I was just trawling through the QUnit source code (1.12.0) and came across a line that confused me. I've done a bit of googling and haven't been able to come up with a reason for it.
Source: http://code.jquery.com/qunit/qunit-1.12.0.js line 520
result = !!result;
A similar thing appears further on in the code, except instead of storing the result in itself, it's storing the double negated variable in JSON.
Source: http://code.jquery.com/qunit/qunit-1.12.0.js line 957
result: !!result
As ! negates, I assume !! will negate then negate again, thus ending up with exactly what you started with. In which case, what is achieved by setting a variable equal to itself, negated twice? (Or in the latter example, returning itself negated twice instead of just returning itself.)