As the amount of JavaScript we use on our pages increases, as does its effect on page performance. Even when browsers manage to produce the same output, how they reach that output can be very different. Some optimisations aren’t obvious, others can be easily broken by seemingly unrelated statements. So, how do we get the most out of our code? This presentation challenges assumptions about performance, using practical code to demonstrate the optimisations that really make a difference, and why they make a difference.