```js
const form = new Form({
  children: [
    {
      name: 'username',
      validate: validation(
        validation.required(),
        validation.async((value) =>
          mockServerCall(2000, () => value === 'john' ? 'username is taken' : null)))
    },
    {
      name: 'password',
      validate: validation(
        validation.required(),
        validation.async((value) =>
          mockServerCall(1000, value => value && value.length < 6 ? 'invalid password' : null)))
    }
  ]
});

const username = form.get('username');
const password = form.get('password');

username.onChange('john'); // initiates call 1 to the server

console.log(username.hasErrors); // false
console.log(username.errors); // null
console.log(password.hasErrors); // false
console.log(password.errors); // null
console.log(form.hasErrors); // false
console.log(form.errors); // null

password.onChange('123'); // initiates call 2 to the server

console.log(username.hasErrors); // false
console.log(username.errors); // null
console.log(password.hasErrors); // false
console.log(password.errors); // null
console.log(form.hasErrors); // false
console.log(form.errors); // null

// ... waiting asynchronously
// a bit later, before responses 1 and 2 come from the server

console.log(username.hasErrors); // false
console.log(username.errors); // null
console.log(password.hasErrors); // false
console.log(password.errors); // null
console.log(form.hasErrors); // false
console.log(form.errors); // null

// ... waiting asynchronously
// later after reponse 2 comes from the server

console.log(username.hasErrors); // false
console.log(username.errors); // null
console.log(password.hasErrors); // true
console.log(password.errors); // ['invalid password']
console.log(form.hasErrors); // true
console.log(form.errors); // ['invalid password']

// ... waiting asynchronously
// later after reponse 1 comes from the server

console.log(username.hasErrors); // true
console.log(username.errors); // ['username is taken']
console.log(password.hasErrors); // true
console.log(password.errors); // ['invalid password']
console.log(form.hasErrors); // true
console.log(form.errors); // ['username is taken', 'invalid password']
```