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                let data = { price: 5, quantity: 2 };
let target = null;

// Our simple Dep class
class Dep {
  constructor() {
    this.subscribers = [];
  depend() {
    if (target && !this.subscribers.includes(target)) {
      // Only if there is a target & it's not already subscribed
  notify() {
    this.subscribers.forEach(sub => sub());

// Go through each of our data properties
Object.keys(data).forEach(key => {
  let internalValue = data[key];

  // Each property gets a dependency instance
  const dep = new Dep();

  Object.defineProperty(data, key, {
    get() {
      dep.depend(); // <-- Remember the target we're running
      return internalValue;
    set(newVal) {
      internalValue = newVal;
      dep.notify(); // <-- Re-run stored functions

// The code to watch to listen for reactive properties
function watcher(myFunc) {
  target = myFunc;
  target = null;

watcher(() => { = data.price * data.quantity;

console.log("total = " +
data.price = 20
console.log("total = " +
data.quantity = 10
console.log("total = " +