type-fest/source/is-never.d.ts

A collection of essential TypeScript types

/**
Returns a boolean for whether the given type is `never`.

@link https://github.com/microsoft/TypeScript/issues/31751#issuecomment-498526919
@link https://stackoverflow.com/a/53984913/10292952
@link https://www.zhenghao.io/posts/ts-never

Useful in type utilities, such as checking if something does not occur.

@example
```
import type {IsNever, And} from 'type-fest';

// https://github.com/andnp/SimplyTyped/blob/master/src/types/strings.ts
type AreStringsEqual<A extends string, B extends string> =
	And<
		IsNever<Exclude<A, B>> extends true ? true : false,
		IsNever<Exclude<B, A>> extends true ? true : false
	>;

type EndIfEqual<I extends string, O extends string> =
	AreStringsEqual<I, O> extends true
		? never
		: void;

function endIfEqual<I extends string, O extends string>(input: I, output: O): EndIfEqual<I, O> {
	if (input === output) {
		process.exit(0);
	}
}

endIfEqual('abc', 'abc');
//=> never

endIfEqual('abc', '123');
//=> void
```

@category Type Guard
@category Utilities
*/
export type IsNever<T> = [T] extends [never] ? true : false;