expect.fail(...)

Explicitly forces failure.

expect.fail()
Explicit failure
expect.fail('Custom failure message')
Custom failure message
expect.fail('{0} was expected to be {1}', 0, 'zero');
0 was expected to be 'zero'

I case you want to rethrow an error, you should always use expect.fail, as it ensures that the error message will be correct for the different error modes.

var error = new Error('throw me');
expect.fail(new Error(error));
Error: throw me

When you want to build a completely custom output, you can call expect.fail with a callback and receive a magicpen instance that the output can be written to.

expect.fail(function (output) {
    'You have been a very bad boy!'.split(/ /).forEach(function (word, index) {
        if (index > 0) { output.sp(); }
        var style = index % 2 === 0 ? 'jsPrimitive' : 'jsString';
        output[style](word);
    });
});
You have been a very bad boy!