Type Keyword

The type keyword defines an alias to a type.

type str = string;
let cheese: str = 'gorgonzola';
let cake: str = 10; // Type 'number' is not assignable to type 'string'

At first glance, this does not appear to be very useful (even the error mentions the original type), but as type annotations become more complex, the benefits of the type keyword become apparent.

Union Types

Union types allow type annotations to specify that a property should be one of a set of types (either/or).

function admitAge (age: number|string): string {
  return `I am ${age}, alright?!`;
}

admitAge(30); // 'I am 30, alright?!'
admitAge('Forty'); // 'I am Forty, alright?!'

The type keyword simplifies annotating and reusing union types.

type Age = number | string;

function admitAge (age: Age): string {
  return `I am ${age}, alright?!`;
}

let myAge: Age = 50;
let yourAge: Age = 'One Hundred';
admitAge(yourAge); // 'I am One Hundred, alright?!'

A union type of string literal types is a very useful pattern, creating what is basically an enum with string values.

type PartyZone = "pizza hut" | "waterpark" | "bowling alley" | "abandoned warehouse";

function goToParty (place: PartyZone): string {
  return `lets go to the ${place}`;
}

goToParty("pizza hut");
goToParty("chuck e. cheese"); // Argument of type `"chuck e. cheese"' is not assignable to parameter of type 'PartyZone'

Intersection Types

Intersection types are the combination of two or more types. Useful for objects and params that need to implement more than one interface.

interface Kicker {
  kick(speed: number): number;
}

interface Puncher {
  punch(power: number): number;
}
// assign intersection type definition to alias KickPuncher
type KickPuncher = Kicker & Puncher;

function attack (warrior: KickPuncher) {
  warrior.kick(102);
  warrior.punch(412);
  warrior.judoChop(); // Property 'judoChop' does not exist on type 'KickPuncher'
}

Function Type Definitions

Function type annotations can get much more specific than typescripts built-in Function type. Function type definitions allow you to attach a function signature to it's own type.

type MaybeError = Error | null;
type Callback = (err: MaybeError, response: Object) => void;

function sendRequest (cb: Callback): void {
  if (cb) {
    cb(null, {});
  }
}

The syntax is similar to ES6 fat-arrow functions. ([params]) => [return type].

To illustrate the how much the type keyword improved the readability of the previous snippet, here is the function type defined inline.

function sendRequest (cb: (err: Error|null, response: Object) => void): void {
  if (cb) {
    cb(null, {});
  }
}