如何判断是否是数组

var arr = [1,2];
var arr2 = "";
Array.isArray(arr);//true
Array.isArray(arr2);//false

var arr = [1,2];
var arr2 = "";
Object.prototype.toString.call(arr);//"[object Array]"
Object.prototype.toString.call(arr2);//"[object String]"

var arr = [1,2];
var arr2 = "";
arr.pop//function pop(){...}
arr2.pop//undefined

var arr = [1,2];
var arr = "";
arr instanceof Array;//true
arr2 instanceof Array;//false

if(typeof Array.isArray === "undefined"){
Array.isArray = function(arr) {
return Object.prototype.toString.call(arr) === "[object Array]";
}
}