Masks email with asterisks/bullets in Node.js

mask-email.js

/** Masks email, e.g. [email protected] as j•••e@g•••m **/
function maskEmail(email) {
  
    function mask(segment) {
        const hider = '•••';
        let result = '';
        if (segment) {
            result = segment[0] + hider;
            if (segment.length > 2) {
                result += segment[segment.length - 1];
            }
        }
        return result;
    }
  
    let i = (email || '').lastIndexOf('@');
    if (i < 0) { 
        return mask(email);
    }
    return mask(email.substring(0, i)) + '@' + mask(email.substring(i + 1));
}

// ==== tests ====
function assert(expected, result) {
    if (expected === result) {
        console.log('ok');
    } else {
        console.log(expected + ' - [' + result + ']');
    }
}
assert('', maskEmail(null));
assert('', maskEmail(''));
assert('@', maskEmail('@'));
assert('t•••t', maskEmail('test'));
assert('t•••t@', maskEmail('test@'));
assert('t•••', maskEmail('t'));
assert('@a•••', maskEmail('@a'));
assert('@e•••', maskEmail('@eu'));
assert('@c•••m', maskEmail('@com'));
assert('a•••@l•••l', maskEmail('a@local'));
assert('i•••l@g•••m', maskEmail('invalid@[email protected]'));
assert('t•••t@g•••m', maskEmail('[email protected]'));
assert('j•••e@m•••t', maskEmail('[email protected]'));
assert('j•••e@m•••m', maskEmail('[email protected]'));
assert('"•••"@i•••g', maskEmail('"first\"last"@iana.org'));
assert('"•••"@i•••g', maskEmail('"first@last"@iana.org'));
assert('f•••t@[•••]', maskEmail('first.last@[12.34.56.78]'));
assert('"•••"@i•••g', maskEmail('"first"."middle"."last"@iana.org'));
assert('y•••s@g•••m', maskEmail('[email protected]'));