Javascript Check Undefined Example

By now we all know that in Javascript we attach types to values and not variables, like in most other programming languages. One of these types is undefined.

All variable declarations are initialized firstly with undefined, until we give them another value. When we don’t declare the variables, they just do not exist, but when we declare them and forget to initialize them?

What about when you have forgotten if you did so, but don’t have the time to go back and search if you initialized it or not? Let’s check!

Strict Equality Operator

When we want to determine whether a variable has a value or not, we use the strict equality operator. We don’t use the equality operator == because it also checks true for null, which is not the same as undefined, although frequently confused.

The code would be like below:

var a;
if (a === undefined) {
   // execute this code
else {
   // execute this code

In the case above, since we haven’t initialized the a variable, the if statement will be true, so the code in the if clause will be executed.

However, when the variable a is not even declared before, this method will leave us with a ReferenceError.

TypeOf Operator

To get rid of the issue we mentioned before about the strict equality operator throwing a ReferenceError we use the typeOf operator, as in the code snippet below:

if (typeof a === 'undefined') { // true if a is not declared
    // execute this code

However, this does not work for objects. In that case you would have to use it together with in, this way:

key in yourObject && typeof yourObject.key === 'undefined';

Which alternatively would be like below, if you’re not familiar with the in operator:

 yourObject.hasOwnProperty(key) && typeof yourObject.key === 'undefined';

This way you can easily understand what in does; namely, returns true if an object has a particular property and false if it doesn’t. Which means that by using this method, we firstly check if the property exists, avoiding the possibility that we get a ReferenceError, and then check the type of that variable.

Void Operator

As an alternative to undefined we might use the void operator like below:

var a;
 if (a === void 0) {
    // execute this code

Note that in this case we have only substituted undefined for void 0, and everything else stays the same. That means that, even while using this method, when the variable is not declared, we have a ReferenceError in our hands.

Whichever method you use, you will be able to check if your variable is undefined or not with efficiency.

Download the source code

This was an example of check undefined in Javascript.

Download the source code for this tutorial:

You can download the full source code of this example here : CheckUndefined

Era Balliu

Era is a Telecommunications Engineering student, with a great passion for new technologies. Up until now she has been coding with HTML/CSS, Bootstrap and other front-end coding languages and frameworks, and her recent love is Angular JS.
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