Detect upload status in javascript

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I am using wfu in an admin page to upload files. The page is generated on-the-fly and the upload is triggered in javascripts with the code fragment:

var elem = document.getElementById(“upload_1”);
if (typeof elem.onclick == “function”) { elem.onclick.apply(elem); }

on the input object (id=”upload_1″)that is the submit button. The upload works, but how do I test for failure in javascript?