Five Common Ajax Anti-Patterns
Not inspecting the return results in the callback. When using the processReqChange()callback on the XMLHTTPRequest object, the readyState and status fields of the callback argument need to be checked. The callback will often be invoked before the results are complete. Failing to check this result can cause errors if code tries to process incomplete data.
York Xyander, Bodo Junglas Jul 31, 2015