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Teksty, liczby, wyrażenia zwracają wartość logiczną
true lub false jeśli są "sprawdzane jako wyrażenia logiczne".
Warto wiedzieć jakie wartości są zwracane.
!! czyli podwójne zaprzeczenie zwraca wartość logiczną danego wyrażenia.
console.log('"" - '+!!"");
console.log('0 - '+!!0);
console.log('null - '+!!null);
console.log('undefined - '+!!undefined);
console.log('abc - '+!!"abc");
console.log('" " - '+!!" ");
console.log('7 - '+!!7);
console.log('-45 - '+!!45);