I was being very heavily tongue in cheek. Your knowledge tests that people understand implicit conversions in Javascript to be able to find the equivalent boolean values.

    !!( "  ")
    !!("")
    !!(undefined === null)

You probably want to test for any falsey value though. So the interviewee understanding the ToBoolean spec [0] would be what you actually want to be testing for.

    if (!x) {
        // interviewee should be able to answer for which values of `x` this condition would run
        // this shows they understand which values are falsey and which values are truthy
    }

The knowledge being tested would be "the same" but in a different, and I feel more direct, way. I felt your test was testing their knowledge of implicit conversion and not truthy/falsey values, so I wrote purposefully confusing code that uses implicit conversion.

"If everyone can easily understand what's going on, there's no harm writing code like that!" /s

[0] http://www.ecma-international.org/ecma-262/6.0/index.html#se...