FileReader: loadstart event
===========================

**Note:** This feature is available in Web Workers

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.


```js
addEventListener("loadstart", (event) => {});

onloadstart = (event) => {};
```

Event type
----------

A `ProgressEvent`. Inherits from `Event`.

Event

ProgressEventEvent 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


```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);
```


#### Result

Specifications
--------------

| Specification |
| --- |
| File API # dfn-loadstart-event |
| File API # dfn-onloadstart |

Browser compatibility
---------------------

BCD tables only load in the browser with JavaScript enabled. Enable JavaScript to view data.

See also
--------

* Related events: `FileReader.load_event`, `FileReader.loadend_event`, `FileReader.progress_event`, `FileReader.error_event`, `FileReader.abort_event`