Write unobtrusive JS. Keep all JS in a few, minified, external files. Don’t use inline script blocks or write inline JS. Only include jQuery when really necessary, since parse and execution time of large JS files can be slow. Prefer vanilla JavaScript code and micro-frameworks. Before including a library to polyfill a feature, considering the cost to your users.

It can help to use a style guide to keep your code consistent and readable (there’s an example file in this Guide). Check your code for errors and complexity using something like JS Hint (there’s an example file in this Guide).