mixin BEM on pugJS

bem.pug

-
  function htmlEscape(str) {
    return str
      .replace(/&/g, '&')
      .replace(/"/g, '"')
      .replace(/'/g, ''')
      .replace(/</g, '&lt;')
      .replace(/>/g, '&gt;')
      .replace(/\//g, '&#x2F;');
  };

  /**
   * @param {string} className
   * @param {string|array} modifiers
   * @return {array}
   */
  function BEM(className, modifiers) {
    var classes = [className];

    if (modifiers !== undefined) {
      if (typeof modifiers === 'string') {
        modifiers = [modifiers];
      }

      classes = classes.concat(
        modifiers
          .filter(item => item !== '' && item !== undefined)
          .map(item => classes[0] + '--' + item)
      );
    }

    return classes;
  }

mixin BEM(classname, modifiers)
  div(class=BEM(classname, modifiers))&attributes(attributes)
    block

// @use
+BEM('btn', 'primary') Enviar
// @output
// <div class="btn btn--primary">Enviar</div>