Use the typeof Method to Determine the undefined type, typeofundefined

Use the typeof Method to Determine the undefined type, typeofundefined

For JavaScript to determine the undefined type, the typeof method is used. typeof returns a string, where one is undefined.

Js determines the undefined type

if (reValue== undefined){alert("undefined");}

The system found that the data could not be determined, and finally found the information to use the typeof method:

if (typeof(reValue) == "undefined"){ alert("undefined");}

Typeof returns a string, which may be "number", "string", "boolean", "object", "function", or "undefined ".
Be sure to pay attention when using it.

JavaScript typeof ()

Arrays are often used in js. For example, if multiple inputs with the same name are generated dynamically, You need to determine whether the input is an array during submission.

If (document. mylist. length! = "Undefined") {} This usage is incorrect.

Correct is if (typeof (document. mylist. length )! = "Undefined "){}

Or if (! IsNaN (document. mylist. length )){}

The number of typeof operations is undefined, and "undefined" is returned ".

Number of operations: number typeof (x) = "number"

String typeof (x) = "string"

Boolean value typeof (x) = "boolean"

Object, array, and null typeof (x) = "object"

Function typeof (x) = "function"

The typeof operator returns a string that represents the Data Type of the expression.
Possible strings include: "number", "string", "boolean", "object", "function", and "undefined ".
For example:
Alert (typeof (123); // typeof (123) returns "number"
Alert (typeof ("123"); // typeof ("123") returns "string"

In java, how does one determine whether the variable is of the int type? If typeof is used, an error is returned?

Java does not contain typerof. You must use instanceof
The basic type does not work. You must use an object.
Integer I = 0;
If (I instanceof Integer ){
System. out. println ("haha ");
}

String str = "abc ";
If (str instanceof Object ){
System. out. println ("haha ");
}
Hope to help you.