I was wondering if there was a way to write JSP code inside a Javascript if-statement?

No, this isn't possible.

At a high-level, once the request lands at the server:

1. Server runs any JSP and produces a new page. After this step, any JSP things are complete.
2. Server sends that generated page to the client
3. Client (browser) sees the page, maybe runs some Javascript (if present)

Your desired behavior is:

1. Javascript code runs first, checks the if statement condition, then proceeds with the body of the if statement only when "condition" was true.
2. JSP code inside the Javascript if block runs

The actual behavior is:

1. JSP code runs first, no matter what your intended code logic is. All JSP throughout the entire page will run first, always.
2. The output of the JSP code is mixed with the rest of the static HTML+Javascript in the apge.
3. The browser gets involved for the first time, seeing the newly-created page: JSP output + static HTML and Javascript.
4. Browser runs any Javascript in the page.

Your example starts like this as JSP:

<script>
if (false) {
<%
  funStuff.testVoid();
%>  
}
console.log('<%= funStuff.retK() %>');
</script>

The server will evaluate the JSP <% ... %> bits, so for example, assume that:

• funStuff.testVoid() returns "xyz"
• funStuff.retK() returns "K"

then the server will produce this:

<script>
if (false) {
  xyz
}
console.log('K');
</script>

The browser will receive the output as above, having no knowledge that "xyz" or "K" were generated on the fly vs. being pre-written into the page, or that JSP scriptlets were involved in any way.