// 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 ${this.fullName} with the following arguments: ${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.