Getting selected item id from SharePoint 2013 list view using JavaScript object model

<input type='button' value='Get Item IDs' onclick="clickMethod();"/>

<script language="javascript" type="text/javascript">

function clickMethod() 

{

   var ctx = SP.ClientContext.get_current();

   var items = SP.ListOperation.Selection.getSelectedItems(ctx);

   var mySeletedItems = '';

   var i;

       for (i in items)

       {

            mySeletedItems += '|' + items[i].id;

       }

         alert (mySeletedItems);

}

</script>

Now Save the page and select few items from the list and then click on the button, it will display you id's of the selected items in a dialog box as shown in the fig below: