W3cubDocs

/Web APIs

FileReader: readAsDataURL() method

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

js

readAsDataURL(blob)

Parameters

blob

The Blob or File from which to read.

Return value

None (undefined).

Examples

Reading a single file

HTML

html

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

JavaScript

js

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

  reader.addEventListener(
    "load",
    () => {
      // convert image file to base64 string
      preview.src = reader.result;
    },
    false,
  );

  if (file) {
    reader.readAsDataURL(file);
  }
}

Result

Reading multiple files

HTML

html

<input id="browse" type="file" multiple />
<div id="preview"></div>

JavaScript

js

function previewFiles() {
  const preview = document.querySelector("#preview");
  const 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)) {
      const reader = new FileReader();

      reader.addEventListener(
        "load",
        () => {
          const image = new Image();
          image.height = 100;
          image.title = file.name;
          image.src = reader.result;
          preview.appendChild(image);
        },
        false,
      );

      reader.readAsDataURL(file);
    }
  }

  if (files) {
    Array.prototype.forEach.call(files, readAndPreview);
  }
}

const picker = document.querySelector("#browse");
picker.addEventListener("change", previewFiles);

Result

Specifications

Specification
File API
# readAsDataURL

Browser compatibility

Desktop Mobile
Chrome Edge Firefox Internet Explorer Opera Safari WebView Android Chrome Android Firefox for Android Opera Android Safari on IOS Samsung Internet
readAsDataURL 6 12 3.6 10 11 6 3 18
32Using the camera in Android 8.x raises an exception. See bug 1511083.
11 6 1.0

See also

© 2005–2023 MDN contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/API/FileReader/readAsDataURL