Defining array with multiple types in TypeScript

I have an array of the form: [ 1, "message" ].

How would I define this in TypeScript?

Solution 1:

Defining array with multiple types in TypeScript

Use a union type (string|number)[] demo:

const foo: (string|number)[] = [ 1, "message" ];

I have an array of the form: [ 1, "message" ].

If you are sure that there are always only two elements [number, string] then you can declare it as a tuple:

const foo: [number, string] = [ 1, "message" ];

IMPORTANT NOTE

This won't work with complex types with different properties, when you want to access a property available on only one of the types.

See this newer answer.

If you're treating it as a tuple (see section 3.3.3 of the language spec), then:

var t:[number, string] = [1, "message"]

interface NumberStringTuple extends Array<string|number>{0:number; 1:string}
var t:NumberStringTuple = [1, "message"];

My TS lint was complaining about other solutions, so the solution that was working for me was:

item: Array<Type1 | Type2>

if there's only one type, it's fine to use:

item: Type1[]

I've settled on the following format for typing arrays that can have items of multiple types.

Array<ItemType1 | ItemType2 | ItemType3>

This works well with testing and type guards. https://www.typescriptlang.org/docs/handbook/advanced-types.html#type-guards-and-differentiating-types

This format doesn't work well with testing or type guards:

(ItemType1 | ItemType2 | ItemType3)[]