function _arrayBufferToBase64( buffer ) {

    var binary = '';

    var bytes = new Uint8Array( buffer );

    var len = bytes.byteLength;

    for (var i = 0; i < len; i++) {

        binary += String.fromCharCode( bytes[ i ] );

    }

    return window.btoa( binary );

}

​

​

var xhr = new XMLHttpRequest();

xhr.open('GET', 'http://jsbin.com/images/favicon.png', true);

xhr.responseType = 'arraybuffer';

​

xhr.onload = function(e) {

        if (this.status == 200) {

          var string_with_bas64_image = _arrayBufferToBase64(this.response);

          document.getElementById("img").src = "data:image/png;base64," + string_with_bas64_image;

        }

};

​

xhr.send();

​

​