Function Composition – 2629

🔗LC2629 🟢 Easy 🧩 Pattern – Function Transformations

📅 Day 8/30 Days of JavaScript

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

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

Solution

Iterate the array in reverse order so that we can call functions from right to left.

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

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