js what does ?? means code example

Example: javascript % meaning

// % is the modulo operator
// It returns the rest of a division
// Example uses:

27 % 5
// returns 2, as 27/2 is 5 with rest 2.

99 % 13
// returns 8, as 99/13 is 7 with rest 8.

420 % 0
// returns NaN, as division by 0 is not calculatable.