forEach()和map()

自写forEach()

Array.prototype.myForEach = function(callback){
  for(var i=0;i<this.length;i++){
    callback(this[i],i,this);
  }
 }

自写map()

Array.prototype.myMap = function(callback){
  var newArr = new Array();
  for(var i=0;i<this.length;i++){
    return newArr = callback(this[i],i,this);
  }
}