Boolean, Null & Undefined - Explained in Javascript

Until now, you have studied about the number and the string data types. Well, there are more data types in JavaScript. Let’s talk about the rest of them one by one. Let us start with the boolean data type.

The boolean data type can only hold two values - true or false. So they are often used as indicators of whether some event has occurred or not, or to check some condition which results in either of the two aforementioned values.

Let’s now move on to the next set of data types, viz. null and undefined.

Null signifies that the container is purposefully kept empty. On the other hand, undefined means that the container does not have anything inside it. Null is like saying I am keeping my tea container empty. Undefined is like saying that I have a container but I have forgotten to put anything inside it. That’s why null is set by the user i.e you. This is not the case with undefined as any value which has not been defined is initialized undefined by the translator.

There is also something known as not defined, which means that something hasn't even been declared or allocated memory. This is like saying that my container doesn't even exist. 

Let's see this with an example that highlights the difference between undefined and not defined:

Code Snippet:

console.log(y);

Output:
Not defined

Code Snippet:

var y;
console.log(y);

Output:
undefined

Well, you might be wondering how can you find what kind of datatype is your variable. Let’s find it out in the next video.

You also learnt about ===, which is known as the strict equality operator.

It returns true only when both the data type as well as the values are equal. 

So while 
 

console.log('2' == 2);

 will output true because the values contained inside both operands is same (notice that type coercion happens here), but 
 

console.log('2' === 2);

will output false because '2' is a string data type but 2 is a number data type.

You can use the typeof operator to find out the data type of any literal or variable or expression in JavaScript.