FileReader.readAsDataURL()

The readAsDataURL method is used to read the contents of the specified Blob or File. When the read operation is finished, the readyState becomes DONE, and the loadend is triggered. At that time, the result attribute contains  the data as a URL representing the file's data as a base64 encoded string.

Syntax

instanceOfFileReader.readAsDataURL(blob);

Parameters

blob
The Blob or File from which to read.

Example

HTML

<input type="file" onchange="previewFile()"><br>
<img src="" height="200" alt="Image preview...">

JavaScript

function previewFile() {
  var preview = document.querySelector('img');
  var file    = document.querySelector('input[type=file]').files[0];
  var reader  = new FileReader();

  reader.onloadend = function () {
    preview.src = reader.result;
  }

  if (file) {
    reader.readAsDataURL(file);
  } else {
    preview.src = "";
  }
}

Live Result

Note: The FileReader() constructor was not supported by Internet Explorer for versions before 10. For a full compatibility code you can see our crossbrowser possible solution for image preview. See also this more powerful example.

Specifications

Specification Status Comment
File API
The definition of 'FileReader' in that specification.
Working Draft Initial definition.

Browser compatibility

Feature Firefox (Gecko) Chrome Internet Explorer* Opera* Safari
Basic support 3.6 (1.9.2) 7 10 12.02 6.0.2
Feature Firefox Mobile (Gecko) Android IE Mobile Opera Mobile Safari Mobile
Basic support 32 3 10 11.5 6.1

Implementation notes

Gecko-specific notes

  • Prior to Gecko 2.0 beta 7 (Firefox 4.0 beta 7), all Blob parameters below were File parameters; this has since been updated to match the specification correctly.
  • Prior to Gecko 13.0 (Firefox 13.0 / Thunderbird 13.0 / SeaMonkey 2.10) the FileReader.error property returned a FileError object. This interface has been removed and FileReader.error is now returning the DOMError object as defined in the latest FileAPI draft.

See also

Document Tags and Contributors

Last updated by: teoli,