A basic implementation of the Array.prototype.reduce method

myArrayReduce.js

Array.prototype.xreduce = myReduce;

function myReduce (action, defaultValue) {
  var result = [];
  
  var isObject = typeof defaultValue === 'object';
  var initialAccVal = isObject ? (defaultValue.constructor === Object ? {} : []) : defaultValue;
  
  var acc = isObject ? Object.assign(initialAccVal, defaultValue) : defaultValue;

  for (var i = 0; i < this.length; i++) {
    var args = [acc, this[i], i, this];
    acc = action.apply(null, args);
  }

  return acc;
}

var res = [1, 2, 3].xreduce((acc, value, index, array) => {
  acc.push(value + 4);
  return acc;
}, []);

console.log(res, 'reduce output')