Object-oriented programming in JavaScript

1. Encapsulation

The core idea in object-oriented programming is to divide programs into smaller pieces and make each piece responsible for managing its own state. This way, some knowledge about the way a piece of the program works can be kept local to that piece. Someone working on the rest of the program does not have to remember or even be aware of that knowledge. When­ever these local details change, only the code directly around it needs to be updated.

Different pieces of such a program interact with each other through interfaces, limited sets of functions or bindings that provide useful functional­ity at a more abstract level, hiding their precise implementation.

Such program pieces are modeled using objects. Their interface consists of a specific set of methods and properties. Properties that are part of the interface are called public. The others, which outside code should not be touching, are called private.

Many languages provide a way to distinguish public and private prop­erties and prevent outside code from accessing the private ones altogether. JavaScript, once again taking the minimalist approach, does not—not yet at least. There is work underway to add this to the language.

Even though the language doesn’t have this distinction built in, JavaScript programmers are successfully using this idea. Typically, the avail­able interface is described in documentation or comments. It is also com­mon to put an underscore character (_) at the beginning of property names to indicate that those properties are private.

Separating interface from implementation is a great idea. It is usually called encapsulation.

2. Methods

Methods are nothing more than properties that hold function values. This is a simple method:

let rabbit = {}; rabbit.speak = function(line) {

console.log(‘The rabbit says ‘${line}”);


rabbit.speak(“I’m alive.”);

// → The rabbit says ‘I’m alive.’

Usually a method needs to do something with the object it was called on. When a function is called as a method—looked up as a property and immediately called, as in object.method()—the binding called this in its body automatically points at the object that it was called on.

function speak(line) {

console.log(‘The ${this.type} rabbit says ‘${line}”);


let whiteRabbit = {type: “white”, speak};

let hungryRabbit = {type: “hungry”, speak};

whiteRabbit.speak(“Oh my ears and whiskers, ” +

“how late it’s getting!”);

// → The white rabbit says ‘Oh my ears and whiskers, how

// late it’s getting!’
hungryRabbit.speak(“I could use a carrot right now.”);

// → The hungry rabbit says ‘I could use a carrot right now.’

You can think of this as an extra parameter that is passed in a different way. If you want to pass it explicitly, you can use a function’s call method, which takes the this value as its first argument and treats further arguments as normal parameters.

speak.call(hungryRabbit, “Burp!”);

// → The hungry rabbit says ‘Burp!’

Since each function has its own this binding, whose value depends on the way it is called, you cannot refer to the this of the wrapping scope in a regular function defined with the function keyword.

Arrow functions are different—they do not bind their own this but can see the this binding of the scope around them. Thus, you can do something like the following code, which references this from inside a local function:

function normalize() {

console.log(this.coords.map(n => n / this.length));


normalize.call({coords: [0, 2, 3], length: 5});

// → [0, 0.4, 0.6]

If I had written the argument to map using the function keyword, the code wouldn’t work.

3. Prototypes

Watch closely.

let empty = {};


// → function toString(){…} console.log(empty.toString());

// → [object Object]

I pulled a property out of an empty object. Magic!

Well, not really. I have simply been withholding information about the way JavaScript objects work. In addition to their set of properties, most objects also have a prototype. A prototype is another object that is used as a fallback source of properties. When an object gets a request for a property that it does not have, its prototype will be searched for the property, then the prototype’s prototype, and so on.

So who is the prototype of that empty object? It is the great ancestral prototype, the entity behind almost all objects, Object.prototype.

console.log(Object.getPrototypeOf({}) == Object.prototype);

// → true


// → null

As you guess, Object.getPrototypeOf returns the prototype of an object.

The prototype relations ofJavaScript objects form a tree-shaped struc­ture, and at the root of this structure sits Object.prototype. It provides a few methods that show up in all objects, such as toString, which converts an object to a string representation.

Many objects don’t directly have Object.prototype as their prototype but instead have another object that provides a different set of default prop­erties. Functions derive from Function.prototype, and arrays derive from Array.prototype.

console.log(Object.getPrototypeOf(Math.max) == Function.prototype);

// → true

console.log(Object.getPrototypeOf([]) == Array.prototype);

// → true

Such a prototype object will itself have a prototype, often Object .prototype, so that it still indirectly provides methods like toString.

You can use Object.create to create an object with a specific prototype.

let protoRabbit = {

speak(line) {

console.log(‘The ${this.type} rabbit says ‘${line}”);



let killerRabbit = Object.create(protoRabbit);

killerRabbit.type = “killer”;


// → The killer rabbit says ‘SKREEEE!’

A property like speak(line) in an object expression is a shorthand way of defining a method. It creates a property called speak and gives it a function as its value.

The “proto” rabbit acts as a container for the properties that are shared by all rabbits. An individual rabbit object, like the killer rabbit, con­tains properties that apply only to itself—in this case its type—and derives shared properties from its prototype.

4. Classes

JavaScript’s prototype system can be interpreted as a somewhat informal take on an object-oriented concept called classes. A class defines the shape of a type of object—what methods and properties it has. Such an object is called an instance of the class.

Prototypes are useful for defining properties for which all instances of a class share the same value, such as methods. Properties that differ per instance, such as our rabbits’ type property, need to be stored directly in the objects themselves.

So to create an instance of a given class, you have to make an object that derives from the proper prototype, but you also have to make sure it, itself, has the properties that instances of this class are supposed to have. This is what a constructor function does.

function makeRabbit(type) {

let rabbit = Object.create(protoRabbit);

rabbit.type = type; return rabbit;


JavaScript provides a way to make defining this type of function easier. If you put the keyword new in front of a function call, the function is treated as a constructor. This means that an object with the right prototype is automat­ically created, bound to this in the function, and returned at the end of the function.

The prototype object used when constructing objects is found by taking the prototype property of the constructor function.

function Rabbit(type) {

this.type = type;


Rabbit.prototype.speak = function(line) {

console.log(‘The ${this.type} rabbit says ‘${line}”);


let weirdRabbit = new Rabbit(“weird”);

Constructors (all functions, in fact) automatically get a property named prototype, which by default holds a plain, empty object that derives from Object.prototype. You can overwrite it with a new object if you want. Or you can add properties to the existing object, as the example does.

By convention, the names of constructors are capitalized so that they can easily be distinguished from other functions.

It’s important to understand the distinction between the way a prototype is associated with a constructor (through its prototype property) and the way objects have a prototype (which can be found with Object.getPrototypeOf).

The actual prototype of a constructor is Function.prototype since constructors are functions. Its prototype property holds the prototype used for instances created through it.

console.log(Object.getPrototypeOf(Rabbit) == Function.prototype);

// → true

console.log(Object.getPrototypeOf(weirdRabbit) == Rabbit.prototype);

// → true

5. Class Notation

So JavaScript classes are constructor functions with a prototype property. That is how they work, and until 2015, that was how you had to write them. These days, we have a less awkward notation.

class Rabbit {

constructor(type) {

this.type = type;


speak(line) {

console.log(‘The ${this.type} rabbit says ‘${line}”);



let killerRabbit = new Rabbit(“killer”);

let blackRabbit = new Rabbit(“black”);

The class keyword starts a class declaration, which allows us to define a constructor and a set of methods all in a single place. Any number of methods may be written inside the declaration’s braces. The one named constructor is treated specially. It provides the actual constructor function, which will be bound to the name Rabbit. The others are packaged into that constructor’s prototype. Thus, the earlier class declaration is equivalent to the constructor definition from the previous section. It just looks nicer.

Class declarations currently allow only methods—properties that hold functions—to be added to the prototype. This can be somewhat inconve­nient when you want to save a non-function value in there. The next version of the language will probably improve this. For now, you can create such properties by directly manipulating the prototype after you’ve defined the class.

Like function, class can be used both in statements and in expressions. When used as an expression, it doesn’t define a binding but just produces the constructor as a value. You are allowed to omit the class name in a class expression.

let object = new class { getWord() { return “hello”; } };


// → hello

6. Overriding Derived Properties

When you add a property to an object, whether it is present in the prototype or not, the property is added to the object itself. If there was already a prop­erty with the same name in the prototype, this property will no longer affect the object, as it is now hidden behind the object’s own property.

Rabbit.prototype.teeth = “small”;


// → small

killerRabbit.teeth = “long, sharp, and bloody”;


// → long, sharp, and bloody console.log(blackRabbit.teeth);

// → small


// → small

The following diagram sketches the situation after this code has run. The Rabbit and Object prototypes lie behind killerRabbit as a kind of backdrop, where properties that are not found in the object itself can be looked up.

Overriding properties that exist in a prototype can be a useful thing to do. As the rabbit teeth example shows, overriding can be used to express exceptional properties in instances of a more generic class of objects, while letting the nonexceptional objects take a standard value from their prototype.

Overriding is also used to give the standard function and array proto­types a different toString method than the basic object prototype.

console.log(Array.prototype.toString ==Object.prototype.toString);

// → false

console.log([1, 2].toString());

// → 1,2

Calling toString on an array gives a result similar to calling .join(“,”) on it—it puts commas between the values in the array. Directly calling Object.prototype.toString with an array produces a different string. That function doesn’t know about arrays, so it simply puts the word object and the name of the type between square brackets.

console.log(Object.prototype.toString.call([1, 2]));

// → [object Array]

7. Maps

We saw the word map used in “Transforming with Map” on page 88 for an operation that transforms a data structure by applying a function to its ele­ments. Confusing as it is, in programming the same word is also used for a related but rather different thing.

A map (noun) is a data structure that associates values (the keys) with other values. For example, you might want to map names to ages. It is possible to use objects for this.

let ages = {

Boris: 39,

Liang: 22,

Julia: 62


console.log(‘Julia is ${ages[“Julia”]}’);

// → Julia is 62

console.log(“Is Jack’s age known?”, “Jack” in ages);

// → Is Jack’s age known? false

console.log(“Is toString’s age known?”, “toString” in ages);

// → Is toString’s age known? true

Here, the object’s property names are the people’s names, and the property values are their ages. But we certainly didn’t list anybody named toString in our map. Yet, because plain objects derive from Object.prototype, it looks like the property is there.

As such, using plain objects as maps is dangerous. There are several possible ways to avoid this problem. First, it is possible to create objects with no prototype. If you pass null to Object.create, the resulting object will not derive from Object.prototype and can safely be used as a map.

console.log(“toString” in Object.create(null));

// → false

Object property names must be strings. If you need a map whose keys can’t easily be converted to strings—such as objects—you cannot use an object as your map.

Fortunately, JavaScript comes with a class called Map that is written for this exact purpose. It stores a mapping and allows any type of keys.

let ages = new Map();

ages.set(“Boris”, 39);

ages.set(“Liang”, 22);

ages.set(“Julia”, 62);

console.log(‘Julia is ${ages.get(“Julia”)}’);

// → Julia is 62

console.log(“Is Jack’s age known?”, ages.has(“Jack”));

// → Is Jack’s age known? false


// → false

The methods set, get, and has are part of the interface of the Map object. Writing a data structure that can quickly update and search a large set of values isn’t easy, but we don’t have to worry about that. Someone else did it for us, and we can go through this simple interface to use their work.

If you do have a plain object that you need to treat as a map for some reason, it is useful to know that Object.keys returns only an object’s own keys, not those in the prototype. As an alternative to the in operator, you can use the hasOwnProperty method, which ignores the object’s prototype.

console.log({x: 1}.hasOwnProperty(“x”));

// → true

console.log({x: 1}.hasOwnProperty(“toString”));

// → false

8. Polymorphism

When you call the String function (which converts a value to a string) on an object, it will call the toString method on that object to try to create a meaningful string from it. I mentioned that some of the standard proto­types define their own version of toString so they can create a string that contains more useful information than “[object Object]”. You can also do that yourself.

Rabbit.prototype.toString = function() {

return ‘a ${this.type} rabbit’;


This is a simple instance of a powerful idea. When a piece of code is written to work with objects that have a certain interface—in this case, a toString method—any kind of object that happens to support this interface can be plugged into the code, and it will just work.

This technique is called polymorphism. Polymorphic code can work with values of different shapes, as long as they support the interface it expects.

I noted in “Array Loops” on page 69 that a for/of loop can loop over several kinds of data structures. This is another case of polymorphism—such loops expect the data structure to expose a specific interface, which arrays and strings do. And we can also add this interface to your own objects! But before we can do that, we need to know what symbols are.

9. Symbols

It is possible for multiple interfaces to use the same property name for dif­ferent things. For example, I could define an interface in which the toString method is supposed to convert the object into a piece of yarn. It would not be possible for an object to conform to both that interface and the standard use of toString.

That would be a bad idea, and this problem isn’t that common. Most JavaScript programmers simply don’t think about it. But the language designers, whose job it is to think about this stuff, have provided us with a solution anyway.

When I claimed that property names are strings, that wasn’t entirely accurate. They usually are, but they can also be symbols. Symbols are values created with the Symbol function. Unlike strings, newly created symbols are unique—you cannot create the same symbol twice.

let sym = Symbol(“name”);

console.log(sym == Symbol(“name”));

// → false

Rabbit.prototype[sym] = 55;


// → 55

The string you pass to Symbol is included when you convert it to a string and can make it easier to recognize a symbol when, for example, showing it in the console. But it has no meaning beyond that—multiple symbols may have the same name.

Being both unique and usable as property names makes symbols suitable for defining interfaces that can peacefully live alongside other properties, no matter what their names are.

const toStringSymbol = Symbol(“toString”);

Array.prototype[toStringSymbol] = function() {

return ‘${this.length} cm of blue yarn’;


console.log([1, 2].toString());

// → 1,2

console.log([1, 2][toStringSymbol]());

// → 2 cm of blue yarn

It is possible to include symbol properties in object expressions and classes by using square brackets around the property name. That causes the property name to be evaluated, much like the square bracket property access notation, which allows us to refer to a binding that holds the symbol.

let stringObject = {

[toStringSymbol]() { return “a jute rope”; }



// → a jute rope

10. The Iterator Interface

The object given to a for/of loop is expected to be iterable. This means it has a method named with the Symbol.iterator symbol (a symbol value defined by the language, stored as a property of the Symbol function).

When called, that method should return an object that provides a sec­ond interface, iterator. This is the actual thing that iterates. It has a next method that returns the next result. That result should be an object with a value property that provides the next value, if there is one, and a done property, which should be true when there are no more results and false otherwise.

Note that the next, value, and done property names are plain strings, not symbols. Only Symbol.iterator, which is likely to be added to a lot of different objects, is an actual symbol.

We can directly use this interface ourselves.

let okIterator = “OK”[Symbol.iterator]();


// → {value: “O”, done: false}


// → {value: “K”, done: false}


// → {value: undefined, done: true}

Let’s implement an iterable data structure. We’ll build a matrix class, acting as a two-dimensional array.

class Matrix {

constructor(width, height, element = (x, y) => undefined) {

this.width = width;

this.height = height;

this.content = [];

for (let y = 0; y < height; y++) {

for (let x = 0; x < width; x++) {

this.content[y * width + x] = element(x, y);




get(x, y) {

return this.content[y * this.width + x];


set(x, y, value) {

this.content[y * this.width + x] = value;



The class stores its content in a single array of width X height elements. The elements are stored row by row, so, for example, the third element in the fifth row is (using zero-based indexing) stored at position 4 X width + 2.

The constructor function takes a width, a height, and an optional con­tent function that will be used to fill in the initial values. There are get and set methods to retrieve and update elements in the matrix.

When looping over a matrix, you are usually interested in the position of the elements as well as the elements themselves, so we’ll have our iterator produce objects with x, y, and value properties.

class MatrixIterator {

constructor(matrix) {

this.x = 0;

this.y = 0;

this.matrix = matrix;


next() {

if (this.y == this.matrix.height) return {done: true};

let value = {x: this.x, y: this.y,

value: this.matrix.get(this.x, this.y)};


if (this.x == this.matrix.width) {

this.x = 0;



return {value, done: false};



The class tracks the progress of iterating over a matrix in its x and y properties. The next method starts by checking whether the bottom of the matrix has been reached. If it hasn’t, it first creates the object holding the current value and then updates its position, moving to the next row if necessary.

Let’s set up the Matrix class to be iterable. Throughout this book, I’ll occasionally use after-the-fact prototype manipulation to add methods to classes so that the individual pieces of code remain small and self-contained. In a regular program, where there is no need to split the code into small pieces, you’d declare these methods directly in the class instead.

Matrix.prototype[Symbol.iterator] = function() {

return new MatrixIterator(this);


We can now loop over a matrix with for/of.

let matrix = new Matrix(2, 2, (x, y) => ‘value ${x},${y}’);

for (let {x, y, value} of matrix) {

console.log(x, y, value);


// → 00 value 0,0

// → 10 value 1,0

// → 01 value 0,1

// → 11 value 1,1

11. Getters, Setters, and Statics

Interfaces often consist mostly of methods, but it is also okay to include properties that hold non-function values. For example, Map objects have a size property that tells you how many keys are stored in them.

It is not even necessary for such an object to compute and store such a property directly in the instance. Even properties that are accessed directly may hide a method call. Such methods are called getters, and they are defined by writing get in front of the method name in an object expres­sion or class declaration.

let varyingSize = {

get size() {

return Math.floor(Math.random() * 100);




// → 73 console.log(varyingSize.size);

// → 49

Whenever someone reads from this object’s size property, the associated method is called. You can do a similar thing when a property is written to, using a setter.

class Temperature {

constructor(celsius) {

this.celsius = celsius;


get fahrenheit() {

return this.celsius * 1.8 + 32;


set fahrenheit(value) {

this.celsius = (value – 32) / 1.8;


static fromFahrenheit(value) {

return new Temperature((value – 32) / 1.8);



let temp = new Temperature(22);


// → 71.6

temp.fahrenheit = 86; console.log(temp.celsius);

// → 30

The Temperature class allows you to read and write the temperature in either degrees Celsius or degrees Fahrenheit, but internally it stores only Celsius and automatically converts to and from Celsius in the fahrenheit get­ter and setter.

Sometimes you want to attach some properties directly to your con­structor function, rather than to the prototype. Such methods won’t have access to a class instance but can, for example, be used to provide additional ways to create instances.

Inside a class declaration, methods that have static written before their name are stored on the constructor. So the Temperature class allows you to write Temperature.fromFahrenheit(100) to create a temperature using degrees Fahrenheit.

12. Inheritance

Some matrices are known to be symmetric. If you mirror a symmetric matrix around its top-left-to-bottom-right diagonal, it stays the same. In other words, the value stored at x,y is always the same as that at y,x.

Imagine we need a data structure like Matrix but one that enforces the fact that the matrix is and remains symmetrical. We could write it from scratch, but that would involve repeating some code very similar to what we already wrote.

JavaScript’s prototype system makes it possible to create a new class, much like the old class, but with new definitions for some of its properties. The prototype for the new class derives from the old prototype but adds a new definition for, say, the set method.

In object-oriented programming terms, this is called inheritance. The new class inherits properties and behavior from the old class.

class SymmetricMatrix extends Matrix {

constructor(size, element = (x, y) => undefined) {

super(size, size, (x, y) => {

if (x < y) return element(y, x);

else return element(x, y);



set(x, y, value) {

super.set(x, y, value);

if (x != y) {

super.set(y, x, value);




let matrix = new SymmetricMatrix(5, (x, y) => ‘${x},${y}’); console.log(matrix.get(2, 3));

// → 3,2

The use of the word extends indicates that this class shouldn’t be directly based on the default Object prototype but on some other class. This is called the superclass. The derived class is the subclass.

To initialize a SymmetricMatrix instance, the constructor calls its super­class’s constructor through the super keyword. This is necessary because if this new object is to behave (roughly) like a Matrix, it is going to need the instance properties that matrices have. To ensure the matrix is symmetrical, the constructor wraps the content method to swap the coordinates for values below the diagonal.

The set method again uses super but this time not to call the constructor but to call a specific method from the superclass’s set of methods. We are redefining set but do want to use the original behavior. Because this.set refers to the new set method, calling that wouldn’t work. Inside class meth­ods, super provides a way to call methods as they were defined in the super­class.

Inheritance allows us to build slightly different data types from existing data types with relatively little work. It is a fundamental part of the object- oriented tradition, alongside encapsulation and polymorphism. But while the latter two are now generally regarded as wonderful ideas, inheritance is more controversial.

Whereas encapsulation and polymorphism can be used to separate pieces of code from each other, reducing the tangledness of the overall program, inheritance fundamentally ties classes together, creating more tangle. When inheriting from a class, you usually have to know more about how it works than when simply using it. Inheritance can be a useful tool, and I use it now and then in my own programs, but it shouldn’t be the first tool you reach for, and you probably shouldn’t actively go looking for opportunities to con­struct class hierarchies (family trees of classes).

13. The instanceof Operator

It is occasionally useful to know whether an object was derived from a spe­cific class. For this, JavaScript provides a binary operator called instanceof.


new SymmetricMatrix(2) instanceof SymmetricMatrix);

// → true

console.log(new SymmetricMatrix(2) instanceof Matrix);

// → true

console.log(new Matrix(2, 2) instanceof SymmetricMatrix);

// → false

console.log([1] instanceof Array);

// → true

The operator will see through inherited types, so a SymmetricMatrix is an instance of Matrix. The operator can also be applied to standard constructors like Array. Almost every object is an instance of Object.

Source: Haverbeke Marijn (2018), Eloquent JavaScript: A Modern Introduction to Programming,

No Starch Press; 3rd edition.

Leave a Reply

Your email address will not be published. Required fields are marked *