The FileReader onerror handler receives an Event object, not an Error object, as a parameter, but an error can be accessed from the FileReader object, as instanceOfFileReader.error
// Callback from a <input type="file" onchange="onChange(event)"> function onChange(event) { var file = event.target.files[0]; var reader = new FileReader(); reader.onerror = function(event) { alert("Failed to read file!\n\n" + reader.error); reader.abort(); // (...does this do anything useful in an onerror handler?) }; reader.readAsText(file); }
onerror by Mozilla Contributors is licensed under CC-BY-SA 2.5.