ron.savage has asked for the wisdom of the Perl Monks concerning the following question:

Hi Folks The JS in question is: unary = empty[ tagName ] || !!unary; where unary in an incoming function parameter and empty is a hash. What I don't get it the idiom !!unary. I'm thinking it could be null upon entry, so the first ! is causing it to be defined if necessary, and the second ! is reverting it to it's 'proper' value.

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Re: Translating JS into Perl
by kyle (Abbot) on Feb 18, 2009 at 22:37 UTC

    In Perl, the "!!(value)" idiom normalizes "(value)" as a boolean. Many different values are considered true, and a few different values are considered false, but the result of "!!(value)" can be only one of two values.

    So yes, this will make an undef value into a defined false value, but it will also make some random true value into a single predictable true value.

    I don't know for sure that it means the same thing in JavaScript, but I'd be pretty surprised if it doesn't.

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