compose.ts

type Fn<A, B> = (a: A) => B;

function compose<A, B>(f1: Fn<A, B>): Fn<A, B>;
function compose<A, B, C>(f1: Fn<B, C>, fn2: Fn<A, B>): Fn<A, C>;
function compose<A, B, C, D>(
  fn1: Fn<C, D>,
  f2: Fn<B, C>,
  fn3: Fn<A, B>
): Fn<A, D>;
function compose<A, B, C, D, E>(
  fn1: Fn<D, E>,
  fn2: Fn<C, D>,
  f2: Fn<B, C>,
  fn3: Fn<A, B>
): Fn<A, E>;
function compose<A>(...fns: any[]) {
  return (a: A) =>
    fns.reverse().reduce((result, current) => current(result), a);
}