You're going to need a function to do that:

function incrementIndex() {

    index += 1;

}

This assumes that your index variable is global.

Next what you'll need to do is wait for the click event. There are three ways to do that in JavaScript.

1. Inline JavaScript

   
   
   

   <button onclick=incrementIndex()>Button 1</button>
   
   <button onclick=incrementIndex()>Button 2</button>
   
   

   
   
   

   This is how Hitler developed websites. Don't do this.




2. Event Handler

   
   
   

   This requires you to wait until the DOM has loaded, and then get your two buttons somehow. If you only have two buttons on the page, you can do something like this (but only if this script comes after the HTML in your document):

   
   
   

   var buttons = document.getElementsByTagName('button');
   
   buttons[0].onclick = buttons[1].onclick = incrementIndex;
   
   

   
   
   

   Note in that example that there are no parentheses () at the end of incrementIndex.




3. Event Listener

   
   
   

   The problem with event handlers is that you can only attach one function at a time. If you're going to have other JavaScript listening for button clicks, you'll have to use event listeners.

   
   
   

   But event listeners are a major pain in the butt because IE6-8 does it wrong, and (depending on the project) you'll probably have to code around it. At it's simplest, here's how to add an event listener in most browsers:

   
   
   

   var addEvent = window.addEventListener ? function (elem, type, method) {
   
       elem.addEventListener(type, method, false);
   
   } : function (elem, type, method) {
   
       elem.attachEvent('on' + type, method);
   
   };
   
   
   
   addEvent(buttons[0], 'click', incrementIndex);
   
   addEvent(buttons[1], 'click', incrementIndex);
   
   

   
   
   

   That is just the surface of cross-browser event listening. If you want to learn more, QuirksMode has a good series of articles on it.