Object-Oriented Programming is a style of programming (a "paradigm") that uses objects to manage state (data) and behavior in an application. While OOP does let us do some new things, more than anything, it helps us write better, more organized code.
It can be defined by its 4 pillars:
Encapsulation - bundling methods with the data they operate on while hiding / protecting access to the data
Abstraction - creating interfaces that hiding complexity behind functions
Inheritance - sharing behavior between classes
Polymorphism - similar objects can be used interchangeably
Throughout this module, we will be learning about these four pillars and how we implement them in JavaScript using the class syntax.
Encapsulation Helps us Create Interfaces for Managing Data
In functional programming, we separate our functions from the data they operate on. Pure functions promote consistency & predictability.
// Functional Programming separates data from functionality
const friends = ['ahmad', 'brandon', 'carmen'];
const getUpdatedFriendList = (friends, newFriend) => {
// keep it pure, make a new list
const newList = [...friends, newFriend];
return newList;
}
const newFriends = getUpdatedFriendList(friends, 'deema');
console.log(newFriends); // ['ahmad', 'brandon', 'carmen', 'deema'];
// the original array is not modified
console.log(friends); // ['ahmad', 'brandon', 'carmen'];
In object-oriented programming, we group functions with the data that they operate on. This is called encapsulation.
In the example below, friendsManager contains both the list of friends as well as the actions that you can perform with that list.
// Object Oriented Programming encapsulates data with functionality
const friendsManager = {
friends: [],
// a "method" is a function stored inside of an object
addFriend(newFriend) {
this.friends.push(newFriend);
// `this` refers to the "owner" of the method.
// i.e. the object on which the method is invoked
},
printFriends() {
console.log(this.friends)
}
}
// Here, friendsManager invokes addFriend
// so `this` === friendsManager
friendsManager.addFriend('ahmad');
friendsManager.addFriend('brandon');
friendsManager.addFriend('carmen');
friendsManager.printFriends();
Encapsulation helps us create interfaces for managing a piece of data. In this case, friendsManager provides the methods addFriend and printFriends for interacting with the friends array.
While we could add to and print friends directly, using the provided methods is more readable and produces more consistent and predictable outcomes:
friendsManager.friends.push('motun');
console.log(friendsManager.friends);
// versus
friendsManager.addFriend('motun');
friendsManager.printFriends();
Q: How would you add a friendsManager.removeLast() method that removes the last friend that was added?
const friendsManager = {
friends: [],
// a "method" is a function stored inside of an object
addFriend(newFriend) {
this.friends.push(newFriend);
// `this` refers to the "owner" of the method.
},
printFriends() {
console.log(this.friends)
},
removeLast() {
this.friends.pop()
}
}
Hiding Data From Direct Access
Consider the friendsManager example again. Notice that we've added a guard clause to ensure that only strings are added as friends.
const friendsManager = {
friends: [],
addFriend(newFriend) {
if (typeof newFriend !== 'string') {
throw Error('new friends must be strings');
};
this.friends.push(newFriend);
},
printFriends() {
console.log(this.friends)
}
}
friendsManager.addFriend('daniel'); // this gets added
friendsManager.addFriend(true); // this does not
// What about this is NOT consistent or predictable?
friendsManager.friends.push('emmaneul');
friendsManager.friends.push(42);
Q: What about the last two lines in the example are NOT consistent or predictable?
You can modify the friendsManager.friends array either through the addFriend() method or by directly mutating the friends array. When modifying the array directly, there are no safeguards.
The friends array can be directly modified from the outside, which leads to uncontrolled additions like friendsManager.friends.push(42). This results in an array that might have invalid data types, such as numbers and booleans mixed with strings, violating the intended structure of the friends list.
This unpredictability leads to bugs and confusion, as the integrity of the data inside friendsManager cannot be guaranteed. How to Make the Code More Consistent Using Closures:
A core tenet of encapsulation in OOP is to restrict access to an object's internal data, like friendsManager.friends. If we are able to ensure friendsManager.friends is only interacted with through the methods addFriend and printFriends, we can guarantee that our rules for adding new friends are enforced.
Closures Let Us Hide Data
One amazing feature of JavaScript is the ability to create a closure. Closures are created when an "inner function" maintains references to variables in its surrounding scope (an "outer function").
We can leverage closures to control access to the friends array by doing the following:
We declare a function makeFriendsManager. This is our "outer" function
Inside, we declare the friendsManager object and return it.
We declare the friends array as its own variable, outside of friendsManager. It can no longer be accessed directly via the object but can be referenced by the methods addFriend and printFriends
// First, we make a function that returns our friendsManager.
// This is an "outer" function
const makeFriendsManager = () => {
// This variable is in the "outer" function's scope.
// friends is referenced by addFriend and printFriends but is not in the friendsManager itself.
const friends = [];
const friendsManager = {
addFriend(newFriend) {
if (typeof newFriend !== 'string') {
throw Error('new friends must be strings');
};
friends.push(newFriend);
},
printFriends() {
console.log(friends);
}
}
return friendsManager;
}
const bensFriendsManager = makeFriendsManager();
bensFriendsManager.addFriend('zo')
bensFriendsManager.addFriend('motun')
bensFriendsManager.printFriends() // ['zo', 'motun']
console.log(bensFriendsManager.friends) // undefined
console.log(friends); // reference error!
The full effect of the closure is seen when we invoke makeFriendsManager. After we return from makeFriendsManager, we can't reference friends anymore. However, the methods addFriend and printFriends keep their references to friends, even after they've been returned.
Because addFriend and printFriends were declared within the same scope as friends, they can continue using their reference to friends even after leaving that scope.
Always Return Copies of Pass-By-Reference Values
Suppose we wanted to provide a way to access the list of friends, we would add a "getter" method like getFriends:
const makeFriendsManager = () => {
const friends = [];
const friendsManager = {
addFriend(newFriend) {
if (typeof newFriend !== 'string') {
throw Error('new friends must be strings');
};
friends.push(newFriend);
},
printFriends() {
console.log(friends);
}
getFriends() {
// return a copy of the array, not the array itself
return [...friends];
},
}
return friendsManager;
}
const myFriendsManager = makeFriendsManager();
myFriendsManager.addFriend('reuben')
myFriendsManager.addFriend('maya')
myFriendsManager.addFriend('carmen')
const allFriends = myFriendsManager.getFriends()
// A copy is returned. We can modify the copy all we want...
allFriends[0] = 'ben';
console.log(allFriends); // ['ben', 'maya', 'carmen']
// ... but the original is not disturbed. It remains the "source of truth"
console.log(myFriendsManager.getFriends()); // ['reuben', 'maya', 'carmen']
To ensure friends remains hidden, the getFriends method returns a copy of the friends array to avoid sharing the array reference.
Whenever you are dealing with a pass-by-reference data type (arrays and objects), always make sure to return a copy of it!
Every Instance Creates a New Closure
The cool thing about closures is that each time we invoke this function, we will create a new friends array and a new object with methods that reference that specific instance of the friends array.
We've seen closures before in non-object-oriented contexts. Consider the functions below. Which of them creates a closure? How?
const sayWordsLoudly = (words) => {
words.forEach((word) => console.log(`${word}!!!`));
}
const addRandomToNumbers = (nums) => {
const randomNum = Math.random();
return nums.map((num) => num + randomNum);
}
const addAmountToNumbers = (nums, amount) => {
return nums.map((num) => num + amount);
}
// This is an "IIFE" (Immediately Invoked Function Expression)
const getId = ((id = 1) => () => id++)();
Answer
The first function DOES NOT create a closure. Even though there is an inner arrow function defined, that function doesn't reference variables in the scope outside of it
The second function DOES create a closure because the inner arrow function passed to nums.map references the randomNum variable in the scope outside of it.
The third function DOES create a closure for the same reason as the function above. Referencing the parameter amount is the same.
The fourth function DOES create a closure because we have an outer arrow function returning an inner arrow function. The inner arrow function () => id++ references the id parameter in the outer arrow function.
Challenge
Below is a counter object. The problem is that the counter.value property is not private — it can be directly mutated. Your challenge is to create a function makeCounter that will protect the value of the counter while still allowing us to increment(), decrement(), and get the current value of the counter.
As a bonus, make the function accept an argument startingValue which sets the starting value of the counter. If no value is provided, start at 0. Then make multiple counters, each starting at a different value.
