for循环

arr= [1,2,3];

for(var i=0;i<arr.length;i++){

    setTimeout(function(){console.log(arr[i])},0);

}

for(var j in arr){

    setTimeout(function(){console.log(arr[j])},0);

}

arr.forEach(function(v,k){

    setTimeout(function(){console.log(arr[k])},0);

})

for(var value of arr){

    setTimeout(function(){console.log(value,0);},0);

}

从上往下执行结果依次如下箭头所示：