XMLHttpRequest: loadstart event - Web APIs 编辑

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

Examples

Live example

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>

CSS

.event-log {
    width: 25rem;
    height: 4rem;
    border: 1px solid black;
    margin: .5rem;
    padding: .2rem;
}

input {
    width: 11rem;
    margin: .5rem;
}

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('https://www.wenjiangs.com/wp-content/uploads/2020/mozilla/DgsZYJNXcAIPwzy.jpg');
});

xhrButtonError.addEventListener('click', () => {
    runXHR('https://somewhere.org/i-dont-exist');
});

xhrButtonAbort.addEventListener('click', () => {
    runXHR('https://www.wenjiangs.com/wp-content/uploads/2020/mozilla/DgsZYJNXcAIPwzy.jpg').abort();
});

Result

Specifications

Browser compatibility

BCD tables only load in the browser

See also

• Related events: loadend, load, progress, error, abort
• Monitoring progress