var id = document.getElementId("id");
if(typeof(id) == "undefined" || id == null) {
console.log("节点不存在");
else{
console.log("节点存在");
}
注意：typeof(id) == "undefined" 而不是 typeof(id) == undefined
一般typeof返回的数据类型：
"undefined"
"string"
"boolean"
"number"
"object"
"function"
1、number
console.log(typeof(123));
console.log(typeof(NaN));
console.log(typeof(0));
2、string
console.log(typeof('a'));
3、boolean
console.log(typeof(true));
console.log(typeof(false));
4、undefined
变量没有对应对象或变量的值为undefined
console.log(typeof(a));
var a = undefined;console.log(typeof(a));
5、object
var a = {}; console.log(typeof(a));//对象
var a = []; console.log(typeof(a));//数组
console.log(typeof(null));
6、function
var a = function(){}; console.log(typeof(a));
//Object.prototype.toString.apply(obj)