function openCity(evt, cityName) {
 // Declare all variables
 var i, tabcontent, tablinks;

 // Get all elements with class="tabcontent" and hide them
 tabcontent = document.getElementsByClassName("tabcontent");
 for (i = 0; i < tabcontent.length; i++) {
 tabcontent[i].style.display = "none";
 }

 // Get all elements with class="tablinks" and remove the class "active"
 tablinks = document.getElementsByClassName("tablinks");
 for (i = 0; i < tablinks.length; i++) {
 tablinks[i].className = tablinks[i].className.replace(" active", "");
 }

 // Show the current tab, and add an "active" class to the button that    opened the tab
 document.getElementById(cityName).style.display = "block";
 evt.currentTarget.className += " active";
 }

<!-- Tab links -->
<div class="tab">
<button class="tablinks" onclick="openCity(event, 'London')">London</button>
<button class="tablinks" onclick="openCity(event, 'Paris')">Paris</button>
</div>

<!-- Tab content -->
<div id="London" class="tabcontent">
<h3>London</h3>
<p>London is the capital city of England.</p>
</div>

<div id="Paris" class="tabcontent">
<h3>Paris</h3>
<p>Paris is the capital of France.</p>
</div>

<script type="text/javascript" src="javascript/ajax_openCity.js"> </script>