The implementation seems pretty straightforward:
type Falsy = Zero | "" | false | null | undefined;
type Truthy<T = Numeric | object | string | symbol | true> = Exclude<T, Falsy>;Falsy should include NaN, but TypeScript does not provide that at the type level.
Technically, Falsy should also include HTMLAllCollection due to the [[IsHTMLDDA]] internal slot. However, that type is only available for DOM code (not NodeJS) and is deprecated anyways, so I think it's okay to exclude it from this union.
sindresorhus / type-fest
A collection of essential TypeScript types
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