2629-FunctionComposition.js

// 2629. Function Composition
// 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

/**
 * @param {Function[]} functions
 * @return {Function}
 */
var compose = function(functions) {
    return function(x) {
        let res = x
        while(functions.length != 0) {
            var fn = functions.pop();
            res = fn(res)
        }
        return res
    }
};

// best solution
var compose = function(functions) {
	return function(x) {
        let res = x
        for (let item = functions.length-1; item >= 0; item--){
            res = functions[item](res)
        }
        return res
    }
};

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