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This one value in Javascript is not equal to itself!

We know that Javascript supports all kinds of values such as strings, numbers, constants etc. All these values are deterministic values in that their weights always remains the same. For example, an integer value of 25 is always equal to 25 in Javascript no matter what. Similarly a string value of “Hello” is also always equal to another string “Hello”.

In other words, these values in Javascript can always be compared with another value to determine if they are the same or different. To understand this better with an example, let us open up our browser console. In my case I am using Google Chrome browser console where we will create 3 variables with these values:

We can note from the above Javascript demo video that when the variable a and c are compared, since both their values are same holding a value of 2, they return true when compared with each other. On the other hand when variable a was compared with b, since their values were different, the comparison resulted in a return value of false.

This is true for all type of values present in Javascript – be it string, integers, floats, booleans anything you can think of.

However, there exists one special value in Javascript that is never equal to another variable having the same value. In other words its value is never equal to itself. This value is the NaN value!

NaN in Javascript stands for “Not a Number” and it is that one special value in Javascript which does not return true if it is compared with itself.

Why does NaN not equal to itself in Javascript?

Now you might be wondering why a NaN value does not equal to itself? The answer for this lies in the way Javascript language has been designed.

NaN or Not a Number is a special value in Javascript which is used to represent a nonsensical value – that is it is the value returned whenever a non sensical operation is performed. Now this is where it gets interesting. Why does a non sensical value not be the same all the time or at all the place? In other words, why is this happening here:

Why is it returning false?

The answer is that NaN as mentioned earlier is a value that is used to represent a non sensical values. So if the result of an operation performed is something that cannot be represented by ordinary or normal values, Javascript returns a value of NaN.

Now, if two operations results in non sensical values, they are not necessarily equal. Each of these operations can be returning two non sensical values of different weights. However, they both need to be represented by the value of NaN. Hence, the Javascript treats two NaNs as two different values and never equal to each other.

I learnt about this and many other similar anomalies in the book Eloquent Javascript. This is a very good book to learn and understand such interesting things about Javascript so I will definitely recommend this to anyone interesting in learning Javascript in depth.

If you are also aware of any other similar interesting things about Javascript do let me know in the comments below. Until then, happy coding! 🙂