state-proxy.js

const initialState = {
  debug: false,
  item: null,
};

function makeProxyContext(currentState, updateState) {
  let cur = Object.assign({}, currentState);
  let proxyState = new Proxy(updateState, {
    get(_, prop, receiver) {
      return cur[prop];
    },
    set(target, prop, value) {
      console.log('updating updateState', prop, value);
      target[prop] = value;
    },
  });

  return proxyState;
}

function action() {
  let previousItem = this.item;

  if ( !previousItem ) {
    let nextItem = 'hello';
    this.item = nextItem;
  }
}

let previousState = Object.assign({}, initialState);
let updateState = Object.create(null);
let updateContext = makeProxyContext(previousState, updateState);
action.apply(updateContext);

console.log(updateState);
console.log(previousState);

let nextState = Object.assign({}, previousState, updateState);
console.log(nextState);