Algorithm
30 Days of JavaScript
Function Composition

2629. Function Composition

Tags

  • Function Transformations

Link

https://leetcode.com/problems/function-composition/description/?envType=study-plan-v2&envId=30-days-of-javascript (opens in a new tab)

Question

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

Answer

JavaScript

/**
 * @param {Function[]} functions
 * @return {Function}
 */
var compose = function (functions) {
  return (x) => functions.reduceRight((accumulator, fn) => fn(accumulator), x);
  // return function (x) {
  //   return functions.reduceRight((acc, fn) => fn(acc), x);
  // };
};
 
/**
 * const fn = compose([x => x + 1, x => 2 * x])
 * fn(4) // 9
 */
/**
 * @param {Function[]} functions
 * @return {Function}
 */
var compose = function (functions) {
  return function (x) {
    let value = x;
    for (let i = functions.length - 1; i >= 0; i--) {
      value = functions[i](value);
    }
    return value;
  };
};
 
/**
 * const fn = compose([x => x + 1, x => 2 * x])
 * fn(4) // 9
 */
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