Testing for equality

This short program tests whether two numbers are equal:

Try this and you'll not get the answers you expect. Let's take a closer look at the line that's causing the problem:

What on earth is the browser to make of this? We've used the equals sign twice in this statement, the first time it means "assign a value to this variable", the second time it means "are these equal?". The browser can't be expected to read our minds so it just took "=" to mean "assign a value." It read the value from secondNumber, stored it in firstNumber and then stored it again in result.

In order to avoid this confusion, JavaScript uses a double equals sign to mean "are these equal?" and the single equals sign only has the meaning "assign a value to this variable". The line which we thought was a comparison was really just an unusual way of assigning values.

Change the line to read:

and it should work as you'd expect.