Form model

To make form handling less UI-dependent, @angular/forms provides a set of primitives for modelling forms: FormControl, FormGroup, and FormArray.

FormControl

FormControl is an indivisible part of the form, an atom. It usually corresponds to a simple UI element, such as an input.

const c = newFormControl('Init Value', Validators.required);

A FormControl has a value, status, and a map of errors:

expect(c.value).toEqual('Init Value');
expect(c.errors).toEqual(null); //null means 'no errors'
expect(c.status).toEqual('VALID');

FormGroup

FormGroup is a fixed-size collection of controls, a record.

const c = new FormGroup({
  login: new FormControl(''),
  password: new FormControl('', Validators.required)
});

A FormGroup is itself a control, and, as such, has the same methods as FormControl.

expect(c.value).toEqual({login: '', password: ''});
expect(c.errors).toEqual(null);
expect(c.status).toEqual('INVALID');

The value of a group is just an aggregation of the values of its children. Any time the...