Web/API/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

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>

CSS

img.preview {
  margin: 1rem 0;
}

.event-log-contents {
  width: 18rem;
  height: 5rem;
  border: 1px solid black;
  margin: .2rem;
  padding: .2rem;
}

.example {
  display: grid;
  grid-template-areas:
              "select  log"
              "preview log";
}

.file-select {
  grid-area: select;
}

.preview {
  grid-area: preview;
}

.event-log {
  grid-area: log;
}

.event-log>label {
  display: block;
}

.event-log-contents {
  resize: none;
}

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);

Specifications

Specification	Status
File API	Working Draft

Browser compatibility

The compatibility table on this page is generated from structured data. If you'd like to contribute to the data, please check out https://github.com/mdn/browser-compat-data and send us a pull request.

Update compatibility data on GitHub

	Desktop						Mobile
	Chrome	Edge	Firefox	Internet Explorer	Opera	Safari	Android webview	Chrome for Android	Firefox for Android	Opera for Android	Safari on iOS	Samsung Internet
loadstart event	Chrome Full support Yes	Edge Full support 12	Firefox Full support 79 Full support 79 No support ? — 79 Notes' Notes' loadstart event dispatches synchronously (should be asynchronously as per spec).	IE Full support Yes	Opera Full support Yes	Safari Full support Yes	WebView Android Full support Yes	Chrome Android Full support Yes	Firefox Android Full support 79 Full support 79 No support ? — 79 Notes' Notes' loadstart event dispatches synchronously (should be asynchronously as per spec).	Opera Android Full support Yes	Safari iOS Full support Yes	Samsung Internet Android Full support Yes

Legend

Full support  

Full support

See implementation notes.'

See implementation notes.

See also

• Related events: FileReader.load_event, FileReader.loadend_event, FileReader.progress_event, FileReader.error_event, FileReader.abort_event

FileReader: loadstart event by Mozilla Contributors is licensed under CC-BY-SA 2.5.