These examples are ordered in approximately increasing order of complexity.

Documentation

Code

Declaration

Use declare namespace to describe types or values accessed by dotted notation.

declare namespace myLib {
  function makeGreeting(s: string): string;
  let numberOfGreetings: number;
}

Overloaded Functions

Documentation

The getWidget function accepts a number and returns a Widget, or accepts a string and returns a Widget array.

Code

let x: Widget = getWidget(43);
let arr: Widget[] = getWidget("all of them");

Declaration

declare function getWidget(n: number): Widget;
declare function getWidget(s: string): Widget[];

Documentation

When specifying a greeting, you must pass a GreetingSettings object. This object has the following properties:

1 - greeting: Mandatory string

2 - duration: Optional length of time (in milliseconds)

Code

greet({
  greeting: "hello world",
  duration: 4000
});

Declaration

Use an interface to define a type with properties.

Reusable Types (Type Aliases)

Documentation

Code

function getGreeting() {
  return "howdy";
}
class MyGreeter extends Greeter {}
greet("hello");
greet(getGreeting);
greet(new MyGreeter());

Declaration

You can use a type alias to make a shorthand for a type:

type GreetingLike = string | (() => string) | MyGreeter;
declare function greet(g: GreetingLike): void;

Documentation

The greeter object can log to a file or display an alert. You can provide LogOptions to .log(...) and alert options to .alert(...)

Code

Declaration

Use namespaces to organize types.

declare namespace GreetingLib {
  interface LogOptions {
    verbose?: boolean;
  }
  interface AlertOptions {
    modal: boolean;
    title?: string;
    color?: string;
  }
}

You can also create nested namespaces in one declaration:

Classes

Code

Declaration

Use declare class to describe a class or class-like object. Classes can have properties and methods as well as a constructor.

declare class Greeter {
  constructor(greeting: string);
  greeting: string;
  showGreeting(): void;
}

Documentation

The global variable foo contains the number of widgets present.

Code

console.log("Half the number of widgets is " + foo / 2);

Declaration

Use declare var to declare variables. If the variable is read-only, you can use declare const. You can also use declare let if the variable is block-scoped.

/** The number of widgets present */
declare var foo: number;

Global Functions

Documentation

Code

Declaration

Use declare function to declare functions.