Which event runs first, javascript or Asp.net?

Question

I have a javascript method on a button, which is triggered with the click of it, but this same button triggers an Asp.net event. The js event executes a method that processes a value and this value I play within a Hidden (Asp). The Asp event uses this value (from Hidden) for another process. My problem is that it seems that the Asp event is processing before JS so when Asp tries to get the value generated by JS (which should be in Hidden), the value is empty.

Any idea how to run js first in this case? or any other idea?

document
      .getElementById("ContentPlaceHolder1_cmdAvancarEnder‌​eco")
      .addEventListen‌​er("click", getHashSender); 

---

Private Sub cmdAvancarEndereco_Click(sender As Object, e As EventArgs)
                                      Handles cmdAvancarEndereco.Click

---

<asp:button ID="cmdAvancarEndereco" runat="server" 
                  cssclass="btn btn-success btn-lg" text="Avançar >">
</asp:button>

Answer 1

The solution found was in the event onClickof the button inform a return false; thus barring the event of codebehind, and only when my Hidden is filled, is the event forced submit form to process the data (in case I could force the event click, but the solution has changed).

Answer 2

You can take control of the .cs and do it just for .js.

Transforms the method Private Sub cmdAvancarEndereco_Click(sender As Object, e As EventArgs) in a normal method Public Sub cmdAvancarEndereco_Click({ parametros_necessarios }). Hence the code asp will be called by your javascript after being sued by him.

document.getElementById("ContentPlaceHolder1_cmdAvancarEnder‌​eco").addEventListen‌​er("click", function(evt) {
   callPostBack(null, this, [urlcmdAvancarEndereco_Click], [parametros]);
}); 

This ensures that before arriving at the code backend will go through .js.