FileReader: progress event

The progress event is fired periodically as the FileReader reads data.

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

Examples

Live example

HTML

<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">
    </div>

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

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

  </div>

JS

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) {
        addListeners(reader);
        reader.readAsDataURL(selectedFile);
    }
}

fileInput.addEventListener('change', handleSelected);

Result

Specifications

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
progress_event
7
12
3.6
10
11
6
≤37
18
32
11
6
1.0

See also

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Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/FileReader/progress_event