Language:
Namespace polluting by creating a large footprint of variables in the global context.
Binding event handlers in the form 'foo.onclick = myFunc' (inextensible, should be using attachEvent/addEventListener).
Using eval in almost any non-JSON context
Almost every use of document.write (use the DOM methods like document.createElement)
Prototyping against the Object object (BOOM!)
A small one this, but doing large numbers of string concats with '+' (creating an array and joining it is much more efficient)
Referring to the non-existent undefined constant
Design/Deployment:
(Generally) not providing noscript support.
Not packaging your code into a single resource
Putting inline (i.e. body) scripts near the top of the body (they block loading)
Ajax specific:
not indicating the start, end, or error of a request to the user
polling
passing and parsing XML instead of JSON or HTML (where appropriate)
Many of these were sourced from the book Learning JavaScript Design by Addy Osmati: https://www.oreilly.com/library/view/learning-javascript-design/9781449334840/ch06.html
edit: I keep thinking of more!
