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---
language: TypeScript
contributors:
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- ["Philippe Vlérick", "https://github.com/pvlerick"]
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filename: learntypescript.ts
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---
TypeScript is a language that aims at easing development of large scale applications written in JavaScript.
TypeScript adds common concepts such as classes, modules, interfaces, generics and (optional) static typing to JavaScript.
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It is a superset of JavaScript: all JavaScript code is valid TypeScript code so it can be added seamlessly to any project. The TypeScript compiler emits JavaScript.
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This article will focus only on TypeScript extra syntax, as opposed to [JavaScript] (../javascript/).
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To test TypeScript's compiler, head to the [Playground] (http://www.typescriptlang.org/Playground) where you will be able to type code, have auto completion and directly see the emitted JavaScript.
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```js
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// There are 3 basic types in TypeScript
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var isDone: boolean = false;
var lines: number = 42;
var name: string = "Anders";
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// When it's impossible to know, there is the "Any" type
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var notSure: any = 4;
notSure = "maybe a string instead";
notSure = false; // okay, definitely a boolean
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// For collections, there are typed arrays and generic arrays
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var list: number[] = [1, 2, 3];
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// Alternatively, using the generic array type
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var list: Array< number > = [1, 2, 3];
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// For enumerations:
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enum Color {Red, Green, Blue};
var c: Color = Color.Green;
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// Lastly, "void" is used in the special case of a function returning nothing
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function bigHorribleAlert(): void {
alert("I'm a little annoying box!");
}
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// Functions are first class citizens, support the lambda "fat arrow" syntax and
// use type inference
// The following are equivalent, the same signature will be infered by the
// compiler, and same JavaScript will be emitted
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var f1 = function(i: number): number { return i * i; }
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// Return type inferred
var f2 = function(i: number) { return i * i; }
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var f3 = (i: number): number => { return i * i; }
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// Return type inferred
var f4 = (i: number) => { return i * i; }
// Return type inferred, one-liner means no return keyword needed
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var f5 = (i: number) => i * i;
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// Interfaces are structural, anything that has the properties is compliant with
// the interface
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interface Person {
name: string;
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// Optional properties, marked with a "?"
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age?: number;
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// And of course functions
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move(): void;
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}
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// Object that implements the "Person" interface
// Can be treated as a Person since it has the name and move properties
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var p: Person = { name: "Bobby", move: () => {} };
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// Objects that have the optional property:
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var validPerson: Person = { name: "Bobby", age: 42, move: () => {} };
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// Is not a person because age is not a number
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var invalidPerson: Person = { name: "Bobby", age: true };
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// Interfaces can also describe a function type
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interface SearchFunc {
(source: string, subString: string): boolean;
}
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// Only the parameters' types are important, names are not important.
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var mySearch: SearchFunc;
mySearch = function(src: string, sub: string) {
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return src.search(sub) != -1;
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}
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// Classes - members are public by default
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class Point {
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// Properties
x: number;
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// Constructor - the public/private keywords in this context will generate
// the boiler plate code for the property and the initialization in the
// constructor.
// In this example, "y" will be defined just like "x" is, but with less code
// Default values are also supported
constructor(x: number, public y: number = 0) {
this.x = x;
}
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// Functions
dist() { return Math.sqrt(this.x * this.x + this.y * this.y); }
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// Static members
static origin = new Point(0, 0);
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}
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var p1 = new Point(10 ,20);
var p2 = new Point(25); //y will be 0
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// Inheritance
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class Point3D extends Point {
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constructor(x: number, y: number, public z: number = 0) {
super(x, y); // Explicit call to the super class constructor is mandatory
}
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// Overwrite
dist() {
var d = super.dist();
return Math.sqrt(d * d + this.z * this.z);
}
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}
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// Modules, "." can be used as separator for sub modules
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module Geometry {
export class Square {
constructor(public sideLength: number = 0) {
}
area() {
return Math.pow(this.sideLength, 2);
}
}
}
var s1 = new Geometry.Square(5);
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// Local alias for referencing a module
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import G = Geometry;
var s2 = new G.Square(10);
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// Generics
// Classes
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class Tuple< T1 , T2 > {
constructor(public item1: T1, public item2: T2) {
}
}
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// Interfaces
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interface Pair< T > {
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item1: T;
item2: T;
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}
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// And functions
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var pairToTuple = function< T > (p: Pair< T > ) {
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return new Tuple(p.item1, p.item2);
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};
var tuple = pairToTuple({ item1:"hello", item2:"world"});
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// Including references to a definition file:
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/// < reference path = "jquery.d.ts" / >
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// Template Strings
var name = 'Tyrone';
var greeting = `Hi ${name}, how are you?`
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```
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## Further Reading
* [TypeScript Official website] (http://www.typescriptlang.org/)
* [TypeScript language specifications (pdf)] (http://go.microsoft.com/fwlink/?LinkId=267238)
* [Anders Hejlsberg - Introducing TypeScript on Channel 9] (http://channel9.msdn.com/posts/Anders-Hejlsberg-Introducing-TypeScript)
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* [Source Code on GitHub] (https://github.com/Microsoft/TypeScript)
* [Definitely Typed - repository for type definitions] (http://definitelytyped.org/)