XMLHttpRequestEventTarget: loadstart event

Baseline Widely available

This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.

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See full compatibility

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.

Examples

Usage with XMLHttpRequest

HTML

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>

.event-log {
  width: 25rem;
  height: 4rem;
  border: 1px solid black;
  margin: 0.5rem;
  padding: 0.2rem;
}

input {
  width: 11rem;
  margin: 0.5rem;
}

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;
});