JavaScript "undefined" keyword not recognized. Null is recognized but not undefined. Syntax: undefined Description undefined is a top-level property and is not associated with any object. A variable that has not been assigned a value is of type undefined. A method or statement also returns undefined if the variable that is being evaluated does not have an assigned value. You can use undefined to determine whether a variable has a value. In the following code, the variable x is not defined, and the if statement evaluates to true. var x if(x == undefined) { // these statements execute } undefined is also a primitive value.