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.

Way 1: only works for dataURL, not for other types of url.

function dataURLtoFile(dataurl, filename) {
    var arr = dataurl.split(','),
        mime = arr[0].match(/:(.*?);/)[1],
        bstr = atob(arr[arr.length - 1]), 
        n = bstr.length, 
        u8arr = new Uint8Array(n);
    while(n--){
        u8arr[n] = bstr.charCodeAt(n);
    }
    return new File([u8arr], filename, {type:mime});
}

//Usage example:
var file = dataURLtoFile('data:text/plain;base64,aGVsbG8=','hello.txt');
console.log(file);

Way 2: works for any type of url, (http url, dataURL, blobURL, etc...)

// return a promise that resolves with a File instance
function urltoFile(url, filename, mimeType){
    if (url.startsWith('data:')) {
        var arr = url.split(','),
            mime = arr[0].match(/:(.*?);/)[1],
            bstr = atob(arr[arr.length - 1]), 
            n = bstr.length, 
            u8arr = new Uint8Array(n);
        while(n--){
            u8arr[n] = bstr.charCodeAt(n);
        }
        var file = new File([u8arr], filename, {type:mime || mimeType});
        return Promise.resolve(file);
    }
    return fetch(url)
        .then(res => res.arrayBuffer())
        .then(buf => new File([buf], filename,{type:mimeType}));
}

//Usage example:
urltoFile('data:text/plain;base64,aGVsbG8=', 'hello.txt','text/plain')
.then(function(file){ console.log(file);});

Modern ES6 way (async/await)

const toBase64 = file => new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = () => resolve(reader.result);
    reader.onerror = reject;
});

async function Main() {
   const file = document.querySelector('#myfile').files[0];
   console.log(await toBase64(file));
}

Main();

UPD:

If you want to catch errors

async function Main() {
   const file = document.querySelector('#myfile').files[0];
   try {
      const result = await toBase64(file);
      return result
   } catch(error) {
      console.error(error);
      return;
   }
   //...
}