43 lines
1.0 KiB
Plaintext
43 lines
1.0 KiB
Plaintext
/**
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Returns a boolean for whether the given type is `never`.
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@link https://github.com/microsoft/TypeScript/issues/31751#issuecomment-498526919
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@link https://stackoverflow.com/a/53984913/10292952
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@link https://www.zhenghao.io/posts/ts-never
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Useful in type utilities, such as checking if something does not occur.
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@example
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```
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import type {IsNever, And} from 'type-fest';
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// https://github.com/andnp/SimplyTyped/blob/master/src/types/strings.ts
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type AreStringsEqual<A extends string, B extends string> =
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And<
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IsNever<Exclude<A, B>> extends true ? true : false,
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IsNever<Exclude<B, A>> extends true ? true : false
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>;
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type EndIfEqual<I extends string, O extends string> =
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AreStringsEqual<I, O> extends true
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? never
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: void;
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function endIfEqual<I extends string, O extends string>(input: I, output: O): EndIfEqual<I, O> {
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if (input === output) {
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process.exit(0);
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}
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}
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endIfEqual('abc', 'abc');
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//=> never
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endIfEqual('abc', '123');
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//=> void
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```
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@category Type Guard
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@category Utilities
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*/
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export type IsNever<T> = [T] extends [never] ? true : false;
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