The following is incorrect usage:
var exp = undefined;
if (exp == undefined)
{
alert("undefined");
}
When exp is null, the same result as undefined will be obtained , although null and undefined are not the same.
The following is the correct usage:
var exp = undefined;
if (typeof(exp) == "undefined")
{
alert("undefined");
}
typeof returns a string , there are six possibilities: "number", "string", "boolean", "object", "function", "undefined"
![[Web front-end] Node.js quick start](https://img.php.cn/upload/course/000/000/067/662b5d34ba7c0227.png)
