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Język JS ma tzw. słabe typowanie tzn. typ zmiennej może się zmieniać w trakcie działania skryptu.
Aktualny typ zmiennej można sprawdzić za pomocą instrukcji - typeof.
Typ zmiennej o nazwie zmienna, zmienia się ;-). Najpierw jest to number, potem string.
let zmienna = 12;
zmienna = 'Język Java Script';