It's often enough to validate each field in isolation:

• email should contain a @;
• password should be more than four characters.

But every once in a while, you need to validate several fields together:

• password confirmation should match password;
• phone number's length depends on the country code from a separate input.

It's hard or impossible implement that with field-level validations without unnecessarily complicating the code. However, with a simple validation architecture from the other articles, it's pretty trivial.

A function that validates the entire form is simple:

function validate(email, password) {
  // true means invalid, so our conditions got reversed
  return {
    email: email.length === 0,
    password: password.length === 0,
  };
}

I mean, what could be simpler? And yet, it's very powerful. Validating several fields together is easily achievable:

function validate(email, password, passwordConfirm) {
  // true means invalid, so our conditions got reversed
  return {
    email: email.length === 0,
    password: password.length === 0,
    passwordConfirm: password !== passwordConfirm,
  };
}

Try entering different passwords now:

Once again, controlled inputs saved our day 🐶