Validating against checkbox and aspnet validation

I'm using asp.net validation controls which work fine. I've since introduced an HTML checkbox which should be ticked before the form can be submitted. My function for this reads as follows:

function terms(form) {
var terms = document.getElementById("chkTerms").checked;
var errorsArray = new Array();

if (terms == false) {
    //alert("Terms not checked (works)");
    errorsArray.push("You must agree to the terms and conditions.");
    if (errorsArray.length) {
        document.getElementById("termsOutput").innerHTML = errorsArray.join("\n") + "<br />";
        return false;
    }
开发者_StackOverflow中文版}
else {
    document.getElementById("termsOutput").innerHTML = "";
}}

It is called as follows:

<asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClientClick="return terms(form)" />

The problem I have is seems I can either use the asp.net validation OR the custom checkbox validation I wrote. After the form posts however, the asp.net validation fires! Anyone know what I'm doing wrong?

The ASP.NET button control uses the WebForm_DoPostBackWithOptions method that subsequently triggers the client-side validation. By Adding return terms(form) to your button client click handler, this statement returns before the postback statement runs (which calls Page_ClientValidate) and so you don't see the validation. Doing an (if !terms(form)) return false) instead will cancel only when false.

HTH.