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

readAsDataURL(blob)

Parameters

blob

The Blob or File from which to read.

Return value

None (undefined).

Examples

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);
  }
}

Live Result

Example reading multiple files

HTML

<input id="browse" type="file" onchange="previewFiles()" 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 = this.result;
          preview.appendChild(image);
        },
        false
      );

      reader.readAsDataURL(file);
    }
  }

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

Specifications

Specification
File API
# readAsDataURL

Browser compatibility

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See also