Problem

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.

Example 1:

Input: functions = [x => x + 1, x => x * x, x => 2 * x], x = 4 Output: 65 Explanation: Evaluating from right to left … Starting with x = 4. 2 _ (4) = 8 (8) _ (8) = 64 (64) + 1 = 65

Example 2:

Input: functions = [x => 10 * x, x => 10 * x, x => 10 * x], x = 1 Output: 1000 Explanation: Evaluating from right to left … 10 _ (1) = 10 10 _ (10) = 100 10 * (100) = 1000

Example 3:

Input: functions = [], x = 42 Output: 42 Explanation: The composition of zero functions is the identity function

Constraints:

-1000 <= x <= 1000 0 <= functions.length <= 1000 all functions accept and return a single integer

Pre analysis

Will use reduce to solve this problem. This will be iterative solution.

Another solution

/**
 * @param {Function[]} functions
 * @return {Function}
 */
var compose = function (functions) {
  return function (x) {
    for (let i = functions.length - 1; i >= 0; i--) {
      x = functions[i](x);
    }
    return x;
  };
};