XMLHttpRequestUpload: loadstart event

The loadstart event is fired when a request has started to load data.


Use the event name in methods like addEventListener(), or set an event handler property.


addEventListener("loadstart", (event) => {});

onloadstart = (event) => {};

Event type

Event properties

In addition to the properties listed below, properties from the parent interface, Event, are available.

lengthComputable Read only

A boolean flag indicating if the total work to be done, and the amount of work already done, by the underlying process is calculable. In other words, it tells if the progress is measurable or not.

loaded Read only

A 64-bit unsigned integer value indicating the amount of work already performed by the underlying process. The ratio of work done can be calculated by dividing total by the value of this property. When downloading a resource using HTTP, this only counts the body of the HTTP message, and doesn't include headers and other overhead.

total Read only

A 64-bit unsigned integer representing the total amount of work that the underlying process is in the progress of performing. When downloading a resource using HTTP, this is the Content-Length (the size of the body of the message), and doesn't include the headers and other overhead.


Using the loadstart event

You can use the loadstart event to detect the beginning of an upload. For a complete code example that uploads a file and displays a progress bar, see the main XMLHttpRequestUpload page.


// When the upload starts, we display the progress bar
xhr.upload.addEventListener("loadstart", (event) => {
  progressBar.value = 0;
  progressBar.max = event.total;
  log.textContent = "Uploading (0%)…";
  abortButton.disabled = false;


XMLHttpRequest Standard
# event-xhr-loadstart
XMLHttpRequest Standard
# handler-xhr-onloadstart

Browser compatibility

BCD tables only load in the browser

See also