Boolean() cannot be used to perform a null check

TypeScript Version: 2.4.0

Apologies for today’s issue raising binge.

Code

// Compiles
const nullCheckOne = (value?: number) => {
    if (!!value) {
        return value.toFixed(0);
    }

    return '';
}

const nullCheckTwo = (value?: number) => {
    if (Boolean(value)) {
        // Object is possibly 'undefined'
        return value.toFixed(0);
    }

    return '';
}

Expected behavior: Both examples compile.

Actual behavior: The latter example fails w/ Object is possibly 'undefined'.

Explanation To my knowledge !!value and Boolean(value) are equivalent. I’m wondering what is the reason behind not supporting the second case. One reason I can think of would be an imported, non-typescript module, globally overriding it to something like: Boolean = (value) => !value.

[Jessidhia]

One common pattern using the boolean constructor is .filter(Boolean) to remove falsy values from an array (including null and undefined).

[Etheryte]

For anyone else running into the specific case of Array.filter(Boolean), adding an overload for Array.prototype.filter() works great. I’ve tested this on 4.1.2 and https://github.com/microsoft/TypeScript/issues/31551#issuecomment-495201128 doesn’t seem to be present anymore. Patching Boolean itself should work too, but I haven’t tried that.

type Falsy = false | 0 | "" | null | undefined;

interface Array<T> {
  filter<S extends T>(predicate: BooleanConstructor, thisArg?: any): Exclude<S, Falsy>[];
}