class A {
  state = {
    x: 0,
    y: 1
  }

  constructor(initialState?: A) {
      if (initialState) {
          this.state = initialState
      }
  }
  setX(x): A {
    return new A({...this.state, x})
  }
  setY(): void {
    // legacy
  }
}

const reducer = (state: A, action) => {
  switch (action.type) {
    case SET_X:
      return state.setX(action.payload)
  }
}