The prefix increment and decrement operators allow you to increment or decrement any given value and will evaluate to the newly incremented value:

let n = 0;

++n; // => 1 (the newly incremented value)
n;   // => 1 (the newly incremented value)

--n; // => 0 (the newly decremented value)
n;   // => 0 (the newly decremented value)

++n would technically be equivalent to the following additive assignment:

n += Number(n);

Note how the current value of n is first converted into Number. This is the nature of both the increment and decrement operators: they operate strictly on numbers. So, if n were String, that could not be coerced successfully, then the new incremented or decremented value of n would be NaN:

let n = 'foo';
++n; // => NaN
n;   // => NaN

Here, we can observe how, since the coercion of foo to a Number fails, the attempted incrementation of it also fails, returning NaN.