type-fest/source/union-to-intersection.d.ts

A collection of essential TypeScript types

/**
Convert a union type to an intersection type using [distributive conditional types](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html#distributive-conditional-types).

Inspired by [this Stack Overflow answer](https://stackoverflow.com/a/50375286/2172153).

@example
```
import type {UnionToIntersection} from 'type-fest';

type Union = {the(): void} | {great(arg: string): void} | {escape: boolean};

type Intersection = UnionToIntersection<Union>;
//=> {the(): void; great(arg: string): void; escape: boolean};
```

@category Type
*/
export type UnionToIntersection<Union> = (
	// `extends unknown` is always going to be the case and is used to convert the
	// `Union` into a [distributive conditional
	// type](https://www.typescriptlang.org/docs/handbook/release-notes/typescript-2-8.html#distributive-conditional-types).
	Union extends unknown
		// The union type is used as the only argument to a function since the union
		// of function arguments is an intersection.
		? (distributedUnion: Union) => void
		// This won't happen.
		: never
		// Infer the `Intersection` type since TypeScript represents the positional
		// arguments of unions of functions as an intersection of the union.
) extends ((mergedIntersection: infer Intersection) => void)
	// The `& Union` is to ensure result of `UnionToIntersection<A | B>` is always assignable to `A | B`
	? Intersection & Union
	: never;

export {};