This doesn't affect the variable outside the function. Primitive values are passed by copy.

Good but this overwrites the original principal. Better save it initially to a temp variable.

A true software engineer.

100 actually

How else would you calculate a percentage?

This is the type of solution that requires you to know math. How the hell am I suppose to know 1000 * 0.05 would give me 5% of 1000? Yeah, only when I know math.

Sneaky way to ensure the response is a boolean, instead of null.

why these !! ?

if the tax is 12% for example (t = 0.12), then what remains is 88% of your earnings. 1-t = 0.88. This is the number by which you should multiply your earnings.

what does (1 - tax) mean in this solution ?
im confused
how does it work ?
1 is used in many solutions. what does it express ? explain to me as if I were a child please

This makes so much sense now. Thank you!

Those are local variables. Mutating them will not have an outside effect. ( Because they are primitive values. You would be right if they were objects. )

The solution being broken would not be the biggest problem in either of those cases.

Mutating the arguments is not Best Practises. So better to have an additional variable.