The loadstart
event is fired when a file read operation has begun.
This event is not cancelable and does not bubble.
The loadstart
event is fired when a file read operation has begun.
This event is not cancelable and does not bubble.
Use the event name in methods like addEventListener()
, or set an event handler property.
js
addEventListener("loadstart", (event) => {}); onloadstart = (event) => {};
A ProgressEvent
. Inherits from Event
.
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 total
by 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.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.
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>
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 += `${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);
Specification |
---|
File API # dfn-loadstart-event |
File API # dfn-onloadstart |
Desktop | Mobile | |||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
Chrome | Edge | Firefox | Internet Explorer | Opera | Safari | WebView Android | Chrome Android | Firefox for Android | Opera Android | Safari on IOS | Samsung Internet | |
loadstart_event |
6 | 12 | 793.6–79loadstart event dispatches synchronously (should be asynchronously as per spec). |
10 | 11 | 6 | 3 | 18 | 7932–79loadstart event dispatches synchronously (should be asynchronously as per spec). |
11 | 6 | 1.0 |
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https://developer.mozilla.org/en-US/docs/Web/API/FileReader/loadstart_event