TypeScript

TypeScript

Paradigm	Multi-paradigm: scripting, object-oriented, structured, imperative, functional, generic
Designed by	Microsoft
Developer	Microsoft
First appeared	October 1, 2012 (2012-10-01)[1]
Stable release	1.8 / January, 2016
License	Apache License 2.0
Filename extensions	.ts
Website	www.typescriptlang.org
Influenced by
JavaScript, Java, C#
Influenced
AtScript

TypeScript is a free and open source programming language developed and maintained by Microsoft. It is a strict superset of JavaScript, and adds optional static typing and class-based object-oriented programming to the language. Anders Hejlsberg, lead architect of C# and creator of Delphi and Turbo Pascal, has worked on the development of TypeScript.^[2][3][4][5] TypeScript may be used to develop JavaScript applications for client-side or server-side (Node.js) execution.

TypeScript is designed for development of large applications and transcompiles to JavaScript.^[6] As TypeScript is a superset of JavaScript, any existing JavaScript programs are also valid TypeScript programs.

TypeScript supports definition files that can contain type information of existing JavaScript libraries, much like C/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 like jQuery, MongoDB, and D3.js. TypeScript headers for the Node.js basic modules are also available, allowing development of Node.js programs within TypeScript.^[7]

The TypeScript compiler is itself written in TypeScript, transcompiled to JavaScript and licensed under the Apache 2 License.

TypeScript is included as a first-class programming language in Microsoft Visual Studio 2013 Update 2 and later, beside C# and other Microsoft languages.^[8] An official extension allows Visual Studio 2012 to support TypeScript as well.^[9]

History

Typescript was first made public in October 2012 (at version 0.8), after two years of internal development at Microsoft.^[10][11] Early after the announcement, Miguel de Icaza praised the language itself, but criticized the lack of mature IDE support apart from Microsoft Visual Studio, which is not available on Linux and OS X.^[12][13] As of 2013 there is support in other IDEs, particularly in Eclipse, via a plug-in contributed by Palantir Technologies.^[14][15] Various text editors, including Emacs, Vim, and Sublime also support TypeScript.^[16]

TypeScript 0.9, released in 2013, added support for generics.^[17] TypeScript 1.0 was released at Build 2014.^[18] Visual Studio 2013 Update 2 provides built-in support for TypeScript.^[19]

In July 2014, the development team announced a new TypeScript compiler, claiming 5× performance gains. Simultaneously, the source code, which was initially hosted on CodePlex, was moved to GitHub.^[20]

Language design

TypeScript originated from the perceived shortcomings of JavaScript for the development of large-scale applications both at Microsoft and among their external customers.^[21] Challenges with dealing with complex JavaScript code led to demand for custom tooling to ease developing of components in the language.^[22]

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 TypeScript is a preview of what to expect of ECMAScript 6. A unique aspect not in the proposal, but added to TypeScript, is optional static typing that enables static language analysis, which facilitates tooling and IDE support.

ECMAScript 6 support

TypeScript adds support for features such as classes, modules and an arrow function syntax as proposed in the upcoming ECMAScript 6 standard.

Language features

TypeScript is a language extension that adds features to ECMAScript 5. Additional features include:

• Type annotations and compile-time type checking
• Type inference
• Type erasure
• Interfaces
• Enumerated type
• Mixin
• Generic
• Namespaces
• Tuple
• Await

The following features are backported from ECMAScript 6:

• Classes
• Modules^[23]
• 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

TypeScript is a strict superset of JavaScript. As such, a JavaScript program is also a valid TypeScript program, and a TypeScript program can seamlessly consume JavaScript. TypeScript compiles to ES3-compatible JavaScript.^[24] By default the compiler targets ECMAScript 3, the current prevailing standard, and is also able to generate constructs used in ECMAScript 5.

With TypeScript, it is possible to use existing JavaScript code, incorporate popular JavaScript libraries, and call TypeScript-generated code from other JavaScript.^[24] 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;
}

The annotations for the primitive types are number, boolean and string. Weakly- or dynamically-typed structures are of type any.

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, then it defaults to the dynamic any type. A value of the any type supports the same operations as a value in JavaScript and minimal static type checking is performed for operations on any values.^[25]

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 file found in C/C++.

declare module 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.

A large collection of declaration files for popular JavaScript libraries is hosted on GitHub by borisyankov in his DefinitelyTyped repository. A command-line utility called tsd is provided by the latter to help search and install declaration files from the repository.

Classes

TypeScript supports ECMAScript 6 classes that integrate the optional type annotations support.

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})`; // As of version 1.4
    }
}

Generics

TypeScript supports generic programming.^[26]

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) utilizes immediately-invoked function expression of JavaScript to encapsulate code, whereas modules (formerly external modules) leverage JavaScript library patterns to do so (AMD or CommonJS).^[27]

Development tools

Compiler

The TypeScript compiler, named tsc, is written in TypeScript that can be compiled into regular JavaScript that can be executed in any JavaScript engine in any host, such as 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.

There is also an alpha version of a client-side compiler in JavaScript, which executes TypeScript code on the fly, upon page load.^[28]

The current version of the compiler supports ECMAScript 3 by default. An option is allowed to target ECMAScript 5 to make use of language features exclusive to that version. Classes, despite being part of the ECMAScript 6 standard, are available in both modes.

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 Sublime Text, Emacs and Vim.^[29]
• Visual Studio Code is an open-source, cross-platform source code editor developed by Microsoft based on Electron. It supports TypeScript in addition to several other languages, and offers features like debugging and intelligent code completion.
• 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,^[30] as well as their Visual Studio Add-in and extension, ReSharper 8.1.^[31]
• Atom editor 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.
• T3S and Better TypeScript are available for Sublime Text.
• The Cross Platform Cloud IDE Codeanywhere supports TypeScript.

Integration with build automation tools

Using plug-ins, TypeScript can be integrated with build automation tools, including Grunt (grunt-ts^[32]), Apache Maven (TypeScript Maven Plugin^[33]) and Gradle (TypeScript Gradle Plugin^[34]).

See also

• Closure Compiler
• JavaScript
• ECMAScript
• Dart

References

External links

• Official website
• TypeScript project at GitHub
• TypeScript Language Specification
• CATS Cross Platform TypeScript Editor
• Build 2013 presentation
• Awesome-Typescript: A community-supported list of tools and resources related to TypeScript

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