How to judge undefined in JS (a good method)

I have encountered some problems in my recent project. I 'd like to share it with you.

After the servlet is assigned to the html page text box value, alert is used to pop up the value. the result shows "undefined ". so I naturally use this value to compare it with the string "undefined". Unfortunately, the comparison result is false.

No way, you only need to google it and look for the gods.

In the past, typeof must be used, which is written as follows:
Copy codeThe Code is as follows:
If (typeof (value) = "undefined "){

Alert ("undefined ");

}

In this way, the "undefined ";

Typeof returns a String. There are six possibilities: "number" "String" "boolean" "object" "function" "undefined"