FileReader: readAsDataURL() method

Available in 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 property becomes DONE, and the loadend event 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

readAsDataURL(blob)

Parameters

• blob
  • : The Blob or File from which to read.

Return value

None (undefined).

Examples

Reading a single file

HTML

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

JavaScript

const preview = document.querySelector("img");
const fileInput = document.querySelector("input[type=file]");

fileInput.addEventListener("change", previewFile);

function previewFile() {
  const file = fileInput.files[0];
  const reader = new FileReader();

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

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

Result

Reading multiple files

HTML

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

JavaScript

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);
      });

      reader.readAsDataURL(file);
    }
  }

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

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

Result

Specifications

Browser compatibility

See also

• FileReader
• URL.createObjectURL()