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

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<head>
  <meta charset="utf-8">
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  <title>JS Bin</title>
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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>