数组 filter
Array.prototype.myFilter = function(callback){
var newArr = []
for(var i = 0; i<this.length;i++){
if(callback(this[i], i)){
newArr.push(this[i])
}
}
return newArr
}
var arr = [1,5,6,7,8]
arr.myFilter((item, index)=>{
if(item>7){
console.log(item)
}
})