<script>
// 原型对象的应用 扩展内置对象方法
console.log(Array().__proto__);
Array.prototype.sum = function() {
var sum = 0;
for (var i = 0; i < this.length; i++) {
sum += this[i];
}
return sum;
}
var arr = [1, 2, 3];
console.log(arr.sum());
</script>