Function composition

86.9% acceptance

Mar 2, 2026

826

Given an array of functions [f1, f2, f3, ..., fn], return a new function fn that is the function composition of the array of functions. The function composition of [f(x), g(x), h(x)] is fn(x) = f(g(h(x))). The function composition of an empty list of functions is the identity function f(x) = x. You may assume each function in the array accepts one integer as input and returns one integer as output.

Solution

TypeScript

Time O(1)

Space O(1)

LeetCode

solution.ts

type F = (x: number) => number;

function compose(functions: F[]): F {
  return function (x: number): number {
  return functions.reduceRight((acc, fn) => fn(acc), x);
  };
}

/**
 * const fn = compose([x => x + 1, x => 2 * x])
 * fn(4) // 9
 */