[afandian]

This is valid JS:

    1;
    2;
    alert("returns undefined");
    confirm("return true or false?")
    (function() {return 3})();
    (function() {})();

What's being done with the values of 1, 2, undefined, true/false, 3 and undefined? Nothing if they're not assigned. But as every function returns a value and invocations are therefore an expression, it would be impossible to disallow expressions to be evaluated but not assigned.