FileReader: loadstart event

The loadstart event is fired when a file read operation has begun.

Bubbles No
Cancelable No
Interface ProgressEvent
Event handler property FileReader.onloadstart


Live example


<div class="example">

    <div class="file-select">
        <label for="avatar">Choose a profile picture:</label>
        <input type="file"
               id="avatar" name="avatar"
               accept="image/png, image/jpeg">

    <img src="" class="preview" height="200" alt="Image preview...">

    <div class="event-log">
        <label>Event log:</label>
        <textarea readonly class="event-log-contents"></textarea>



const fileInput = document.querySelector('input[type="file"]');
const preview = document.querySelector('img.preview');
const eventLog = document.querySelector('.event-log-contents');
const reader = new FileReader();

function handleEvent(event) {
    eventLog.textContent = eventLog.textContent + `${event.type}: ${event.loaded} bytes transferred\n`;

    if (event.type === "load") {
        preview.src = reader.result;

function addListeners(reader) {
    reader.addEventListener('loadstart', handleEvent);
    reader.addEventListener('load', handleEvent);
    reader.addEventListener('loadend', handleEvent);
    reader.addEventListener('progress', handleEvent);
    reader.addEventListener('error', handleEvent);
    reader.addEventListener('abort', handleEvent);

function handleSelected(e) {
    eventLog.textContent = '';
    const selectedFile = fileInput.files[0];
    if (selectedFile) {

fileInput.addEventListener('change', handleSelected);



Specification Status
File API Working Draft

Browser compatibility

Update compatibility data on GitHub
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
loadstart eventChrome Full support YesEdge Full support 12Firefox Full support 79
Full support 79
No support ? — 79
Notes loadstart event dispatches synchronously (should be asynchronously as per spec).
IE Full support YesOpera Full support YesSafari Full support YesWebView Android Full support YesChrome Android Full support YesFirefox Android Full support YesOpera Android Full support YesSafari iOS Full support YesSamsung Internet Android Full support Yes


Full support  
Full support
See implementation notes.
See implementation notes.

See also