// for FormState
const state = new FormState({
  foo: new FielState(1),
  bar: new FieldState(2)
})
state.onChange({ foo: 3, bar: 4 })
state.value // { foo: 3, bar: 4 }

// for ArrayFormState:
const state = new ArrayFormState([1, 2], v => new FieldState(v))
state.onChange([3, 4])
state.value // [3, 4]

const state = new ArrayFormState(['a', 'b', 'c', 'd'], v => new FieldState(v))

state.remove(from, num) // remove items from specific index
state.remove(1, 2) // [a, b, c, d] => [a, d]

state.insert(from, ...items) // insert items from specific index
state.insert(0, 'e', 'f') // [a, b, c, d] => [e, f, a, b, c, d]

state.append(...items) // insert items at the end of current field list, shorthand for `state.insert(state.$.length, ...items)`
state.append('e', 'f') // [a, b, c, d] => [a, b, c, d, e, f]

state.move(from, to) // move item, with specific from index and target index
state.move(0, 2) // [a, b, c, d] => [b, c, a, d]

// for FormState
state.$.foo = ... // assignment to property

// for ArrayFormState:
state.$[1] = ...  // assignment to index
state.$.push(...) // array methods which causes mutation