Understanding QUnit exception testing

Question

While writing QUnit tests I was suprised by the behaviour of 'throws'. Regarding the following code (http://jsfiddle.net/DuYAc/75/), could anyone please answer my questions:

    function subfunc() {
        throw "subfunc error";
    }

    function func() {
        try {
            subfunc();
        } catch (e) {}
    }

    test("official cookbook example", function () {
        throws(function () {
            throw "error";
        }, "Must throw error to pass.");
    });

    test("Would expect this to work", function () {
        throws(subfunc(), "Must throw error to pass.");
    });

    test("Why do I need this encapsulation?", function () {
        throws(function(){subfunc()}, "Must throw error to pass.");
    });

    test("Would expect this to fail, because func does not throw any exception.", function () {
        throws(func(), "Must throw error to pass.");
    });

Only the second test fails, although this would have been my natural choice of writing this test...

Questions:

1) Why do I have to use an inline function to surround my tested function?

2) Why does the last test not fail? 'func' does not throw any exception.

Would be grateful to read any explanation.

Autolithography

Answer 1

1) Why do I have to use an inline function to surround my tested function?

You don't. When you write throws(subfunc(), [...]), subfunc() is evaluated first. As subfunc() throws outside the throws function, the test fails immediately. In order to fix it, you have to pass throws a function. function(){subfunc()} works, but so does subfunc:

test("This works", function () {
    throws(subfunc, "Must throw error to pass.");
});

2) Why does the last test not fail? 'func' does not throw any exception.

For the same reason. func() is evaluated first. As there is no explicit return statement, it returns undefined. Then, throws tries to call undefined. As undefined is not callable, an exception is thrown and the test passes.

test("This doesn't work", function () {
    throws(func, "Must throw error to pass.");
});

Farquhar

Great explanation. Do not call the function but pass it to throws! Thanks a lot. I learnt some JS today. ;) – Autolithography