14: True and False Tests

A common operation in JavaScript is to test an equality of two variables (or values). You might want to know if a variable x is less than, equal to, or greater than another variable y. There are a large number of comparison operations that all fall into the category of "boolean logic", but in programming we'll call this a test. The end result of performing a test is true or false. You perform a test in JavaScript by using a "comparison operator" such as x == y or y != 1. Your computer's CPU even has a TEST instruction that does this.

Equal

In JavaScript, you can test the equality of two variables using == (double equals) or === (triple equals). You can think of == as "could be equal" and === as "definitely equal". You can play with node and try these examples out:

> 1 == '1'
true
> 1 === '1'
false
> '' == 0
true
> '' === 0
false

You can see that with == JavaScript will let you compare things that are seemingly very different like a numeric 1 and the string '1'. However, with === you'll be held to a stricter requirement that both sides of the === are exactly the same to be true. You should also look at the way I compare 0 to '' (an empty string). I explain this in the next part, but try to figure out why this happens in this example.