bem classes modifiers

modslasses.js

(function() {
  'use strict';

  var delimiter = '--';
  
  DOMTokenList.prototype.modify = function(mod) {
    if (false === tag.classList.modified(mod)) {
      this.add([this[0], delimiter, mod].join(''));
    }
  };

  DOMTokenList.prototype.unmodify = function(mod) {
    if (true === tag.classList.modified(mod)) {
      this.remove([this[0], delimiter, mod].join(''));
    }
  };

  DOMTokenList.prototype.modified = function(mod) {
    return 'string' === typeof this[0] && this.contains([this[0], delimiter, mod].join(''));
  };
})();


var tag = document.createElement('div');

tag.classList.add('baseclass');

console.log(tag, tag.classList.modified('mod'));

tag.classList.modify('mod');

console.log(tag, tag.classList.modified('mod'));

tag.classList.modify('foo');

console.log(tag, tag.classList.modified('foo'));

tag.classList.unmodify('mod');

console.log(tag, tag.classList.modified('mod'));

tag.classList.unmodify('foo');

console.log(tag, tag.classList.modified('foo'));