Type alias TupleToUnion<ArrayType>

Convert a tuple/array into a union type of its elements.

This can be useful when you have a fixed set of allowed values and want a type defining only the allowed values, but do not want to repeat yourself.

Example

import type {TupleToUnion} from 'type-fest';

const destinations = ['a', 'b', 'c'] as const;

type Destination = TupleToUnion<typeof destinations>;
//=> 'a' | 'b' | 'c'

function verifyDestination(destination: unknown): destination is Destination {
    return destinations.includes(destination as any);
}

type RequestBody = {
    deliverTo: Destination;
};

function verifyRequestBody(body: unknown): body is RequestBody {
    const deliverTo = (body as any).deliverTo;
    return typeof body === 'object' && body !== null && verifyDestination(deliverTo);
}
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Alternatively, you may use typeof destinations[number]. If destinations is a tuple, there is no difference. However if destinations is a string, the resulting type will the union of the characters in the string. Other types of destinations may result in a compile error. In comparison, TupleToUnion will return never if a tuple is not provided.

Example

const destinations = ['a', 'b', 'c'] as const;

type Destination = typeof destinations[number];
//=> 'a' | 'b' | 'c'

const erroringType = new Set(['a', 'b', 'c']);

type ErroringType = typeof erroringType[number];
//=> Type 'Set<string>' has no matching index signature for type 'number'. ts(2537)

const numberBool: { [n: number]: boolean } = { 1: true };

type NumberBool = typeof numberBool[number];
//=> boolean
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