FileReader.readAsDataURL() - Web APIs 编辑

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 data: URL representing the file's data as a base64 encoded string.

Note: The blob's result cannot be directly decoded as Base64 without first removing the Data-URL declaration preceding the Base64-encoded data. To retrieve only the Base64 encoded string, first remove data:*/*;base64, from the result.

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

  reader.addEventListener("load", function () {
    // convert image file to base64 string
    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);
  }

}

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 'readAsDataURL()' in that specification.	Working Draft	Initial definition

Browser compatibility

BCD tables only load in the browser

See also

• FileReader
• URL.createObjectURL()