The correct method for JavaScript to determine the undefined type

The correct method for JavaScript to determine the undefined type

This article mainly introduces the correct method of JavaScript to judge the undefined type. I believe many people will use the wrong method when encountering this problem. The correct method is also very simple. For more information, see

The Code is as follows:

If (reValue = undefined ){

Alert ("undefined ");

}

Unable to judge, found the information to use typeof

Method:

The Code is as follows:

If (typeof (reValue) = "undefined "){

Alert ("undefined ");

}

Typeof returns a string, which may be "number", "string", "boolean", "object", "function", or "undefined"