OnClientClick prevent page submission if validation errors

I've written a JavaScript function which validates form input and highlights the input fields and displays any error messages, which I have tested and all works fine.

I have called this function from the OnClientClick attribute of an asp:Button tag. However, if the JavaScript function determines there to be validation errors, I want to stop the form from submitting. How do I do this?

I am a VB.NET newbie, so any help would be appreciated. Thanks.

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<asp:Button ID="btnSubmit" Text="Save Changes" runat="server" OnClientClick="validateEMI();" />

JavaScript

function validateEMI() {

     // a big bunch of validation checks...

     if (errorOnForm == true) {
         document.getElementById("message").innerHTML = errorMsg;
         return;
     }
}

if (errorOnForm == true) {
     document.getElementById("message").innerHTML = errorMsg;
     return false;
}

return false;

EDIT: and validateEMI should be hooked with onsubmit event

use stopPropagation

function validateEMI(event) {
     event.stopPropagation()
     // a big bunch of validation checks...

     if (errorOnForm == true) {
         document.getElementById("message").innerHTML = errorMsg;
         return;
     }
}