You cannot evaluate an anonymous function directly, it yields a syntax error but the expected result is true wrapped in a QJSValue. Note the function call directly after the function declaration.

Code to reproduce the error:

QJSEngine engine;
QJSValue result = engine.evaluate("function(){return true}()");
if (result.isError()) {
    qWarning("%s", qPrintable(result.toString()));
}

Output:

SyntaxError: unexpected token

It works fine if I surround the function declaration with parenthesis, but other engines support the other way, though:

(function(){return true})();