let elem = document.createElement('div');
elem.classList.toggle('test-class', false);
if (elem.classList.contains('test-class')) {
  let _toggle = DOMTokenList.prototype.toggle;
  DOMTokenList.prototype.toggle = function(token, force) {
    if (arguments.length > 1 && !this.contains(token) === !force) {
      return force;
    } else {
      return _toggle.call(this, token);
    }
  };
}

if (!String.prototype.startsWith) {
  String.prototype.startsWith = function(searchString, position){
    position = position || 0;
    return this.substr(position, searchString.length) === searchString;
  };
}

if (!String.prototype.endsWith) {
  String.prototype.endsWith = function(searchString, position) {
    var subjectString = this.toString();
    if (typeof position !== 'number' || !isFinite(position) || Math.floor(position) !== position || position > subjectString.length) {
      position = subjectString.length;
    }
    position -= searchString.length;
    var lastIndex = subjectString.indexOf(searchString, position);
    return lastIndex !== -1 && lastIndex === position;
  };
}

let elem = document.createElement('div');
elem.classList.toggle('test-class', false);
if (elem.classList.contains('test-class')) {
  let _toggle = DOMTokenList.prototype.toggle;
  DOMTokenList.prototype.toggle = function(token, force) {
    if (arguments.length > 1 && !this.contains(token) === !force) {
      return force;
    } else {
      return _toggle.call(this, token);
    }
  };
}

if (!String.prototype.startsWith) {
  String.prototype.startsWith = function(searchString, position){
    position = position || 0;
    return this.substr(position, searchString.length) === searchString;
  };
}

if (!String.prototype.endsWith) {
  String.prototype.endsWith = function(searchString, position) {
    var subjectString = this.toString();
    if (typeof position !== 'number' || !isFinite(position) || Math.floor(position) !== position || position > subjectString.length) {
      position = subjectString.length;
    }
    position -= searchString.length;
    var lastIndex = subjectString.indexOf(searchString, position);
    return lastIndex !== -1 && lastIndex === position;
  };
}

if (!Array.prototype.find) {
  Array.prototype.find = function(predicate) {
    if (this === null) {
      throw new TypeError('Array.prototype.find called on null or undefined');
    }
    if (typeof predicate !== 'function') {
      throw new TypeError('predicate must be a function');
    }
    var list = Object(this);
    var length = list.length >>> 0;
    var thisArg = arguments[1];
    var value;

    for (var i = 0; i < length; i++) {
      value = list[i];
      if (predicate.call(thisArg, value, i, list)) {
        return value;
      }
    }
    return undefined;
  };
}

add polyfill for Array find