常见的回调函数

1.forEach(Es5)   (遍历数组)

 let arr =[10,11,12,13,14];           
 arr.forEach(function (item,index){
 console.log(index);
 });

2.map 与forEach用法一致    对数组中每一个元素执行相同的操作，返回一个新的数组

 let arr =[10,11,12,13,14];      
 let newArr =  arr.map(function(item , index){
     return item * 2;
 });
 console.log(newArr);

3. let arr =[10,11,12,13,14];        过滤，筛选
   let newArr = arr.filter(function (item,index){
     return item %2 ===0;
   });
   console.log(newArr);

 let arr =[10,11,12,13,14,11,12];         去重
 let newArr = arr.filter(function (item,index){
     return arr.indexOf(item)===index;
 });
 console.log(newArr);

4.reduce
 let arr =[10,11,12,13,14];       比大小
 let result = arr.reduce(function(a,b){
      console.log(a,b);
     return a>b?a:b;
 });
 console.log(result);
5.every
 let arr = [10, 11, 12, 13, 14];  对每一个元素进行判断，一假全假
 let result = arr.every(function (item, index) {
      console.log(item,index);
     return item > 3;
 })
 console.log(result);

6.some
 let arr = [10, 11, 12, 13, 14];  对每一个元素进行判断，一真全真
 let result = arr.some(function (item, index) {
      console.log(item,index);
     return item >14 ;
 });
 console.log(result)