Originally posted on: http://geekswithblogs.net/Vipin/archive/2013/07/24/retrieve-.net-control-id-in-javascript.aspx

If you need to retrieve a client ID of an asp:net control in a javascript function, then you can use the below function -

function $$(id, context) {

        var el = $("#" + id, context);

        if (el.length < 1)

            el = $("[id$=_" + id + "]", context);

        return el;

    }

var tempDotNetControl = 'aspTextTemporary';

var ClientSideID = $$(aspTextTemporary);

Please bear in mind, this function is useful if you want to retrieve client ID of a different DotNet control based on some condition, otherwise if it’s always static then you can just use <%= aspTextTemporary.ClientID %>"