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javascripttruthiness

Why does this semi-colon cause a incorrect falsey result

Strange truth test results

filter = /rob/gi
>> /rob/gi
filter.test('hey')
>> false
filter.test('rob')
>> true
true && filter.test('rob');
>> false
true && filter.test('rob') ;
>> true
(true && filter.test('rob'));
>> false
(true && filter.test('rob')) ;
>> true

Reproducible in Firefox and Chrome

Solution

  • That's because .test behaves as .exec() and maintains state (position) between calls

    As with exec() (or in combination with it), test() called multiple times on the same global regular expression instance will advance past the previous match.

    So for the 'rob' input it matches it. Then on the second call it tries to match whatever left after the first match: it's an empty string, so it fails and rewinds.

    To see it in action try to match 'robrobrob' - there will be 3 true followed by false.

    References:

    UPD:

    • In this particular case it happens because you use the g modified (credits to Barmar)