Get the selected item's ID in 2 steps on AllItems.aspx or any view page.

Step 1 - Include SP Services in your javascript file.

<script type="text/javascript" src="../../SiteAssets/jquery.SPServices-2014.02.min.js"></script>

Step 2 -

        var clientContext = SP.ClientContext.get_current();
        var website = clientContext.get_web();
        clientContext.load(website);
        var items = SP.ListOperation.Selection.getSelectedItems(clientContext);
        var k;
        var idList = new Array();
        for (k in items) {
        var  thisID = items[k].id;
          }

Here , "thisID" will give you the selected item's ID on the View.