FileReader: readAsDataURL() method - Web APIs | MDN (original) (raw)

Baseline

Widely available

Note: This feature is available in Web Workers.

The readAsDataURL() method of the FileReader interface is used to read the contents of the specifiedBlob or File. When the read operation is finished, thereadyState property becomes DONE, and theloadend event is triggered. At that time, theresult 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

Parameters

blob

The Blob or File from which to read.

Return value

Examples

Reading a single file

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

  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);
        },
        false,
      );

      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