Type Guards

typeGuards.ts

/** A `TypeGuard` is a function that checks that an unknown value is of certain type. */
export type TypeGuard<A> =
    (value: unknown) => value is A;

const number: TypeGuard<number> = (value: unknown): value is number =>
    typeof value === 'number';

const string: TypeGuard<string> = (value: unknown): value is string =>
    typeof value === 'string';

const null_: TypeGuard<null> = (value: unknown): value is null =>
    value === null;

const boolean: TypeGuard<boolean> = (value: unknown): value is boolean =>
    typeof value === 'boolean';

function literal<Literal extends string | number | boolean | null | undefined>(literal: Literal): TypeGuard<Literal> {
    return (value: unknown): value is Literal =>
        value === literal;
}

function array<A>(typeGuard: TypeGuard<A>): TypeGuard<Array<A>> {
    return (value: unknown): value is Array<A> =>
        value instanceof Array
            && value.every(typeGuard);
}

function object<A>(shape: { [key in keyof A]: TypeGuard<A[key]> }): TypeGuard<A> {
    return (value: unknown): value is A =>
        typeof value === 'object'
            && value !== null
            && Object.entries(shape).every(([key, typeGuard]) =>
                key in value
                    && (typeGuard as any)(value[key])
            );
}

type ToUnion<TypeGuards> =
    TypeGuards extends [TypeGuard<infer First>, ...infer Others]
        ? First | ToUnion<Others>
        : never;

function union<
    TypeGuards extends TypeGuard<any>[]
>(
    ...args: TypeGuards
): TypeGuard<ToUnion<TypeGuards>> {
    return (value: unknown): value is ToUnion<TypeGuards> =>
        args.some(typeGuard => (typeGuard as any)(value));
}

type ToTuple<TypeGuards> =
    TypeGuards extends [TypeGuard<infer First>, ...infer Others]
        ? [First, ...ToTuple<Others>]
        : [];

function tuple<
    TypeGuards extends TypeGuard<any>[]
>(
    ...args: TypeGuards
): TypeGuard<ToTuple<TypeGuards>> {
    return (value: unknown): value is ToTuple<TypeGuards> =>
        value instanceof Array
            && args.every((typeGuard, index) => typeGuard(value[index]));
}

export default {
    number,
    string,
    null_,
    boolean,
    literal,
    array,
    object,
    union,
    tuple
};