Here are two reasons why you need to be careful when doing browser-based mathematical comparison operations in JavaScript:

You can try it for yourself. Open Chrome, press F12 (opens Developer Tools) and navigate to the Console tab. Type in the above equations and see the results for yourself.

It looks broken. But apparently, it's working as expected.

If you happen to be doing some JavaScript validation and working with numbers, this small discrepancy can be a real PITA.

Image you have a bit of code like this:

if ( (x + y) > 0.3 ){
  alert("You've exceeded the limit of 0.3");
}

Now, imagine x and y come from user input and they inputted 0.2 and 0.1. They'll be wondering a big WTF.

Instead you need to introduce the concept of Epsilon. Like so:

if ( (x + y) - 0.3 > Epsilon ){
  alert("You've reached the limit");
}

Where Epsilon is a very, very small number. How small? Typically something like 0.00000000001.

You've been warned.