How do I get the the value of an ASP.NET hiddenfield in a typescript file

Here is the control on the aspx page:

<asp:HiddenField ID="hdnGOfromLocalStorage" runat="server" Value="-1" />

Here is the code that is not working in Typescript:

let xxx: string = document.getElementById('<%= hdnGOfromLocalStorage.ClientID %>');

Your expression and code does not really make sense, since you attempting to assign the value to itself???

Your question is one of you attempting to get a value from the hidden field, not set a value.

Hence the following code should work:

 <asp:HiddenField ID="hdnGOfromLocalStorage" runat="server" Value="-1" />

 <asp:Button ID="cmdTest" runat="server" Text="Show Hidden value"
            OnClientClick="mytest();return false;" />

        <script>

            function mytest() {

                var MyValue 
                MyValue = document.getElementById('<%= hdnGOfromLocalStorage.ClientID %>').value

                alert("Value of hidden field = " + MyValue)

            }

        </script>

And when we run the above, we see this:

So, your expression does not retrieve any value, and looks to be an attempt to "assign" a value to the hidden field, which of course is a different question.

To assign a value to the hidden field, the following code would work:

document.getElementById('<%= hdnGOfromLocalStorage.ClientID %>').value = "Hello world"