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return !1 in javascript

In a JavaScript file I had to read today, there was a line where a variable was declared like a factorial, like this :

var myVariable = !1;

and then something similar was used as parameter in a function like this :

return variable.myFunction(!0);

Can anyone explain me what the exclamation mark means in this context and eventually, why this is generally used for (benefits) ?

Thank you in advance !