Encode images to base64 using FileReader

encodeImage.js

var encodeImage = (function() {
  var DEFAULT_IMAGE = 'data:image/png;base64,',
    prefix = "_",
    cache = {};

  function encode(url, callback) {
    var key = prefix + url, value = cache[key];
    if (value) {
      callback(value);
    } else {
      var xhr = new XMLHttpRequest();
      xhr.onreadystatechange = function() {
        if (this.readyState == 4 && this.status == 200) {
          if (this.response.type.indexOf("image/") === 0) {
            var reader = new FileReader();
            reader.onload = function (e) {
              callback(e.target.result);
            };
            reader.readAsDataURL(this.response);
          } else {
            callback(DEFAULT_IMAGE);
          }
        }
      };
      xhr.onerror = function() {
        callback(DEFAULT_IMAGE);
      }
      xhr.open('GET', url);
      xhr.responseType = 'blob';
      xhr.send();
    }
  }

  return function(url, callback) {
    url = typeof url === 'string' ? url.trim() : "";
    if (url.length > 1) {
      if (url.indexOf('data') === 0) {
        callback(url);
      } else {
        encode(url, callback);
      }
    } else {
      callback(DEFAULT_IMAGE);
    }
  }
}());