<form runat="server">
<asp:HiddenField ID="HiddenField1" runat="server" />
</form>
<script>
var hiddenFieldId = '<%= HiddenField1.ClientID%>';
</script>
<script src="test.js"></script>
test.js
if (typeof hiddenFieldId !== 'undefined') {
var el = document.getElementById(hiddenFieldId);
alert(el.value);
}
There is probably a better solution depending on what your javascript is doing though. If you're writing js components that attach to elements it's generally better to read the required ids from data attributes, or to target items in your js via attributes rather than IDs
<form runat="server">
<input type="hidden" id="HiddenField1" data-targetid runat="server" />
</form>
<script src="test.js"></script>
test.js
var el = document.querySelector("[data-targetid]");
alert(el.value);