Proposal: Add not

[younho9]

const isNullable = (value: unknown): value is null | undefined =>
	value === null || value === undefined;

const isNonNullable = not(isNullable);

declare const someValue: string | null | undefined;

if (isNonNullable(someValue)) {
	someValue;
	//=> string
}

It could be useful to reuse existing predicate.

use-case: TypeScript Playground

[jonahsnider]

Is inverting the return type of a type guard possible with TypeScript's type system?

[younho9]

It depends on Exclude type. Type guard using Exclude can exclude some types from a given type.

microsoft/TypeScript#4183 (comment)

type Predicate<Type> = (value: unknown) => value is Type;

function not<Type>(predicate: Predicate<Type>) {
    return <Value>(value: Value): value is Exclude<Value, Type> => !predicate(value);
};

const isNullable = (value: unknown): value is null | undefined =>
	value === null || value === undefined;

const isNonNulluable = not(isNullable);
//=> <Value>(value: Value) => value is Exclude<Value, null | undefined>

use-cases:

• Nullable <-> NonNullable
• Falsy <-> Truthy

[tychenjiajun]

@younho9 I don't think this will work with isEmpty or isInfinite. It's not only depends on Exclude but also depends on negated type. microsoft/TypeScript#29317

[younho9]

@younho9 I don't think this will work with isEmtpy or isInfinite. It's not only depends on Exclude but also depends on negated type. microsoft/TypeScript#29317

Yes, when this PR merge, the negated type guards will be more useful.

Negative type guards (with Exclude) do not narrow types in some situations.

• Exclude unexpected behavior  microsoft/TypeScript#44901
• Negative type guards (with Exclude) do not narrow types in some situations microsoft/TypeScript#43589

But I still think some types guards with Exclude are useful.

[fregante]

Related type-only request:

• Add Not<string, 'not me'> type-fest#417