function getBase64(file) {
  return new Promise((resolve, reject) => {
    const reader = new FileReader();
    reader.readAsDataURL(file);
    reader.onload = () => resolve(reader.result);
    reader.onerror = error => reject(error);
  });
}

var file = document.querySelector('#files > input[type="file"]').files[0];
getBase64(file).then(
  data => console.log(data)
);

var fileInput = document.getElementById('#myInputID');

var reader = new FileReader();
reader.readAsDataURL(fileInput.files[0]);

reader.onload = function () {
	console.log(reader.result);//base64encoded string
};
reader.onerror = function (error) {
	console.log('Error: ', error);
};