Y fixed combinator with TypeScript.

y.ts

export type Paradoxical<A, R> = (f: Paradoxical<A, R>) => (a: A) => R;

export const Y = <A, R>(f: (g: (a: A) => R) => (a: A) => R): ((a: A) => R) =>
    ((x: Paradoxical<A, R>) => f((lazy) => x(x)(lazy)))(
        (x: Paradoxical<A, R>) => f((lazy) => x(x)(lazy)),
    );