Jon Raasch's notes on Nicholas Zakas's Google Tech Talk "Speed Up Your JavaScript" in which he covers basic performance improvements like local variables for more expensive lookups, avoiding for in loops, casting DOM array-like objects into proper arrays, avoid touching the DOM, use classes to change CSS instead of inline styles.
As web pages become more and more like applications, code performance becomes more and more important. This article looks at a number of performance issues to avoid, in EcmaScript, DOM and AJAX requests. Covers eval, the with keyword, try/catch in performance-critical code, global variables, implicit object conversion, string concatenation, primitive operations over function calls, repainting and reflowing documents, modifying elements, using XPath.