HTML

<img id="imageid" src="https://www.google.de/images/srpr/logo11w.png">

JavaScript

function getBase64Image(img) {
  var canvas = document.createElement("canvas");
  canvas.width = img.width;
  canvas.height = img.height;
  var ctx = canvas.getContext("2d");
  ctx.drawImage(img, 0, 0);
  var dataURL = canvas.toDataURL("image/png");
  return dataURL.replace(/^data:image\/?[A-z]*;base64,/);
}

var base64 = getBase64Image(document.getElementById("imageid"));

Special thanks to @Md. Hasan Mahmud for providing an improved regex that works with any image mime type in my comments!

This method requires the canvas element, which is perfectly supported.

• The MDN reference of HTMLCanvasElement.toDataURL().
• And the official W3C documentation.

There are multiple approaches you can choose from:

1. Approach: FileReader

Load the image as blob via XMLHttpRequest and use the FileReader API (readAsDataURL()) to convert it to a dataURL:

function toDataURL(url, callback) {
  var xhr = new XMLHttpRequest();
  xhr.onload = function() {
    var reader = new FileReader();
    reader.onloadend = function() {
      callback(reader.result);
    }
    reader.readAsDataURL(xhr.response);
  };
  xhr.open('GET', url);
  xhr.responseType = 'blob';
  xhr.send();
}

toDataURL('https://www.gravatar.com/avatar/d50c83cc0c6523b4d3f6085295c953e0', function(dataUrl) {
  console.log('RESULT:', dataUrl)
})

This code example could also be implemented using the WHATWG fetch API:

const toDataURL = url => fetch(url)
  .then(response => response.blob())
  .then(blob => new Promise((resolve, reject) => {
    const reader = new FileReader()
    reader.onloadend = () => resolve(reader.result)
    reader.onerror = reject
    reader.readAsDataURL(blob)
  }))


toDataURL('https://www.gravatar.com/avatar/d50c83cc0c6523b4d3f6085295c953e0')
  .then(dataUrl => {
    console.log('RESULT:', dataUrl)
  })

These approaches:

• have better compression
• work for other file types as well

Browser Support:

• http://caniuse.com/#feat=filereader
• http://caniuse.com/#feat=fetch

Tip: To convert local files, you can use live-server. Once started on the folder that contains the picture to transform, open the url in browser and using developer console you can convert the image to base 64.

2. Approach: Canvas (for legacy browsers)

Load the image into an Image-Object, paint it to a nontainted canvas and convert the canvas back to a dataURL.

function toDataURL(src, callback, outputFormat) {
  var img = new Image();
  img.crossOrigin = 'Anonymous';
  img.onload = function() {
    var canvas = document.createElement('CANVAS');
    var ctx = canvas.getContext('2d');
    var dataURL;
    canvas.height = this.naturalHeight;
    canvas.width = this.naturalWidth;
    ctx.drawImage(this, 0, 0);
    dataURL = canvas.toDataURL(outputFormat);
    callback(dataURL);
  };
  img.src = src;
  if (img.complete || img.complete === undefined) {
    img.src = "data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///ywAAAAAAQABAAACAUwAOw==";
    img.src = src;
  }
}

toDataURL(
  'https://www.gravatar.com/avatar/d50c83cc0c6523b4d3f6085295c953e0',
  function(dataUrl) {
    console.log('RESULT:', dataUrl)
  }
)

In detail

Supported input formats:

image/png, image/jpeg, image/jpg, image/gif, image/bmp, image/tiff, image/x-icon, image/svg+xml, image/webp, image/xxx

Supported output formats:

image/png, image/jpeg, image/webp(chrome)

Browser Support:

• http://caniuse.com/#feat=canvas

• Internet Explorer 10 (Internet Explorer 10 just works with same origin images)

3. Approach: Images from the local file system

If you want to convert images from the users file system you need to take a different approach. Use the FileReader API:

function encodeImageFileAsURL(element) {
  var file = element.files[0];
  var reader = new FileReader();
  reader.onloadend = function() {
    console.log('RESULT', reader.result)
  }
  reader.readAsDataURL(file);
}

<input type="file" onchange="encodeImageFileAsURL(this)" />