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XMLHttpRequestEventTarget: loadstart event

Note: This feature is available in Web Workers, except for Service Workers.

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

Syntax

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

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

onloadstart = (event) => { }

Event type

A ProgressEvent. Inherits from Event.

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 proportion of work done can be calculated by dividing the value of this property by total. 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.

Examples

Usage with XMLHttpRequest

HTML

<div class="controls">
  <input
    class="xhr success"
    type="button"
    name="xhr"
    value="Click to start XHR (success)" />
  <input
    class="xhr error"
    type="button"
    name="xhr"
    value="Click to start XHR (error)" />
  <input
    class="xhr abort"
    type="button"
    name="xhr"
    value="Click to start XHR (abort)" />
</div>

<textarea readonly class="event-log"></textarea>

JavaScript

const xhrButtonSuccess = document.querySelector(".xhr.success");
const xhrButtonError = document.querySelector(".xhr.error");
const xhrButtonAbort = document.querySelector(".xhr.abort");
const log = document.querySelector(".event-log");

function handleEvent(e) {
  log.textContent = `${log.textContent}${e.type}: ${e.loaded} bytes transferred\n`;
}

function addListeners(xhr) {
  xhr.addEventListener("loadstart", handleEvent);
  xhr.addEventListener("load", handleEvent);
  xhr.addEventListener("loadend", handleEvent);
  xhr.addEventListener("progress", handleEvent);
  xhr.addEventListener("error", handleEvent);
  xhr.addEventListener("abort", handleEvent);
}

function runXHR(url) {
  log.textContent = "";

  const xhr = new XMLHttpRequest();
  addListeners(xhr);
  xhr.open("GET", url);
  xhr.send();
  return xhr;
}

xhrButtonSuccess.addEventListener("click", () => {
  runXHR("example-image.jpg");
});

xhrButtonError.addEventListener("click", () => {
  runXHR("https://example.com/notfound.jpg");
});

xhrButtonAbort.addEventListener("click", () => {
  runXHR("example-image.jpg").abort();
});

Result

Usage with XMLHttpRequestUpload

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.classList.add("visible");
  progressBar.value = 0;
  progressBar.max = event.total;
  log.textContent = "Uploading (0%)…";
  abortButton.disabled = false;
});

Specifications

Specification
XMLHttpRequest> # event-xhr-loadstart>
XMLHttpRequest> # handler-xhr-onloadstart>

Browser compatibility

api.XMLHttpRequest.loadstart_event

	Desktop					Mobile
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	WebView Android	WebView on iOS
`loadstart_event`	1	12	3.5	≤12.1	1.3	18	4	≤12.1	1	1.0	4.4	1

api.XMLHttpRequestUpload.loadstart_event

	Desktop					Mobile
	Chrome	Edge	Firefox	Opera	Safari	Chrome Android	Firefox for Android	Opera Android	Safari on IOS	Samsung Internet	WebView Android	WebView on iOS
`loadstart_event`	2	12	3.5	≤12.1	4	18	4	≤12.1	3	1.0	4.4	3