Function Composition.js

// 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

// code 

/**
 * @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; 
};
};

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