old_contents: $('<section class="min-chat"> \
  <header class="min-chat__header"> \
      <b class="min-chat__header__title">\
              <img src="http://i.imgur.com/RKJMsl8.png" class="min-chat__icon" >\
              Czatrepajr</b> \
   <button id="min-chat-switch" class="min-chat__header__switch">Wyłącz</button> \
  </header> \
  <div id="min-chat-content"> \
      <div class="min-chat__notify min-chat__notify--left min-chat__notify--hidden"></div> \
      <div class="min-chat__notify min-chat__notify--join min-chat__notify--hidden"></div> \
      <div class="min-chat__content"></div> \
      <div class="min-chat__fieldset"> \
          <input type="text" id="chat-input" class="min-chat__fieldset__input" spellcheck="true"> \
      </div> \
  </div> \
  </section>').appendTo('body');
  
new_contents: $(`<section class="min-chat">
  <header class="min-chat__header">
      <b class="min-chat__header__title">
              <img src="http://i.imgur.com/RKJMsl8.png" class="min-chat__icon" >
              Czatrepajr</b>
   <button id="min-chat-switch" class="min-chat__header__switch" title="Wyłącz">↓</button> 
  </header>
  <div id="min-chat-content">
      <div class="min-chat__notify min-chat__notify--left min-chat__notify--hidden"></div>
      <div class="min-chat__notify min-chat__notify--join min-chat__notify--hidden"></div>
      <div class="min-chat__content"></div>
      <div class="min-chat__fieldset">
          <input type="text" id="chat-input" class="min-chat__fieldset__input" spellcheck="true">
      </div>
  </div>
  </section>`).appendTo('body');
  
subject: Change syntax to template strings.