Filtering Nullish Values

type Truthy<T> = T extends "" | false | 0 | 0n ? never : T;

interface Array<T> {
  filter(predicate: BooleanConstructor): Truthy<NonNullable<T>>[];
}

interface ReadonlyArray<T> {
  filter(predicate: BooleanConstructor): Truthy<NonNullable<T>>[];
}

// as const creates a readonly tuple
const array = [0, 1, 2, 3, ``, -0, 0n, false, undefined, null] as const;

// const filtered: (1 | 2 | 3)[]
const filtered = array.filter(Boolean);

const nullOrOne: Array<0 | 1> = [0, 1, 0, 1];

// const onlyOnes: 1[]
const onlyOnes = nullOrOne.filter(Boolean);