Form validation: a `validator()` function

[vlukashov]

• A validator() funciton can be defined for an entire form or for individual form fields.
• By default Vaadin Form runs validator functions on every state change (on every key stroke).
• The validator() function could be used in the same way with any form API or rendering approach.
• validator() functions can be combined in chains to create complex validation rules.

const form = new VaadinForm();
// init the form

const passwordRepeatedCorrectly = values => {
  if (values.password !== values.passwordRepeated) {
    return `Please check that you've repeated the password correctly.`;
  }
};

form.validator = [passwordRepeatedCorrectly];