XMLHttpRequest: loadstart event
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.
js
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 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.
Examples
Live example
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>
JavaScript
js
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
Specifications
Specification |
---|
XMLHttpRequest Standard # event-xhr-loadstart |
XMLHttpRequest Standard # handler-xhr-onloadstart |
Browser compatibility
BCD tables only load in the browser