JS Bin example1 // source https://jsbin.com/kuhukal

<!DOCTYPE html>
<html>
<head>
<meta name="description" content="example1">
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>JS Bin</title>
</head>
<body>
  <h3>Check your console to see the output, press F12 and go to the console tab.</h3>
  
  You can also <a href="https://jsbin.com/kuhukal/edit">view and edit the code</a> if you want.
<script id="jsbin-javascript">
// Note: ES6 code, works in modern browsers

const makeAjaxRequest = () => { /* production code */ };

// say we have a User class somewhere
class User {
  trackAction(action, category) {
    makeAjaxRequest({ action, category, isLoggedIn: this.loggedIn });  
  }
}

const loggedInUser = new User();

// elsewhere in the code..
loggedInUser.fullName = 'Bob Jenkins';
loggedInUser.id = 184643;
loggedInUser.trackAction('add-product', 'shoes');

const originalFn = loggedInUser.trackAction;

// print the arguments of the function to the console
// yet still call original function with right context and arguments
loggedInUser.trackAction = function() {
  // convert function arguments to array
  const args = Array.prototype.slice.call(arguments);
  
  console.log('trackAction called for %s with the following arguments: %s.', this.fullName, args.join(', '));
  
  // being a good citizen means returning the result of the original function
  // and making sure it is executed with the correct context and arguments
  return originalFn.apply(this, args);
};

loggedInUser.trackAction('add-product', 'shoes');
// trackAction called for Bob Jenkins with the following arguments: add-product, shoes.
</script>

const makeAjaxRequest = () => { /* production code */ };

// say we have a User class somewhere
class User {
  trackAction(action, category) {
    makeAjaxRequest({ action, category, isLoggedIn: this.loggedIn });  
  }
}

const loggedInUser = new User();

// elsewhere in the code..
loggedInUser.fullName = 'Bob Jenkins';
loggedInUser.id = 184643;
loggedInUser.trackAction('add-product', 'shoes');

const originalFn = loggedInUser.trackAction;

loggedInUser.trackAction('add-product', 'shoes');
// trackAction called for Bob Jenkins with the following arguments: add-product, shoes.</script></body>
</html>