The new class syntax is mostly, though not entirely, syntactic sugar (but, you know, the good kind of sugar). It markedly simplifies writing constructor functions and the objects they assign as prototypes to the objects they create, especially when setting up inheritance hierarchies, which was error-prone with the ES5 syntax. But unlike the old way, class syntax also enables super.example() for supercalls (which are notoriously hard to do the old way) as well as property declarations, private fields, and private methods (including static ones).
(Sometimes people say you have to use class syntax if you want to subclass Error or Array [which couldn’t be properly subclassed in ES5]. That’s not true, you can use a different ES2015 feature, Reflect.construct [spec, MDN], if you don’t want to use class syntax.¹)
Moreover, is
classa different kind of OOP or it still JavaScript’s prototypical inheritance?
It’s the same prototypical inheritance we’ve always had, just with cleaner, more convenient, and less error-prone syntax if you like using constructor functions (new Foo, etc.), plus some added features.
Can I modify it using
.prototype?
Yes, you can still modify the prototype object on the class’s constructor once you’ve created the class. E.g., this is perfectly legal:
class Foo {
constructor(name) {
this.name = name;
}
test1() {
console.log("test1: name = " + this.name);
}
}
Foo.prototype.test2 = function() {
console.log("test2: name = " + this.name);
};
Are there speed benefits?
By providing a specific idiom for this, I suppose it’s possible that the engine may be able to do a better job optimizing. But they’re awfully good at optimizing already, I wouldn’t expect a significant difference. One thing in particular about class syntax is that if you use property declarations, you can minimize the number of shape changes an object goes through when being constructed, which can make interpreting and later compiling the code a bit faster. But again, it’s not going to be big.
What benefits does ES2015 (ES6)
classsyntax provide?
Briefly: If you don’t use constructor functions in the first place, preferring Object.create or similar, class isn’t useful to you.
If you do use constructor functions, there are some benefits to class:
-
The syntax is simpler and less error-prone.
-
It’s much easier (and again, less error-prone) to set up inheritance hierarchies using the new syntax than with the old.
-
classdefends you from the common error of failing to usenewwith the constructor function (by having the constructor throw an exception). -
Calling the parent prototype’s version of a method is much simpler with the new syntax than the old (
super.method()instead ofParentConstructor.prototype.method.call(this)orObject.getPrototypeOf(Object.getPrototypeOf(this)).method.call(this)). -
Property declarations can make the shape of the instances being created clearer, separating it from the constructor logic.
-
You can use private fields and methods (both instance and static) with
classsyntax, and not with ES5 syntax.
Here’s a syntax comparison (without private members) for a hierarchy:
// ***ES2015+**
class Person {
constructor(first, last) {
this.first = first;
this.last = last;
}
personMethod() {
// ...
}
}
class Employee extends Person {
constructor(first, last, position) {
super(first, last);
this.position = position;
}
employeeMethod() {
// ...
}
}
class Manager extends Employee {
constructor(first, last, position, department) {
super(first, last, position);
this.department = department;
}
personMethod() {
const result = super.personMethod();
// ...use `result` for something...
return result;
}
managerMethod() {
// ...
}
}
Example:
// ***ES2015+**
class Person {
constructor(first, last) {
this.first = first;
this.last = last;
}
personMethod() {
return `Result from personMethod: this.first = ${this.first}, this.last = ${this.last}`;
}
}
class Employee extends Person {
constructor(first, last, position) {
super(first, last);
this.position = position;
}
personMethod() {
const result = super.personMethod();
return result + `, this.position = ${this.position}`;
}
employeeMethod() {
// ...
}
}
class Manager extends Employee {
constructor(first, last, position, department) {
super(first, last, position);
this.department = department;
}
personMethod() {
const result = super.personMethod();
return result + `, this.department = ${this.department}`;
}
managerMethod() {
// ...
}
}
const m = new Manager("Joe", "Bloggs", "Special Projects Manager", "Covert Ops");
console.log(m.personMethod());vs.
// **ES5**
var Person = function(first, last) {
if (!(this instanceof Person)) {
throw new Error("Person is a constructor function, use new with it");
}
this.first = first;
this.last = last;
};
Person.prototype.personMethod = function() {
// ...
};
var Employee = function(first, last, position) {
if (!(this instanceof Employee)) {
throw new Error("Employee is a constructor function, use new with it");
}
Person.call(this, first, last);
this.position = position;
};
Employee.prototype = Object.create(Person.prototype);
Employee.prototype.constructor = Employee;
Employee.prototype.employeeMethod = function() {
// ...
};
var Manager = function(first, last, position, department) {
if (!(this instanceof Manager)) {
throw new Error("Manager is a constructor function, use new with it");
}
Employee.call(this, first, last, position);
this.department = department;
};
Manager.prototype = Object.create(Employee.prototype);
Manager.prototype.constructor = Manager;
Manager.prototype.personMethod = function() {
var result = Employee.prototype.personMethod.call(this);
// ...use `result` for something...
return result;
};
Manager.prototype.managerMethod = function() {
// ...
};
Live Example:
// **ES5**
var Person = function(first, last) {
if (!(this instanceof Person)) {
throw new Error("Person is a constructor function, use new with it");
}
this.first = first;
this.last = last;
};
Person.prototype.personMethod = function() {
return "Result from personMethod: this.first = " + this.first + ", this.last = " + this.last;
};
var Employee = function(first, last, position) {
if (!(this instanceof Employee)) {
throw new Error("Employee is a constructor function, use new with it");
}
Person.call(this, first, last);
this.position = position;
};
Employee.prototype = Object.create(Person.prototype);
Employee.prototype.constructor = Employee;
Employee.prototype.personMethod = function() {
var result = Person.prototype.personMethod.call(this);
return result + ", this.position = " + this.position;
};
Employee.prototype.employeeMethod = function() {
// ...
};
var Manager = function(first, last, position, department) {
if (!(this instanceof Manager)) {
throw new Error("Manager is a constructor function, use new with it");
}
Employee.call(this, first, last, position);
this.department = department;
};
Manager.prototype = Object.create(Employee.prototype);
Manager.prototype.constructor = Manager;
Manager.prototype.personMethod = function() {
var result = Employee.prototype.personMethod.call(this);
return result + ", this.department = " + this.department;
};
Manager.prototype.managerMethod = function() {
// ...
};
var m = new Manager("Joe", "Bloggs", "Special Projects Manager", "Covert Ops");
console.log(m.personMethod());As you can see, there’s lots of repeated and verbose stuff there which is easy to get wrong and boring to retype (I used to use a script for it, back in the day, before class came along).
I should note that in the ES2015 code, the Person function is the prototype of the Employee function, but that’s not true in the ES5 code. In ES5, there’s no way to do that; all functions use Function.prototype as their prototype. Some environments supported a __proto__ pseudo-property that might have allowed changing that, though. In those environments, you could do this:
Employee.__proto__ = Person; // Was non-standard in ES5
If for some reason you wanted to do this with function syntax instead of class in an ES2015+ environment, you’d use the standard Object.setPrototypeOf instead:
Object.setPrototypeOf(Employee, Person); // Standard ES2015+
But I can’t see any strong motivation for using the old syntax in an ES2015+ environment (other than to experiment with understanding how the plumbing works).
(ES2015 also defines a __proto__ accessor property that is a wrapper for Object.setPrototypeOf and Object.getPrototypeOf so that code in those non-standard environments becomes standard, but it’s only defined for legacy code and is “normative optional” meaning an environment is not required to provide it.)
¹ Here’s how you’d use Reflect.construct to subclass Error (for instance) if you didn’t want to use class syntax:
// Creating an Error subclass:
function MyError(...args) {
return Reflect.construct(Error, args, this.constructor);
}
MyError.prototype = Object.create(Error.prototype);
MyError.prototype.constructor = MyError;
MyError.prototype.myMethod = function() {
console.log(this.message);
};
// Example use:
function outer() {
function inner() {
const e = new MyError("foo");
console.log("Callng e.myMethod():");
e.myMethod();
console.log(`e instanceof MyError? ${e instanceof MyError}`);
console.log(`e instanceof Error? ${e instanceof Error}`);
throw e;
}
inner();
}
outer();.as-console-wrapper {
max-height: 100% !important;
}