if (checkPassword() == true || checkUserName() == true)
    event.preventDefault();

if ([checkPassword(), checkUserName()].some(Boolean)) {
    // ...
}

// checkPassword and checkUserName have side effects, both need to be run
const invalidPassword = checkPassword()
const invalidUserName = checkUserName()

if (invalidPassword || invalidUserName) {
  ...
}

0 0 | 0
0 1 | 1
1 0 | 1
1 1 | 1

0 0 | 0
0 1 | 0
1 0 | 0
1 1 | 1