concatAll, map, filter, forEach

es6-function-of-Array

Array.prototype.forEach = function(cb) {
  let len = this.length
  for(let i = 0; i < len; i++) {
    cb(this[i], i, this)
  }
}

Array.prototype.concatAll = function() {
  let ret = []
  // [].push.apply(ret, item);
  this.forEach(item => ret.push(...item))
  return ret
}

Array.prototype.map = function(cb) {
  let ret = []
  this.forEach((item, index, array) => ret.push(cb(item, index, array)))
  return ret
}

Array.prototype.filter = function(cb) {
  let ret = []
  this.forEach((item, index, array) => cb(item, index, array) && ret.push(item))
  return ret
}

演示练习： https://codepen.io/flcwl/pen/LYPNayR