FileReader: progress event
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since January 2020.
Note: This feature is available in Web Workers.
The progress
event of the FileReader
interface is fired periodically as the FileReader
reads data.
This event is not cancelable and does not bubble.
Syntax
Use the event name in methods like addEventListener()
, or set an event handler property.
addEventListener("progress", (event) => {});
onprogress = (event) => {};
Event type
A ProgressEvent
. Inherits from Event
.
Event properties
Also inherits properties from its parent Event
.
ProgressEvent.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.
ProgressEvent.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 the value of this property by
total
. When downloading a resource using HTTP, this only counts the body of the HTTP message, and doesn't include headers and other overhead. ProgressEvent.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
<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 for="eventLog">Event log:</label>
<textarea readonly class="event-log-contents" id="eventLog"></textarea>
</div>
</div>
JavaScript
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 += `${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
Specification |
---|
File API # dfn-progress-event |
File API # dfn-onprogress |
Browser compatibility
BCD tables only load in the browser