Get the element type of each index of an array with mixed types

Question

I have a mixed array like:

const array = [false, 1, '', class T {}];

Whose type is:

type arrayType = typeof array; // (string | number | boolean | typeof T) []

And the type of an object in any index is:

string | number | boolean | typeof T

How can I get the type of the object from a specific index, as below, instead of the union of the types?

const a = array [0] // should be boolean
const b = array [1] // should be number
const c = array [2] // should be string
const d = array [3] // should be typeof T

TS Playground

Titian Cernicova-Dragomir · Accepted Answer · 2020-07-13 08:31:10Z

5

+50

You need to use a tuple type. You can either be explicit about the type, or you can make TS infer a tuple type by using an as const assertion:

const array = [false,1,''] as const;


type arrayType = typeof array; /// readonly [false, 1, ""]

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answered Jul 13, 2020 at 8:31

Titian Cernicova-Dragomir

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1 Comment

Titian Cernicova-Dragomir Over a year ago

@MuratKaragöz it is a marker for the TS compiler to change the way it does inference, namely, it will keep literal type, it will keep tuples, and it will make the tuple readonly github.com/Microsoft/TypeScript/pull/29510

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