Logo Questions Linux Laravel Mysql Ubuntu Git Menu
 

What does the ampersand (&) mean in a TypeScript type definition?

On line 60359 of this type definition file, there is the following declaration:

type ActivatedEventHandler = (ev: Windows.ApplicationModel.Activation.IActivatedEventArgs & WinRTEvent<any>) => void;

What does the & sigil mean in this context?

like image 450
Carl Patenaude Poulin Avatar asked Jul 11 '16 23:07

Carl Patenaude Poulin


People also ask

What does the ampersand symbol mean?

An ampersand is a sign for the word and. It's written or typed as the symbol &. It's a modification of the term “and per se and,” which has Latin origins. The ampersand can indicate that the listed items are grouped together as part of a name.

Why do people decorate with ampersands?

First, it is seen as a symbol of family, as in “You &…”, we're all together. It brings words like “you & me” closer than using “and” would, so it's that close-knit family relationship. And then secondly, some consider the ampersand design a symbol of discovery. Of hope of more – of what's coming next.


2 Answers

& in a type position means intersection type.

More from typescript docs on Intersection Types:

https://www.typescriptlang.org/docs/handbook/2/objects.html#intersection-types

Citation from the docs linked above:

Intersection types are closely related to union types, but they are used very differently. An intersection type combines multiple types into one. This allows you to add together existing types to get a single type that has all the features you need. For example, Person & Serializable & Loggable is a type which is all of Person and Serializable and Loggable. That means an object of this type will have all members of all three types.

For example, if you had networking requests with consistent error handling then you could separate out the error handling into it’s own type which is merged with types which correspond to a single response type.

interface ErrorHandling {
  success: boolean;
  error?: { message: string };
}

interface ArtworksData {
  artworks: { title: string }[];
}

interface ArtistsData {
  artists: { name: string }[];
}

// These interfaces are composed to have
// consistent error handling, and their own data.

type ArtworksResponse = ArtworksData & ErrorHandling;
type ArtistsResponse = ArtistsData & ErrorHandling;

const handleArtistsResponse = (response: ArtistsResponse) => {
  if (response.error) {
    console.error(response.error.message);
    return;
  }

  console.log(response.artists);
};
like image 114
basarat Avatar answered Oct 08 '22 08:10

basarat


Intersection type in Typescript

  • A & in TS in the context of a types means an intersection type.
  • It merges all properties of 2 object types together and creates a new type

Example:

type dog = {age: number, woof: Function};
type cat = {age: number, meow: Function};

// Type weird is an intersection of cat and dog
// it needs to have all properties of them combined
type weird = dog & cat;

const weirdAnimal: weird = {age: 2, woof: () => {'woof'}, meow: () => {'meow'}}

interface extaprop {
    color: string
}

type catDog = weird & extaprop; // type now also has added color
const weirdAnimal2: catDog = {age: 2, woof: () => {'woof'}, meow: () => {'meow'}, color: 'red'}


// This is different form a union type
// The type below means either a cat OR a dog
type dogOrCat = dog | cat;
like image 35
Willem van der Veen Avatar answered Oct 08 '22 10:10

Willem van der Veen