What is the difference between "defined" and defined in javascript?

Question

When writing this in javascript, I have seen it written in two different ways:

if (typeof x === "undefined") {
// execute code here
}
if (typeof x === undefined) {
// execute code here
}

my question here is:

what is the difference between "undefined" and undefined. One is enclosed in quotes and the other is not.

Can anybody clarify this for me?

Thanks!

• Marc B: "undefined" is a string containing the letters u, n, d, etc... undefined (no quotes) is a javascript variable.
• zerkms: The first is a string, the second is a primitive value of undefined type

Answer 1

undefined is a value, 'undefined' is a string literal. The typeof operator returns a string that gives you the type. So typeof x returns the string name of the type of x.

Use if( x === undefined ) or if( typeof x === 'undefined' ) but never if( typeof x === undefined ) because typeof x will always return a string (which will never equal undefined).

• trex005: actually, undefined could be defined as "undefined" in which case if(typeof x === undefined) would evaluate identically to if(typeof x === "undefined")
• error: If you do this, I will find you.

Answer 2

"undefined" is a string, and undefined is a variable containing the primitive value undefined (Thank you elclanrs).

if(typeof x === undefined) should only ever be able to return true if undefined is reassigned to a string matching the type of x.

• zerkms: "is a variable, which is probably undefined" --- that's not correct
• JCOC611: You can't set a variable with name undefined, it's a reserved global that represents undefinedness...
• elclanrs: undefined is indeed a variable, that you can give any value you want, unless you use strict
• zerkms: @elclanrs "is indeed a varaible" --- as per specification or implementation?
• elclanrs: @zerkms, actually, seems to be defined in the spec. ecma-international.org/ecma-262/5.1/#sec-15.1.1. I think we are talking about different things. undefined is a variable with value undefined which is of type Undefined