var selector=document.querySelectorAll('.selected_icon');
console.log(selector);

<div class="menu-icon-bar">
   <div class="material-icons menu-icon select selected_icon" > format_bold</div>
   <div class="material-icons menu-icon select selected_icon">format_italic</div>
   <div class="material-icons menu-icon select selected_icon">format_italic</div>
   <div class="material-icons menu-icon select selected_icon">format_underlined</div>
   <div class="material-icons menu-icon select selected_icon">content_cut</div>
   <div class="material-icons menu-icon select selected_icon">content_copy</div>
   <div class="material-icons menu-icon select selected_icon">content_paste</div>
   <select class="font_family_selector">
      <option value="Hello" selected>Hello</option>
      <option value="Hello1">Hello2</option>
      <option value="Hello3">Hello3</option>
      <option value=""></option>
   </select>
   <div class="material-icons menu-icon select selected_icon ">format_align_left</div>
   <div class="material-icons menu-icon select selected_icon">format_align_right</div>
   <div class="material-icons menu-icon select selected_icon">format_align_justify</div>
   <div class="material-icons menu-icon select selected_icon">format_color_text</div>
   <div class="material-icons menu-icon select selected_icon">format_color_fill</div>
</div>