]).
TSLint[47] 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.[48] 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.[49]
CodeDOM[50] 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.[51] Programmers use CodeDOM and a CodeDOM provider to construct a code generator that generates codes for an application domain. TypeScript CodeDOM Provider[52] generates TypeScript codes according to a CodeDOM.
TypeScript was directly influenced by F#: one of the originators of TypeScript was Luke Hoban, who began TypeScript (then called Strada) immediately after working on F# 2.0. Recently he noted the influence of F# on early parts of the TypeScript design [Hoban 2017].
But TypeScript only delivers half of the value in using a strongly typed language to Unix developers: strong typing. Intellisense, code completion and refactoring are tools that are only available to Visual Studio Professional users on Windows. There is no Eclipse, MonoDevelop or Emacs support for any of the language features
And I think this is a pretty big misstep. If you're building web apps that run on anything other than Windows, you're likely using a Mac and most likely not using Visual Studio. You need the Visual Studio plug-in to get the IntelliSense. All you get without Visual Studio is the strong-typing. You don't get the productivity benefits you get from IntelliSense..
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