FileReader: loadstart event

The loadstart event is fired when a file read operation has begun.

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("loadstart", (event) => {});

onloadstart = (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 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.

Examples

Live example

HTML

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>

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-loadstart-event
File API # dfn-onloadstart

Browser compatibility

BCD tables only load in the browser