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.addEventListener("load", function () {
    preview.src = reader.result;
  }, false);
  if (file) {
    reader.readAsDataURL(file);
  }
}

Live Result

Example reading multiple files

HTML

<input id="browse" type="file" onchange="previewFiles()" multiple>
<div id="preview"></div>

JavaScript

function previewFiles() {
  var preview = document.querySelector('#preview');
  var files   = document.querySelector('input[type=file]').files;
  function readAndPreview(file) {
    // Make sure `file.name` matches our extensions criteria
    if ( /\.(jpe?g|png|gif)$/i.test(file.name) ) {
      var reader = new FileReader();
      reader.addEventListener("load", function () {
        var image = new Image();
        image.height = 100;
        image.title = file.name;
        image.src = this.result;
        preview.appendChild( image );
      }, false);
      reader.readAsDataURL(file);
    }
  }
  if (files) {
    [].forEach.call(files, readAndPreview);
  }
}

Specifications

Browser compatibility

[1] 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.

[2] IE9 has a File API Lab.

[3] Opera has partial support in 11.1.

See also

• FileReader