FileReader: readAsDataURL() method

Note: This feature is available in Web Workers

The readAsDataURL method of the FileReader interface 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

BCD tables only load in the browser

See also