JavaScript Null & Undefined

Today will be a shorter post to touch on the types null and undefined. These are sometimes referred to as non-values. undefined and null are both the type and value. They are to be treated as empty or 'non' values. Specifically, null is a JavaScript literal representing null or an "empty" value, whereas undefined is a global property that represents the primitive value undefined.

These types are sometimes interchangeable, so some developers prefer specificity in distinguishing them:

  • undefined hasn't had a value yet
  • null had a value and doesn't anymore


  • null is an empty value
  • undefined is a missing value

Go with whatever works for your way of thinking.


Let's focus on null real quick. There's really not much to it. It's basically a placeholder for something with no value, but there's a major caveat to null, and that is typeof null returns 'object'. This is a bug that has been around for so long that fixing it would cause too much code to break. So, if we want to check if something is null we have to do the following:

var nothing = null
(!nothing && typeof nothing === 'object') // true

JavaScript evaluates anything in parentheses (). In the above snippet we evaluate that nothing has no value, and it is of the type object. If both these things are true, this expression will evaluate to true, meaning our variable is null.


Variables that have no value are undefined. Variables that are declared, but not assigned a value, are given the value and type undefined. We can see calling typeof on such variables will return 'undefined'.

var a
typeof a // 'undefined'

Similarly to undefined we might see undeclared, most likely in the context of a ReferenceError. If something goes wrong in your program, it will display an error. Errors are a large subject, and so we will just peek at ReferenceError.

var a
console.log(a) // undefined
console.log(b) // ReferenceError: b is not defined

The language is not the most clear here, but we can see in the above example that a logged undefined because we had declared it, but left no assignment. Then we attempted to log the variable b, and we got ReferenceError: b is not defined. This is telling us that the program attempted to do something with variable b, but it was not defined (undeclared), and it threw an error. The term throw is often used around errors, and basically just means log some details about what went wrong, so we can hopefully use it for debugging and to fix the issue. If you find yourself having good reason to avoid this error from throwing, here is a solution:

// Safety check to avoid a ReferenceError for undeclared.

// will error
if (DEBUGGER) console.log('Debugging started.')
// safe existence check
if (typeof DEBUGGER !== 'undefined') console.log('Debugging started.')
// feature check for API
if (typeof yourFeature === 'undefined') yourFeature = function(){/*...*/}
// or check the global object. window is the global in browsers.
if (!window.yourFeature) yourFeature = function() {/*...*/}

Here's a little bonus for the day. Don't sweat this one. It's not very common, but neat to know about. JavaScript has a void operator. It evaluates the given expression and then returns undefined. void will void out any value, so the result is always undefined. Note, this does not affect the original value.

var num = 999
console.log(void num) // undefined
console.log(num) // 999

Just remember that null and undefined represent 'non' or empty values, and you'll be OK. We'll be at you with Objects and Numbers this week. Thanks for joining, and see you then!