Tools such as dynaTrace AJAX Edition or SpeedTracer from Google have some unique capabilities to trace all JavaScript execution on the web page. dynaTrace also traces calls into the Browser DOM (Document Object Model) and is able to capture method arguments and return values. The following illustration shows a JavaScript trace of a script execution in the PurePath view of the dynaTrace AJAX Edition
By getting this level of details on JavaScript execution it is easy to identify slow running JavaScript handlers, custom javascript code, slow access to the DOM and expensive or inefficient calls into 3rd party frameworks such as jQuery
In the Best Practices for JavaScript and AJAX Performance we list all common problems we have seen when working with our AJAX Edition users, people like Steve Souders or John Resig and our commercial customers over the last couple of months. It includes recommendations on how to avoid blocking script blocks, inefficient CSS Selectors with frameworks such as jQuery, reducing DOM manipulations and XHR calls. The latest beta version of the dynaTrace AJAX Edition implements some of these Best Practices to automatically analyze slow running JavaScript code.