imagetobase64.js

function imageToBase64(URL) {
    var imagem = new Image();
    imagem.src = URL;
    imagem.onload = function () {
    var canvas = document.createElement("canvas");
    canvas.width = this.width;
    canvas.height = this.height;
    var cvx = canvas.getContext("2d");
    cvx.drawImage(this, 0, 0);
    var dataURL = canvas.toDataURL("image/png");
    console.log(  dataURL.replace(/^data:image\/(png|jpg);base64,/, ""));
    }
}
//ex
imageToBase64('http://jsfiddle.net/img/logo.png')