Finally there is an standard way to meta-program for javascript objects.
Proxy object is more generalized way of overriding or extending the behavior of some fundamental operations on object.

For starter consider a problem if the given property available in object invoke a given callback function.
Simple solution: keep running an infinite loop and keep checking.

I know you hate this solution. And there is a better way to achieve this with proxy.
Let me give a quick solution to this problem.

Define a proxy object

  let user = {};
  user = new Proxy(user, {});

From the code above, its very easy to define the proxy functions. It take two parameters,

• First is the target object whose we want to extend the behavior.

• Second is handler object. - handler object defines the traps. Yes these are called traps.

     let handler = {
       set: function(val){
          // call the callback function.
       }
     }
  

  Let’s see how it can be a reusable function

     const whenAvailable = (target, property, callback) => 
       new Proxy(target, {
         set: (object, property, value) => { 
           // store the value in property of object.
           object[property] = value;
  
           //call the callback
           callback(value);
  
           //return success
           return true;
         } 
     });
  
     // Lets define a new object user object. If the user is set to loggedIn, do some stuff.
     let user = {};
     user = whenAvailable(user, 'isLoggedIn', (isLoggedInStatus) => {
       if(isLoggedInStatus){
         // do some stuff.
       }
     })
  
     // some where else, if user is set to LoggedIn
     user.isLoggedIn = true;
  

  Now you can understand how powerful Proxy is. We have seen the use of set trap. There are many more of these traps.

• handler.get

• handler.set

• handler.has

• handler.apply

• handler.construct

• handler.ownKeys

• handler.deleteProperty

• handler.defineProperty

• handler.isExtensible

• handler.preventExtensions

• handler.getPrototypeOf

• handler.setPrototypeOf

• handler.getOwnPropertyDescriptor

Read about other trap on MDN