Why does this semi-colon cause a incorrect falsey result

Question

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

It has nothing to do with the semicolons. Just repeat the same test multiple times and you'll see the same behavior. — Barmar, May 01 '15 at 00:39
OH WOW, YES! I feel like I should change the question title now. — Parris, May 01 '15 at 00:40

score 4 · Accepted Answer · edited May 23 '17 at 11:58

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:

https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/RegExp/test

UPD:

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

You should mention that this is specifically because of the `g` modifier. — Barmar, May 01 '15 at 00:39

Why does this semi-colon cause a incorrect falsey result

1 Answers1