I’ve been reading about undefined in JavaScript and now I am not sure if my understanding is correct. There is a lot of talk around how to check for undefined but somehow I couldn’t find any mentioning of something that to me seems fundamental to understanding of how undefined actually works (undefined being property on host object). This is the reason for this question, I need to confirm that what I understand is correct and if I’m wrong I would appreciate clarification.

Okay, first of all, undefined is property on host object (window in browsers) so it’s perfectly legal to use:

window.undefined

The value of this property is type "undefined". This is one of the JavaScript types along with Object, String, Number and Null. So if I do:

if(someVar===undefined) {}

I’m actually checking against window.undefined property, whatever it contains, is that right?

So this code below would be pretty dumb as this would check someVar only against the string "undefined", not the type nor the property of the window object, right?

if(someVar==='undefined') {}

This below would be also incorrect as this would check against the window.undefined property (whatever it contains):

if(typeof someVar===undefined) {}

So, to sum it up, the only proper and cross-browser way to check for undefined is to use typeof e.g.:

if(typeof someVar==='undefined')

Is that right?

Also in ES5 window.undefined cannot be reassigned but it’s perfectly legal in older browsers right?

This however can still be done and is evil if my understanding is right:

(function() { 
  var undefined=66;
  alert(undefined);
})()

I would appreciate clarification if I misunderstood how undefined works in JavaScript.

I’ve been reading about undefined in JavaScript and now I am not sure if my understanding is correct. There is a lot of talk around how to check for undefined but somehow I couldn’t find any mentioning of something that to me seems fundamental to understanding of how undefined actually works (undefined being property on host object). This is the reason for this question, I need to confirm that what I understand is correct and if I’m wrong I would appreciate clarification.

Okay, first of all, undefined is property on host object (window in browsers) so it’s perfectly legal to use:

window.undefined

The value of this property is type "undefined". This is one of the JavaScript types along with Object, String, Number and Null. So if I do:

if(someVar===undefined) {}

I’m actually checking against window.undefined property, whatever it contains, is that right?

So this code below would be pretty dumb as this would check someVar only against the string "undefined", not the type nor the property of the window object, right?

if(someVar==='undefined') {}

This below would be also incorrect as this would check against the window.undefined property (whatever it contains):

if(typeof someVar===undefined) {}

So, to sum it up, the only proper and cross-browser way to check for undefined is to use typeof e.g.:

if(typeof someVar==='undefined')

Is that right?

Also in ES5 window.undefined cannot be reassigned but it’s perfectly legal in older browsers right?

This however can still be done and is evil if my understanding is right:

(function() { 
  var undefined=66;
  alert(undefined);
})()

I would appreciate clarification if I misunderstood how undefined works in JavaScript.

• javascript
• undefined

Share Improve this question Follow edited Sep 11, 2015 at 18:50 Sebastian Simon 19.6k88 gold badges6161 silver badges8484 bronze badges asked Dec 17, 2013 at 1:57 spirytusspirytus 10.9k1414 gold badges6363 silver badges7777 bronze badges 3

• 4 Yes to everything you said. I think you understood all there is to know about undefined. – Felix Kling Commented Dec 17, 2013 at 2:00
• 1 The statement "undefined is property on host object (window in browsers)" is sort of correct. There is an undefined Type whose only value is the undefined value. There is also an undefined property of the Global object, which is a built–in object. In browsers, the window object (a host object) is effectively an alias for the Global object. – RobG Commented Dec 17, 2013 at 2:44
• Null is not a JavaScript type. – Sebastian Simon Commented Sep 11, 2015 at 18:50

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4 Answers 4

Sorted by: Reset to default 4

You're almost correct. Except for this:

The value of [window.undefined] is type "undefined". This is one of Javascriupt types along with Object, String, Number, and Null

There are 3 undefined in javascript. The global variable undefined, the value undefined and the type undefined.

Even if the global variable undefined is overridden, the value undefined still exists. There are several ways to get it one of which is an empty argument to a function, another is a variable declaration without assigning anything:

// Note: Probably need older browsers to assign to undefined:
window.undefined = 1;

(function(foo){ // the value of foo is undefined;
    var bar;    // the value of bar is undefined;

    return [foo === bar, foo === window.undefined]; // returns [true,false]
})();

Note carefully that in the example above we're checking the value, not the type. Yes === checks type and value but if you replace === with == the result would be the same.

The value undefined has type undefined ('Undefined' in the spec and documentation but typeof returns 'undefined') and type undefined is only valid for the value undefined.

That's all fine, plus:

• you can use void 0 to reliably "generate" the real undefined value (or not-a-value; it's kind-of zen)

• in a function, you can reference an argument that you know isn't supplied to get a reliable undefined

  (function( undefined ) {
    // ...
  })();
  

  This second example is not really the clearest code in the world, but you'll see it sometimes in mon public codebases, tutorials, etc.

So if I do:

if(someVar===undefined) {}

I'm actually checking against window.undefined property whatever it contains is that right?

Right.

So this code below would be pretty dumb as this would check someVar only against the string undefined, not the type nor the property of window object right?

if(someVar==='undefined') {}

Right.

This below would be also incorrect as this would check against the window.undefined property (whatever it contains):

if(typeof someVar===undefined) {}

Right.

So to sum it up, the only proper and cross-browser way to check for undefined is to use typeof e.g.:

if(typeof someVar==='undefined')

Is that right?

Yes, though it is error-prone because you may mis-type that string and get no error (even in strict mode) to indicate the mistake.

So it's better to call some method, especially if you're already using some framework e.g. in AngularJS - angular.isUndefined

Also in ES5 window.undefined cannot be reassigned but its perfectly legal in older browsers right?

Right.

This however can still be done and is evil if I my understanding is right:

(function() { 
var undefined=66;
alert(undefined);
})()

I believe so.

So to sum it up, the only proper and cross-browser way to check for undefined is to use typeof e.g.:

if(typeof someVar==='undefined')

No, the direct parison somevar === undefined is fine.

There are any number of global variables that can be overwritten or shadowed that will break code. There's no way to protect against them all, except to simply not allow bad code.

---

What's nice about the direct parison (aside from being shorter and cleaner) is that it's must more natural and intuitive, whereas people often get the other syntax wrong. They end up accidentally using the other examples you gave:

if (somevar === 'undefined')

if (typeof somevar === undefined)

These are very mon errors, and are much more mon than people redefining undefined.

---

Furthermore, you'll see things like this:

if (typeof somevar === 'undefiend')

This is much more subtle, and is hard to spot when surrounded by a bunch of other code. Again, it's a mon mistake.

---

Probably the worst is when you see this:

if (typeof somevar === 'undefined')
    somevar = "foobar";

What's wrong with that? Well, if somevar had not been declared, we've now created an implicit global variable. This can be really bad. If we had done a simple parison, we would have been alerted to the problem with a ReferenceError.