FileReader: load event
Baseline Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since July 2015.
Note: This feature is available inWeb Workers.
Theload event of theFileReader interface is fired when a file has been read successfully.
This event is not cancelable and does not bubble.
Syntax
Use the event name in methods likeaddEventListener(), or set an event handler property.
addEventListener("load", (event) => { })onload = (event) => { }Event type
AProgressEvent. Inherits fromEvent.
Event properties
Also inherits properties from its parentEvent.
ProgressEvent.lengthComputableRead onlyA 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.loadedRead onlyA 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
totalby the value of this property. When downloading a resource using HTTP, this only counts the body of the HTTP message, and doesn't include headers and other overhead.ProgressEvent.totalRead onlyA 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> <div> <label for="avatar">Choose a profile picture:</label> <input type="file" name="avatar" accept="image/png, image/jpeg" /> </div> <img src="" height="200" alt="Image preview" /> <div> <label for="eventLog">Event log:</label> <textarea readonly></textarea> </div></div>img.preview { margin: 1rem 0;}.event-log-contents { width: 18rem; height: 5rem; border: 1px solid black; margin: 0.2rem; padding: 0.2rem; resize: none;}.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;}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-load-event |
| File API # dfn-onload |
Browser compatibility
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