void is not the same as undefined.
void represents not returning a value. It's the inferred type if no return statements exist in a function. In JavaScript, a function that doesn’t return any value will implicitly return the value undefined.
A function that returns void can still return a value, but the value return should not be observed.
This is why Array.forEach uses void for the callback function. The callback can return any value, but you're promising not to use that value for anything.