The purpose of this guide is to teach you how to write a high-quality definition file. This guide is structured by showing documentation for some API, along with sample usage of that API, and explaining how to write the corresponding declaration.

These examples are ordered in approximately increasing order of complexity.

Objects with Properties

Documentation

The global variable myLib has a function makeGreeting for creating greetings, and a property numberOfGreetings indicating the number of greetings made so far.

Code

ts
let result = myLib.makeGreeting("hello, world");
console.log("The computed greeting is:" + result);

let count = myLib.numberOfGreetings;

Declaration

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

ts
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

ts
let x: Widget = getWidget(43);

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

Declaration

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

Reusable Types (Interfaces)

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)

3 - color: Optional string, e.g. ‘#ff00ff’

Code

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

Declaration

Use an interface to define a type with properties.

ts
interface GreetingSettings {
  greeting: string;
  duration?: number;
  color?: string;
}

declare function greet(setting: GreetingSettings): void;

Reusable Types (Type Aliases)

Documentation

Anywhere a greeting is expected, you can provide a string, a function returning a string, or a Greeter instance.

Code

ts
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:

ts
type GreetingLike = string | (() => string) | MyGreeter;

declare function greet(g: GreetingLike): void;

Organizing Types

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

ts
const g = new Greeter("Hello");
g.log({ verbose: true });
g.alert({ modal: false, title: "Current Greeting" });

Declaration

Use namespaces to organize types.

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

You can also create nested namespaces in one declaration:

ts
declare namespace GreetingLib.Options {
  // Refer to via GreetingLib.Options.Log
  interface Log {
    verbose?: boolean;
  }
  interface Alert {
    modal: boolean;
    title?: string;
    color?: string;
  }
}

Classes

Documentation

You can create a greeter by instantiating the Greeter object, or create a customized greeter by extending from it.

Code

ts
const myGreeter = new Greeter("hello, world");
myGreeter.greeting = "howdy";
myGreeter.showGreeting();

class SpecialGreeter extends Greeter {
  constructor() {
    super("Very special greetings");
  }
}

Declaration

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

ts
declare class Greeter {
  constructor(greeting: string);

  greeting: string;
  showGreeting(): void;
}

Global Variables

Documentation

The global variable foo contains the number of widgets present.

Code

ts
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.

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

Global Functions

Documentation

You can call the function greet with a string to show a greeting to the user.

Code

ts
greet("hello, world");

Declaration

Use declare function to declare functions.

ts
declare function greet(greeting: string): void;