/**
 * @param {Function[]} functions
 * @return {Function}
 */
var colutation = function(functions) {
  return function(x) {
    let result = x;
    for (let i = 0; i < functions.length; i++) {
      result = functions[i](result);
    }
    return result;
  };
};

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