FileReader: readAsDataURL() method

{{APIRef("File API")}}{{AvailableInWorkers}}

The readAsDataURL() method of the {{domxref("FileReader")}} interface is used to read the contents of the specified {{domxref("Blob")}} or {{domxref("File")}}. When the read operation is finished, the {{domxref("FileReader.readyState","readyState")}} property becomes DONE, and the {{domxref("FileReader/loadend_event", "loadend")}} event is triggered. At that time, the {{domxref("FileReader.result","result")}} attribute contains the data as a data: URL representing the file's data as a base64 encoded string.

[!NOTE] The blob's {{domxref("FileReader.result","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 {{domxref("Blob")}} or {{domxref("File")}} from which to read.

Return value

None ({{jsxref("undefined")}}).

Examples

Reading a single file

HTML

<input type="file" />

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

{{EmbedLiveSample("Reading a single file", "100%", 240)}}

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

{{EmbedLiveSample("Reading multiple files", "100%", 240)}}

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• {{domxref("FileReader")}}
• {{domxref("URL.createObjectURL_static", "URL.createObjectURL()")}}