js大神进来看看+++++++++++++++++

Question

怎么会出来函数？哪里错了？

 arr = [5,5,5,5];
Array.prototype.sum = function(){
    var sum = 0;
    for(var i in this){
        sum += this[i];
    }
    return sum;
}

document.write(arr.sum()+"</br>");

Answer 1

跪求讲解 纠结中。。。。。