```js
const form = new Form({
  children: [
    {
      name: 'username',
      validate: validation(
        validation.required(),
        validation.async((value) =>
          mockServerCall(1000, () => 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('test'); // 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

username.onChange(''); // no call to the server

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

// ... waiting asynchronously
// a bit later, before response 1 comes from the server (which is ignored anyway)

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

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

// ... waiting asynchronously
// a bit later, before response 2 comes from the server

console.log(username.hasErrors); // true
console.log(username.errors); // ['Required']
console.log(password.hasErrors); // false
console.log(password.errors); // null
console.log(form.hasErrors); // false, because it waits for the async validated child
console.log(form.errors); // null

username.onChange('andrew'); // initiates call 3 to the server
username.onChange('john'); // initiates call 4 to the server

// ... waiting asynchronously
// a bit later, before responses 2, 3 and 4 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 3 comes from the server, which is ignored

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 4 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']
```