asp OnClientClick not executing - asp.net

Question

This button:
<asp:Button ID="btnUpload" runat="server" Height="26px" Text="Upload" Width="86px" OnClick="btnUpload_Click" />
With added attribute in Page_Load:
btnUpload.Attributes.Add("OnClientClick", "javascript:alert('Test');")
Shows in browser Inspector:
<input type="submit" name="ctl00$MainContent$btnUpload" value="Upload" id="btnUpload" class="aspNetDisabled" onclientclick="javascript:alert('Test');" style="height:26px;width:86px;">
but never fires onClientClick. Tried calling function, preceding with 'javascript', many things.... but it never executes what's in onClientClick.
Note: The button's regular OnClick="btnUpload_Click" executes fine.
Any ideas?

Answer 1

This just adds a raw attribute to the resulting HTML:
btnUpload.Attributes.Add("OnClientClick", "javascript:alert('Test');")
And there is no OnClientClick in HTML, so the browser has no reason to make any use of it. That's something the ASP.NET server-side controls use and translate into client-side code. Add it directly to the server-side control instead:
<asp:Button
ID="btnUpload"
runat="server"
Height="26px"
Text="Upload"
Width="86px"
OnClick="btnUpload_Click"
OnClientClick="alert('Test');"
/>
Alternatively (and I don't have a way to test this at the moment), if it's a property on the control then you may be able to add it as such:
btnUpload.OnClientClick = "alert('Test');"
As an aside... If OnClientClick is continuing to cause problems then I'd recommend abandoning it altogether. (I honestly don't know why it even exists, other than as a vestige from a time when ASP.NET was trying to take the web out of web development.) If you just want to attach a click handler in JavaScript, you don't need C#'s permission to do that. Just attach a click handler in JavaScript:
const btn = document.querySelector('#btnUpload');
if (btn) {
btn.addEventListener('click', function () {
alert('Test');
});
}
Adding that to a <script> element at the end of the page would add that function as a click handler to the #btnUpload element.