bootstrap 4 center div

<div class="d-flex justify-content-start">...</div>
<div class="d-flex justify-content-end">...</div>
<div class="d-flex justify-content-center">...</div>
<div class="d-flex justify-content-between">...</div>
<div class="d-flex justify-content-around">...</div>

<!-- Center columns in a Row -->
<div class="row d-flex justify-content-center text-center">
    <div class="col-4">
        // Add Content
    </div>
    <div class="col-4">
        // Add Content
    </div>
    <div class="col-4">
        // Add Content
    </div>
</div>

<!-- Classic -->
<div class="justify-content-center"></div>
<!-- Small -->
<div class="justify-content-sm-center"></div>
<!-- Medium --> 
<div class="justify-content-md-center"></div>
<!-- Large -->
<div class="justify-content-lg-center"></div>
<!-- XL -->
<div class="justify-content-xl-center"></div>

<div class="row justify-content-center">
  ...inner divs and content...
</div>

<div class="my-auto">
  ...inner divs and content...
</div>

class="text-center"