TypeScript

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TypeScript
TypeScript Logo.png
Paradigm(s) Multi-paradigm: scripting, object-oriented, structured, imperative, functional
Designed by Microsoft
Developer Microsoft
Appeared in 2012
Stable release 1.0.1 / May 13, 2014; 56 days ago (2014-05-13)
Preview release 0.9.5 / 5 December 2013; 6 months ago (2013-12-05)
Influenced by JavaScript, Java, C#
License Apache License 2.0
Filename extension(s) .ts
Website www.typescriptlang.org

TypeScript is a free and open source programming language developed 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 development of TypeScript.[1][2][3][4]

TypeScript is a strict superset of JavaScript, so any existing JavaScript programs are also valid TypeScript programs. TypeScript is designed for development of large applications and compiles down to JavaScript.[5]

TypeScript supports header files which can contain type information of existing JavaScript libraries, enabling other programs to use objects defined in the header files as if they were strongly typed TypeScript objects. There are third-party header files for popular libraries like jQuery, MongoDB, Node.js, and D3.js.[6]

Background[edit]

TypeScript originates from the need to develop large-scale JavaScript applications. The people behind the language at Microsoft have said [7] that internal as well as external customers expressed problems structuring their code in JavaScript.

Internally at Microsoft the challenges with dealing with complex JavaScript code led to demand for custom tooling to ease the writing of components in the language.[8] Microsoft has stated that they did not have the intention of making something that could break compatibility with the standard and its cross-platform support. Knowing that the current ECMAScript standard proposal would bring support for class-based programming in the future, they decided to build 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.

Language features[edit]

TypeScript is a language extension that adds features 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.

Type annotations[edit]

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 will default 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.[10]

Declaration files[edit]

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.

ECMAScript 6 support[edit]

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

Although the standard is not ready, Microsoft has said that it aims to align TypeScript's features with the proposed standard.

Classes[edit]

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 + ")";
    }
}

Generics[edit]

TypeScript supports generic programming.[11]

Modules[edit]

Using modules, TypeScript supports encapsulation of classes, interfaces, functions and variables into namespaces. TypeScript distinguishes between internal and external modules. Internal modules are based on the module syntax of ECMAScript 6, whereas external modules leverage a JavaScript library (AMD or CommonJS).[12]

Compatibility with JavaScript[edit]

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.[13] 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.[13] Type declarations for these libraries are provided with the source code.

Development tools[edit]

Compiler[edit]

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 is using 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.[14]

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[edit]

Integration with build automation tools[edit]

Using plug-ins, TypeScript can be integrated with build automation tools, including Grunt (grunt-ts[18]), Apache Maven (TypeScript Maven Plugin[19]) and Gradle (TypeScript Gradle Plugin[20]).

Open source[edit]

TypeScript is free open source software. It is licensed under the Apache 2 License and is source hosted on CodePlex. The project is maintained by Microsoft.

Early after the creation of the language, Miguel de Icaza praised the language itself, and criticized the lack of mature IDE support apart from Microsoft Visual Studio, which is not available on Linux and OS X.[21] ZDNet repeated this sentiment shortly later.[22]

See also[edit]

Other languages that compile to JavaScript[edit]

References[edit]

  1. ^ Microsoft takes the wraps off TypeScript, a superset of JavaScript
  2. ^ TypeScript: JavaScript Development at Application Scale
  3. ^ Microsoft TypeScript: Can the father of C# save us from the tyranny of JavaScript?
  4. ^ Microsoft Augments Javascript for Large-scale Development
  5. ^ Microsoft TypeScript: the JavaScript we need, or a solution looking for a problem?
  6. ^ DefinitelyTyped contains type information for many major libraries.
  7. ^ Anders Hejlsberg (2012-10-05). "What is TypeScript and why with Anders Hejlsberg". www.hanselminutes.com. Retrieved 2014-01-15. 
  8. ^ S. Somasegar (2012-10-01). "TypeScript: JavaScript Development at Application Scale". msdn.com. Retrieved 2013-11-27. 
  9. ^ Klint Finley (2012-10-01). "Microsoft Previews New JavaScript-Like Programming Language TypeScript". TechCrunch. Retrieved 2013-11-27. 
  10. ^ TypeScript Language Specification p.24
  11. ^ Jonathan Turner (2013-06-18). "Announcing TypeScript 0.9". msdn.com. Retrieved 2013-11-27. 
  12. ^ Sönke Sothmann (2014-01-31). "An introduction to TypeScript’s module system". blog.oio.de. Retrieved 2014-02-21. 
  13. ^ a b Official TypeScript Home Page
  14. ^ TypeScript Compile
  15. ^ Olivier Bloch (2012-10-01). "Sublime Text, Vi, Emacs: TypeScript enabled!". Microsoft. Retrieved 2012-10-28. 
  16. ^ "TypeScript support in WebStorm 6". JetBrains. 
  17. ^ "TypeScript support in ReSharper 8.1". JetBrains. 
  18. ^ grunt-ts
  19. ^ TypeScript Maven plugin
  20. ^ TypeScript Gradle plugin
  21. ^ Miguel de Icaza (2012-10-01). "TypeScript: First Impressions". Retrieved 2012-10-12. "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" 
  22. ^ "Microsoft TypeScript: Can the father of C# save us from the tyranny of JavaScript?". ZDNet. 2012-10-01. Retrieved 2012-10-12. "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.." 

External links[edit]