Object-oriented programming

Objects, abstract types, sharing, callbacks

Srini Devadas and Daniel Jackson

Part 1: Review

  • Object literals: for packaging values
  • Slots can hold any value, including functions
  • Closures: functions on out-of-scope variables


  • i18n stands for internationalization
  • "Factor out" choice of language
  • Here's a tiny example using an object literal
  • Can you modify it to speak French instead?

Answer: i18n

  • Just switch the object assigned to g
  • We're swapping one "module" for another

Function Slots

  • Slots can hold any kind of value, including functions

Slot machine

  • Here's a function that returns an object
  • On object-oriented settings, called a constructor
  • Can you change it to greet you by name instead?

Answer: Slot machine

  • Just change the argument to Greeter

Counting closures

  • Our old friend the counter, with an initial value
  • Note update to i, which is out of scope when Counter returns
  • Can you make it count backwards from 10?

Answer: Counting closures

  • Here's the answer: subtract one in the closure instead
  • Start at 11, since it subtracts before printing

Part 2: Objects and Methods

  • Methods: slots holding functions
  • Accessing an object via methods alone
  • Encapsulation of state

Up and down

  • An object with slots holding functions
  • These functions are called methods


  • Same idea, this time for a bank account
  • Try and withdraw too much money and see what happens

Answer: Banking

  • If you withdraw too much, prints "no" and doesn't reduce balance

Attacking the Bank

  • Can a robber reduce the balance without a withdrawal?
  • No way to set bal from outside; it's encapsulated

An Aside: Secure Banking

  • Can't modify balance, but can break the bank anyway
  • To fix this, can call Object.freeze(a): try it!

More: Secure Banking

  • Here's the same code, with freeze
  • In practice, call freeze inside the constructor

Part 3: Sharing and Aliasing

  • When two variables point to one object
  • Idea is often confusing to novices
  • Unexpected sharing is often confusing to experts

Bank Accounts

  • How many account objects are there?
  • What happens to a2 when you deposit in a1?

Answer: Bank Accounts

  • How many account objects are there? Two!
  • What happens to a2 when you deposit in a1? Nothing!

Multiple Variables

  • How many account objects are there?
  • What happens to a2 when you deposit in a1?

Answer: Multiple Variables

  • How many account objects are there? One!
  • What happens to a2 when you deposit in a1? It changes too!


  • Variable a1 and a2 are aliases or aliased
  • The account object is shared between a1and a2
  • Assignment x = e: make x point to the value of e
  • Results in two names for one object

Aliasing, Visually

  • Look at the picture
  • Think of variables as pointers, not slots
  • After the assignment, the situation is symmetrical!

Part 4: Callbacks

  • APIs: using objects through specs
  • Callbacks: functions that an object calls
  • Registration methods

A Traffic light API

  • To use an object, you just need specifications of methods
  • Called an API: application programmers' interface
  • Specifications of traffic light methods:
    • red: switch to red light
    • yellow: switch to yellow light
    • green: switch to green light
    • next: switch to next light in sequence
  • Can you turn the light to green?
  • Can you cycle through the colors on each click of run?

Answer: A Traffic light API

  • Easy: just use the green method, and click run
  • Then replace code by t.next() and click run

Don't call me, I'll call you

  • We've seen that functions can be passed as arguments
  • Callback or listener: function passed as argument, called later
  • Example: account that lets you choose how success is reported

Traffic Light in the Bank

  • Let's hook the account up to the traffic light
  • Recall that t.green if a function that turns t green
  • Can you change the withdrawal amount so that a red light shows?
  • Can you make it show yellow for a failed account operation?

Answer: Traffic Light in the Bank

  • To show yellow for failed account operation, just pass t.yellow instead

On the Button

  • Often want to choose the callback dynamically
  • Common idiom: provide a registration method
  • Our button object has method onclick(f) to register f for clicks
  • Run the code, and click the button to see what happens

Controlling the Lights

  • How does this work?

Answer: Controlling the Lights

  • The call to onclick binds the function t.next to the button
  • Now every time the button is clicked, the traffic light switches to next color

Exercise: Action & Reaction

  • Build a reaction tester
  • Should go green after random delay
  • Then you click button, and it reports your reaction time
  • Hint: now() returns current time in ms

Some Functions

  • Functions for the next exercise

Exercise: Action & Reaction Trace

  • This time, compute average over multiple tries
  • Complete the code inside the function report
  • Hint: use some of map, reduce, zip, filter and pos, plus, max