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Classes in ECMAScript 6 (final semantics)

Updates:

Recently, TC39 decided on the final semantics of classes in ECMAScript 6 [2]. This blog post explains how their final incarnation works. The most significant recent changes were related to how subclassing is handled.

Overview

    class Point {
        constructor(x, y) {
            this.x = x;
            this.y = y;
        }
        toString() {
            return '(' + this.x + ', ' + this.y + ')';
        }
    }
    
    class ColorPoint extends Point {
        constructor(x, y, color) {
            super(x, y);
            this.color = color;
        }
        toString() {
            return super.toString() + ' in ' + this.color;
        }
    }
    
    let cp = new ColorPoint(25, 8, 'green');
    cp.toString(); // '(25, 8) in green'
    
    console.log(cp instanceof ColorPoint); // true
    console.log(cp instanceof Point); // true

The essentials

Base classes

A class is defined like this in ECMAScript 6 (ES6):

    class Point {
        constructor(x, y) {
            this.x = x;
            this.y = y;
        }
        toString() {
            return '(' + this.x + ', ' + this.y + ')';
        }
    }

You use this class just like an ES5 constructor function:

    > var p = new Point(25, 8);
    > p.toString()
    '(25, 8)'

In fact, the result of a class definition is a function:

    > typeof Point
    'function'

However, you can only invoke a class via new, not via a function call (Sect. 9.2.2 in the spec):

    > Point()
    TypeError: Classes can’t be function-called
Class declarations are not hoisted

Function declarations are hoisted: When entering a scope, the functions that are declared in it are immediately available – independently of where the declarations happen. That means that you can call a function that is declared later:

    foo(); // works, because `foo` is hoisted
    
    function foo() {}

In contrast, class declarations are not hoisted. Therefore, a class only exists after execution reached its definition and it was evaluated. Accessing it beforehand leads to a ReferenceError:

    new Foo(); // ReferenceError
    
    class Foo {}

The reason for this limitation is that classes can have an extends clause whose value is an arbitrary expression. That expression must be evaluated in the proper “location”, its evaluation can’t be hoisted.

Not having hoisting is less limiting than you may think. For example, a function that comes before a class declaration can still refer to that class, but you have to wait until the class declaration has been evaluated before you can call the function.

    function functionThatUsesBar() {
        new Bar();
    }
    
    functionThatUsesBar(); // ReferenceError
    class Bar {}
    functionThatUsesBar(); // OK
Class expressions

Similarly to functions, there are two kinds of class definitions, two ways to define a class: class declarations and class expressions.

Also similarly to functions, the identifier of a class expression is only visible within the expression:

    const MyClass = class Me {
        getClassName() {
            return Me.name;
        }
    };
    let inst = new MyClass();
    console.log(inst.getClassName()); // Me
    console.log(Me.name); // ReferenceError: Me is not defined

Inside the body of a class definition

A class body can only contain methods, but not data properties. Prototypes having data properties is generally considered an anti-pattern, so this just enforces a best practice.

constructor, static methods, prototype methods

Let’s examine three kinds of methods that you often find in class literals.

    class Foo {
        constructor(prop) {
            this.prop = prop;
        }
        static staticMethod() {
            return 'classy';
        }
        prototypeMethod() {
            return 'prototypical';
        }
    }
    let foo = new Foo(123);

The object diagram for this class declaration looks as follows. Tip for understanding it: [[Prototype]] is an inheritance relationship between objects, while prototype is a normal property whose value is an object. The property prototype is only special because the new operator uses its value as the prototype for instances it creates.

First, the pseudo-method constructor. This method is special, as it defines the function that represents the class:

    > Foo === Foo.prototype.constructor
    true
    > typeof Foo
    'function'

It is sometimes called a class constructor. It has features that normal constructor functions don’t have (mainly the ability to constructor-call its super-constructor via super(), which is explained later).

Second, static methods. Static properties (or class properties) are properties of Foo itself. If you prefix a method definition with static, you create a class method:

    > typeof Foo.staticMethod
    'function'
    > Foo.staticMethod()
    'classy'

Third, prototype methods. The prototype properties of Foo are the properties of Foo.prototype. They are usually methods and inherited by instances of Foo.

    > typeof Foo.prototype.prototypeMethod
    'function'
    > foo.prototypeMethod()
    'prototypical'
Getters and setters

The syntax for getters and setters is just like in ECMAScript 5 object literals:

    class MyClass {
        get prop() {
            return 'getter';
        }
        set prop(value) {
            console.log('setter: '+value);
        }
    }

You use MyClass as follows.

    > let inst = new MyClass();
    > inst.prop = 123;
    setter: 123
    > inst.prop
    'getter'
Computed method names

You can define the name of a method via an expression, if you put it in square brackets. For example, the following ways of defining Foo are all equivalent.

    class Foo() {
        myMethod() {}
    }
    
    class Foo() {
        ['my'+'Method']() {}
    }
    
    const m = 'myMethod';
    class Foo() {
        [m]() {}
    }

Several special methods in ECMAScript 6 have keys that are symbols [3]. Computed method names allow you to define such methods. For example, if an object has a method whose key is Symbol.iterator, it is iterable [4]. That means that its contents can be iterated over by the for-of loop and other language mechanisms.

    class IterableClass {
        [Symbol.iterator]() {
            ···
        }
    }
Generator methods

If you prefix a method definition with an asterisk (*), it becomes a generator method [4]. Among other things, a generator is useful for defining the method whose key is Symbol.iterator. The following code demonstrates how that works.

    class IterableArguments {
        constructor(...args) {
            this.args = args;
        }
        * [Symbol.iterator]() {
            for (let arg of this.args) {
                yield arg;
            }
        }
    }
    
    for (let x of new IterableArguments('hello', 'world')) {
        console.log(x);
    }
    
    // Output:
    // hello
    // world

Subclassing

The extends clause lets you create a subclass of an existing constructor (which may or may not have been defined via a class):

    class Point {
        constructor(x, y) {
            this.x = x;
            this.y = y;
        }
        toString() {
            return '(' + this.x + ', ' + this.y + ')';
        }
    }
    
    class ColorPoint extends Point {
        constructor(x, y, color) {
            super(x, y); // (A)
            this.color = color;
        }
        toString() {
            return super.toString() + ' in ' + this.color; // (B)
        }
    }

Again, this class is used like you’d expect:

    > let cp = new ColorPoint(25, 8, 'green');
    > cp.toString()
    '(25, 8) in green'
    
    > cp instanceof ColorPoint
    true
    > cp instanceof Point
    true

There are two kinds of classes:

There are two ways of using super:

The prototype of a subclass is the superclass

The prototype of a subclass is the superclass in ECMAScript 6:

    > Object.getPrototypeOf(ColorPoint) === Point
    true

That means that static properties are inherited:

    class Foo {
        static classMethod() {
            return 'hello';
        }
    }
    
    class Bar extends Foo {
    }
    Bar.classMethod(); // 'hello'

You can even super-call static methods:

    class Foo {
        static classMethod() {
            return 'hello';
        }
    }
    
    class Bar extends Foo {
        static classMethod() {
            return super.classMethod() + ', too';
        }
    }
    Bar.classMethod(); // 'hello, too'
Super-constructor calls

In a derived class, you must call super() before you can use this:

    class Foo {}
    
    class Bar extends Foo {
        constructor(num) {
            let tmp = num * 2; // OK
            this.num = num; // ReferenceError
            super();
            this.num = num; // OK
        }
    }

Implicitly leaving a derived constructor without calling super() also causes an error:

    class Foo {}
    
    class Bar extends Foo {
        constructor() {
        }
    }
    
    let bar = new Bar(); // ReferenceError
Overriding the result of a constructor

Just like in ES5, you can override the result of a constructor by explicitly returning an object:

    class Foo {
        constructor() {
            return Object.create(null);
        }
    }
    console.log(new Foo() instanceof Foo); // false

If you do so, it doesn’t matter whether this has been initialized or not. In other words: you don’t have to call super() in a derived constructor if you override the result in this manner.

Default constructors for classes

If you don’t specify a constructor for a base class, the following definition is used:

    constructor() {}

For derived classes, the following default constructor is used:

    constructor(...args) {
        super(...args);
    }
Subclassing built-in constructors

In ECMAScript 6, you can finally subclass all built-in constructors (there are work-arounds for ES5, but these have significant limitations).

For example, you can now create your own exception classes (that will inherit the feature of having a stack trace in most engines):

    class MyError extends Error {    
    }
    throw new MyError('Something happened!');

You can also create subclasses of Array whose instances properly handle length:

    class MyArray extends Array {
        constructor(len) {
            super(len);
        }
    }
    
    // Instances of of `MyArray` work like real arrays:
    let myArr = new MyArray(0);
    console.log(myArr.length); // 0
    myArr[0] = 'foo';
    console.log(myArr.length); // 1

Note that subclassing built-in constructors is something that engines have to support natively, you won’t get this feature via transpilers.

The details of classes

What we have seen so far are the essentials of classes. You only need to read on if you are interested how things happen under the hood. Let’s start with the syntax of classes. The following is a slightly modified version of the syntax shown in Sect. A.4 of the ECMAScript 6 specification.

    ClassDeclaration:
        "class" BindingIdentifier ClassTail
    ClassExpression:
        "class" BindingIdentifier? ClassTail
    
    ClassTail:
        ClassHeritage? "{" ClassBody? "}"
    ClassHeritage:
        "extends" AssignmentExpression
    ClassBody:
        ClassElement+
    ClassElement:
        MethodDefinition
        "static" MethodDefinition
        ";"
    
    MethodDefinition:
        PropName "(" FormalParams ")" "{" FuncBody "}"
        "*" PropName "(" FormalParams ")" "{" GeneratorBody "}"
        "get" PropName "(" ")" "{" FuncBody "}"
        "set" PropName "(" PropSetParams ")" "{" FuncBody "}"
    
    PropertyName:
        LiteralPropertyName
        ComputedPropertyName
    LiteralPropertyName:
        IdentifierName  /* foo */
        StringLiteral   /* "foo" */
        NumericLiteral  /* 123.45, 0xFF */
    ComputedPropertyName:
        "[" Expression "]"

Two observations:

Various checks

Attributes of properties

Class declarations create (mutable) let bindings. For a given class Foo:

Note that method definitions in object literals produce enumerable properties.

The details of subclassing

In ECMAScript 6, subclassing looks as follows.

    class Point {
        constructor(x, y) {
            this.x = x;
            this.y = y;
        }
        ···
    }
    
    class ColorPoint extends Point {
        constructor(x, y, color) {
            super(x, y);
            this.color = color;
        }
        ···
    }
    
    let cp = new ColorPoint(25, 8, 'green');

This code produces the following objects.

The next subsection examines the prototype chains (in the two columns), the subsection after that examines how cp is allocated and initialized.

Prototype chains

In the diagram, you can see that there are two prototype chains (objects linked via the [[Prototype]] relationship, which is an inheritance relationship):

The prototype chain in the left column leads to static properties being inherited.

Allocating and initializing the instance object

The data flow between class constructors is different from the canonical way of subclassing in ES5. Under the hood, it roughly looks as follows.

    // Instance is allocated here
    function Point(x, y) {
        // Performed before entering this constructor:
        this = Object.create(new.target.prototype);
    
        this.x = x;
        this.y = y;
    }
    ···
    
    function ColorPoint(x, y, color) {
        // Performed before entering this constructor:
        this = uninitialized;
    
        this = Reflect.construct(Point, [x, y], new.target); // (A)
            // super(x, y);
    
        this.color = color;
    }
    Object.setPrototypeOf(ColorPoint, Point);
    ···
    
    let cp = Reflect.construct( // (B)
                 ColorPoint, [25, 8, 'green'],
                 ColorPoint);
        // let cp = new ColorPoint(25, 8, 'green');

The instance object is created in different locations in ES6 and ES5:

The previous code uses two new ES6 features:

The advantage of this way of subclassing is that it enables normal code to subclass built-in constructors (such as Error and Array). A later section explains why a different approach was necessary.

Safety checks
The extends clause

Let’s examine how the extends clause influences how a class is set up (Sect. 14.5.14 of the spec).

The value of an extends clause must be “constructible” (invocable via new). null is allowed, though.

    class C {
    }

    class C extends B {
    }

    class C extends Object {
    }

Note the following subtle difference with the first case: If there is no extends clause, the class is a base class and allocates instances. If a class extends Object, it is a derived class and Object allocates the instances. The resulting instances (including their prototype chains) are the same, but you get there differently.

    class C extends null {
    }

Such a class is not very useful: new-calling it leads to an error, because the default constructor makes a super-constructor call and Function.prototype (the super-constructor) can’t be constructor-called. The only way to make the error go away is by adding a constructor that returns an object.

Why can’t you subclass built-in constructors in ES5?

In ECMAScript 5, most built-in constructors can’t be subclassed (several work-arounds exist).

To understand why, let’s use the canonical ES5 pattern to subclass Array. As we shall soon find out, this doesn’t work.

    function MyArray(len) {
        Array.call(this, len); // (A)
    }
    MyArray.prototype = Object.create(Array.prototype);

Unfortunately, if we instantiate MyArray, we find out that it doesn’t work properly: The instance property length does not change in reaction to us adding array elements:

    > var myArr = new MyArray(0);
    > myArr.length
    0
    > myArr[0] = 'foo';
    > myArr.length
    0

There are two obstracles that prevent myArr from being a proper array.

First obstacle: initialization. The this you hand to the constructor Array (in line A) is completely ignored. That means you can’t use Array to set up the instance that was created for MyArray.

    > var a = [];
    > var b = Array.call(a, 3);
    > a !== b  // a is ignored, b is a new object
    true
    > b.length // set up correctly
    3
    > a.length // unchanged
    0

Second obstacle: allocation. The instance objects created by Array are exotic (a term used by the ECMAScript specification for objects that have features that normal objects don’t have): Their property length tracks and influences the management of array elements. In general, exotic objects can be created from scratch, but you can’t convert an existing normal object into an exotic one. Unfortunately, that is what Array would have to do, when called in line A: It would have to turn the normal object created for MyArray into an exotic array object.

The solution: ES6 subclassing

In ECMAScript 6, subclassing Array looks as follows:

    class MyArray extends Array {
        constructor(len) {
            super(len);
        }
    }

This works (but it’s not something that ES6 transpilers can support, it depends on whether a JavaScript engine supports it natively):

    > let myArr = new MyArray(0);
    > myArr.length
    0
    > myArr[0] = 'foo';
    > myArr.length
    1

We can now see how the ES6 approach to subclassing circumvents the obstacles:

Referring to super-properties in methods

The following ES6 code makes a super-method call in line B.

    class Point {
        constructor(x, y) {
            this.x = x;
            this.y = y;
        }
        toString() { // (A)
            return '(' + this.x + ', ' + this.y + ')';
        }
    }
    
    class ColorPoint extends Point {
        constructor(x, y, color) {
            super(x, y);
            this.color = color;
        }
        toString() {
            return super.toString() // (B)
                   + ' in ' + this.color;
        }
    }
    
    let cp = new ColorPoint(25, 8, 'green');
    console.log(cp.toString()); // (25, 8) in green

To understand how super-calls work, let’s look at the object diagram of cp:

ColorPoint.prototype.toString makes a super-call (line B) to the method (starting in line A) that it has overridden. Let’s call the object, in which a method is stored, the home object of that method. For example, ColorPoint.prototype is the home object of ColorPoint.prototype.toString().

The super-call in line B involves three steps:

  1. Start your search in the prototype of the home object of the current method.

  2. Look for a method whose name is toString. That method may be found in the object where the search started or later in the prototype chain.

  3. Call that method with the current this. The reason for doing so is: the super-called method must be able to access the same instance properties (in our example, the properties of cp).

Note that even if you are only getting or setting a property (not calling a method), you still have to consider this in step #3, because the property may be implemented via a getter or a setter.

Let’s express these steps in three different, but equivalent, ways:

    // Variation 1: super-method calls in ES5
    var result = Point.prototype.toString.call(this) // steps 1,2,3
    
    // Variation 2: ES5, refactored
    var superObject = Point.prototype; // step 1
    var superMethod = superObject.toString; // step 2
    var result = superMethod.call(this) // step 3
    
    // Variation 3: ES6
    var homeObject = ColorPoint.prototype;
    var superObject = Object.getPrototypeOf(homeObject); // step 1
    var superMethod = superObject.toString; // step 2
    var result = superMethod.call(this) // step 3

Variation 3 is how ECMAScript 6 handles super-calls. This approach is supported by two internal bindings that the environments of functions have (environments provide storage space, so-called bindings, for the variables in a scope):

A method definition in a class literal that uses super is now special: Its value is still a function, but it has the internal property [[HomeObject]]. That property is set up by the method definition and can’t be changed in JavaScript. Therefore, you can’t meaningfully move such a method to a different object.

Using super to refer to a property is not allowed in function declarations, function expressions and generator functions.

Referring to super-properties is handy whenever prototype chains are involved, which is why you can use it in method definitions inside object literals and class literals (the class can be derived or not, the method can be static or not).

Constructor calls explained via JavaScript code

The JavaScript code in this section is a much simplified version of how the specification describes constructor calls and super-constructor calls. It may be interesting to you if you prefer code to explanations in human language. Before we can delve into the actual functionality, we need to understand a few other mechanisms.

Internal variables and properties

The specification writes internal variables and properties in double brackets ([[Foo]]). In the code, I use double underscores, instead (__Foo__).

Internal variables used in the code:

Internal properties used in the code:

Environments

Environments provide storage space for variables, there is one environment per scope. Environments are managed as a stack. The environment on top of that stack is considered active. The following code is a sketch of how environments are handled.

    /**
     * Function environments are special, they have a few more
     * internal variables than other environments.
     * (`Environment` is not shown here)
     */
    class FunctionEnvironment extends Environment {
        constructor(Func) {
            // [[FunctionObject]] is a function-specific
            // internal variable
            this.__FunctionObject__ = Func;
        }    
    }
    
    /**
     * Push an environment onto the stack
     */
    function PushEnvironment(env) { ··· }
    
    /**
     * Pop the topmost environment from the stack
     */
    function PopEnvironment() { ··· }
    
    /**
     * Find topmost function environment on stack
     */
    function GetThisEnvironment() { ··· }

Constructor calls

Let’s start with the default way (ES6 spec Sect. 9.2.3) in which constructor calls are handled for functions:

    /**
     * All constructible functions have this own method,
     * it is called by the `new` operator
     */
    AnyFunction.__Construct__ = function (args, newTarget) {
        let Constr = this;
        let kind = Constr.__ConstructorKind__;
    
        let env = new FunctionEnvironment(Constr);
        env.__NewTarget__ = newTarget;
        if (kind === 'base') {
            env.__thisValue__ = Object.create(newTarget.prototype);
        } else {
            // While `this` is uninitialized, getting or setting it
            // throws a `ReferenceError`
            env.__thisValue__ = uninitialized;
        }
    
        PushEnvironment(env);
        let result = Constr(...args);
        PopEnvironment();
    
        // Let’s pretend there is a way to tell whether `result`
        // was explicitly returned or not
        if (WasExplicitlyReturned(result)) {
            if (isObject(result)) {
                return result;
            }
            // Explicit return of a primitive
            if (kind === 'base') {
                // Base constructors must be backwards compatible
                return env.__thisValue__; // always initialized!
            }
            throw new TypeError();
        }
        // Implicit return
        if (env.__thisValue__ === uninitialized) {
            throw new ReferenceError();
        }
        return env.__thisValue__;
    }

Super-constructor calls

Super-constructor calls are handled as follows (ES6 spec Sect. 12.3.5.1).

    /**
     * Handle super-constructor calls
     */
    function super(...args) {
        let env = GetThisEnvironment();
        let newTarget = env.__NewTarget__;
        let activeFunc = env.__FunctionObject__;
        let superConstructor = Object.getPrototypeOf(activeFunc);
    
        env.__thisValue__ = superConstructor
                            .__Construct__(args, newTarget);
    }

The species pattern

One more mechanism of built-in constructors has been made extensible in ECMAScript 6: If a method such as Array.prototype.map() returns a fresh instance, what constructor should it use to create that instance? The default is to use the same constructor that created this, but some subclasses may want it to remain a direct instance of Array. ES6 lets subclasses override the default, via the so-called species pattern:

You can override the default, via a static getter (line A):

    class MyArray1 extends Array {
    }
    let result1 = new MyArray1().map(x => x);
    console.log(result1 instanceof MyArray1); // true
    
    class MyArray2 extends Array {
        static get [Symbol.species]() { // (A)
            return Array;
        }
    }
    let result2 = new MyArray2().map(x => x);
    console.log(result2 instanceof MyArray2); // false

An alternative is to use Object.defineProperty() (you can’t use assignment, as that would trigger a setter, which doesn’t exist):

    Object.defineProperty(
        MyArray2, Symbol.species, {
            value: Array
        });

The following getters all return this, which means that methods such as Array.prototype.map() use the constructor that created the current instance for their results.

Conclusion

The specialization of functions

There is an interesting trend in ECMAScript 6: Previously, a single kind of function took on three roles: real function, method and constructor. In ES6, there is specialization:

The future of classes

The design maxim for classes was “maximally minimal”. Several advanced features were discussed, but ultimately discarded in order to get a design that would be unanimously accepted by TC39.

Upcoming versions of ECMAScript can now extend this minimal design – classes will provide a foundation for features such as traits (or mixins), value objects (where different objects are equal if they have the same content) and const classes (that produce immutable instances).

Does JavaScript need classes?

Classes are controversial within the JavaScript community. On one hand, people coming from class-based languages are happy that they don’t have to deal with JavaScript’s unorthodox inheritance mechanisms, anymore. On the other hand, there are many JavaScript programmers who argue that what’s complicated about JavaScript is not prototypal inheritance, but constructors [6].

ES6 classes provide a few clear benefits:

I have made my peace with classes and am glad that they are in ES6. I would have preferred them to be prototypal (based on constructor objects [6], not constructor functions), but I also understand that backwards compatibility is important.

Further reading

Acknowledgement: #1 was an important source of this blog post.

  1. “Instantiation Reform: One last time”, slides by Allen Wirfs-Brock.
  2. “Exploring ES6: Upgrade to the next version of JavaScript”, book by Axel
  3. Symbols in ECMAScript 6
  4. Iterators and generators in ECMAScript 6
  5. Meta programming with ECMAScript 6 proxies
  6. Prototypes as classes – an introduction to JavaScript inheritance