Additional Client side validations in ASP.net controls
===

I am trying to hack some additional javascript to an old ASP.net form, for validations.
The form makes use of ASP.net validators like `asp:RangeValidator` and `asp:RequiredFieldValidator`.

This is the javascript generated for the 'Save' button.

    <input type="submit" name="ctl00$cphContent$btnSave" value="Save"
     onclick="javascript:if(!validateFirstDate('ctl00_cphContent_txtDate', 
     'ctl00_cphContent_txtDateRepayment', 'ctl00_cphContent_lblRepaymentMode'))
      return false;
    WebForm_DoPostBackWithOptions(new 
     WebForm_PostBackOptions(&quot;ctl00$cphContent$btnSave&quot;, &quot;&quot;,
    true, &quot;&quot;, &quot;&quot;, false, false))" id="ctl00_cphContent_btnSave" class="ButtonStyle">

As shown, the first part which uses `'validateFirstDate'` is my new function which I have added using `btnSave.Attributes.Add("onclick", "javascript:if(!validateFirstRepaymentDate...`
while `WebForm_PostBackOptions...` is generated by ASP.net for validations.

Problem: I want my additional script to run after the ASP.net script and only if the default validations fail (instead of the first function to be executed, as it is now). I am unable to figure out how to do so.

I would really appreciate any help.

(I usually work in Java, but I have been asked to fix this old project which is in ASP.net)