Proposal: `is.falsy()` as a type guard

[younho9]

Although there are restrictions on using is.truthy() as a type guard function, is.falsy() could be a type guard function.

type Falsy = false | 0 | 0n | '' | null | undefined;

is.falsy = (value: unknown): value is Falsy => !value;

[sindresorhus]

Sure, it could, but in what cases would it be useful though? The narrowing is still pretty wide.

[younho9]

Your point is correct.

If change signature to use Extract, it would be useful for extracting in a Falsy type from a given value type.

type Falsy = false | 0 | 0n | '' | null | undefined;

is.falsy = <T>(value: T): value is Extract<Falsy, T> => !value;

But, I found there are some bug in extracting from union type.

microsoft/TypeScript#31156

TypeScript Playground

If this problem is solved, It would be useful to use is.falsy as a type guard function, but this problem seems difficult to solve.

So, if you think this type of guard is unnecessary, I will close the issue.

[sindresorhus]

We can keep the issue open for now to gather more feedback and also wait for the TS issue.