In object-oriented programming, a class is an extensible program-code-template for creating objects, providing initial values for state (member variables) and implementations of behavior (member functions or methods).
In practice, we often need to create many objects of the same kind, like users, or goods or whatever.
As we already know from the chapter Constructor, operator "new", new function
can help with that.
But in the modern JavaScript, there’s a more advanced “class” construct, that introduces great new features which are useful for object-oriented programming.
The “class” syntax
The basic syntax is:
class
MyClass
{
// class methods
constructor
(
)
{
...
}
method1
(
)
{
...
}
method2
(
)
{
...
}
method3
(
)
{
...
}
...
}
Then use new MyClass()
to create a new object with all the listed methods.
The constructor()
method is called automatically by new
, so we can initialize the object there.
For example:
class
User
{
constructor
(
name
)
{
this
.
name =
name;
}
sayHi
(
)
{
alert
(
this
.
name)
;
}
}
// Usage:
let
user =
new
User
(
"John"
)
;
user.
sayHi
(
)
;
When new User("John")
is called:
- A new object is created.
- The
constructor
runs with the given argument and assigns it tothis.name
.
…Then we can call object methods, such as user.sayHi()
.
A common pitfall for novice developers is to put a comma between class methods, which would result in a syntax error.
The notation here is not to be confused with object literals. Within the class, no commas are required.
What is a class?
So, what exactly is a class
? That’s not an entirely new language-level entity, as one might think.
Let’s unveil any magic and see what a class really is. That’ll help in understanding many complex aspects.
In JavaScript, a class is a kind of function.
Here, take a look:
class
User
{
constructor
(
name
)
{
this
.
name =
name;
}
sayHi
(
)
{
alert
(
this
.
name)
;
}
}
// proof: User is a function
alert
(
typeof
User)
;
// function
What class User {...}
construct really does is:
- Creates a function named
User
, that becomes the result of the class declaration. The function code is taken from theconstructor
method (assumed empty if we don’t write such method). - Stores class methods, such as
sayHi
, inUser.prototype
.
After new User
object is created, when we call its method, it’s taken from the prototype, just as described in the chapter F.prototype. So the object has access to class methods.
We can illustrate the result of class User
declaration as:
Here’s the code to introspect it:
class
User
{
constructor
(
name
)
{
this
.
name =
name;
}
sayHi
(
)
{
alert
(
this
.
name)
;
}
}
// class is a function
alert
(
typeof
User)
;
// function
// ...or, more precisely, the constructor method
alert
(
User ===
User
.
prototype.
constructor)
;
// true
// The methods are in User.prototype, e.g:
alert
(
User
.
prototype.
sayHi)
;
// the code of the sayHi method
// there are exactly two methods in the prototype
alert
(
Object.
getOwnPropertyNames
(
User
.
prototype)
)
;
// constructor, sayHi
Not just a syntactic sugar
Sometimes people say that class
is a “syntactic sugar” (syntax that is designed to make things easier to read, but doesn’t introduce anything new), because we could actually declare the same thing without using the class
keyword at all:
// rewriting class User in pure functions
// 1. Create constructor function
function
User
(
name
)
{
this
.
name =
name;
}
// a function prototype has "constructor" property by default,
// so we don't need to create it
// 2. Add the method to prototype
User
.
prototype.
sayHi
=
function
(
)
{
alert
(
this
.
name)
;
}
;
// Usage:
let
user =
new
User
(
"John"
)
;
user.
sayHi
(
)
;
The result of this definition is about the same. So, there are indeed reasons why class
can be considered a syntactic sugar to define a constructor together with its prototype methods.
Still, there are important differences.
-
First, a function created by
class
is labelled by a special internal property[[IsClassConstructor]]: true
. So it’s not entirely the same as creating it manually.The language checks for that property in a variety of places. For example, unlike a regular function, it must be called with
new
:class
User
{
constructor
(
)
{
}
}
alert
(
typeof
User)
;
// function
User
(
)
;
// Error: Class constructor User cannot be invoked without 'new'
Also, a string representation of a class constructor in most JavaScript engines starts with the “class…”
class
User
{
constructor
(
)
{
}
}
alert
(
User)
;
// class User { ... }
There are other differences, we’ll see them soon.
-
Class methods are non-enumerable. A class definition sets
enumerable
flag tofalse
for all methods in the"prototype"
.That’s good, because if we
for..in
over an object, we usually don’t want its class methods. -
Classes always
use strict
. All code inside the class construct is automatically in strict mode.
Besides, class
syntax brings many other features that we’ll explore later.
Class Expression
Just like functions, classes can be defined inside another expression, passed around, returned, assigned, etc.
Here’s an example of a class expression:
let
User =
class
{
sayHi
(
)
{
alert
(
"Hello"
)
;
}
}
;
Similar to Named Function Expressions, class expressions may have a name.
If a class expression has a name, it’s visible inside the class only:
// "Named Class Expression"
// (no such term in the spec, but that's similar to Named Function Expression)
let
User =
class
MyClass
{
sayHi
(
)
{
alert
(
MyClass)
;
// MyClass name is visible only inside the class
}
}
;
new
User
(
)
.
sayHi
(
)
;
// works, shows MyClass definition
alert
(
MyClass)
;
// error, MyClass name isn't visible outside of the class
We can even make classes dynamically “on-demand”, like this:
function
makeClass
(
phrase
)
{
// declare a class and return it
return
class
{
sayHi
(
)
{
alert
(
phrase)
;
}
}
;
}
// Create a new class
let
User =
makeClass
(
"Hello"
)
;
new
User
(
)
.
sayHi
(
)
;
// Hello
Getters/setters
Just like literal objects, classes may include getters/setters, computed properties etc.
Here’s an example for user.name
implemented using get/set
:
class
User
{
constructor
(
name
)
{
// invokes the setter
this
.
name =
name;
}
get
name
(
)
{
return
this
.
_name;
}
set
name
(
value
)
{
if
(
value.
length <
4
)
{
alert
(
"Name is too short."
)
;
return
;
}
this
.
_name =
value;
}
}
let
user =
new
User
(
"John"
)
;
alert
(
user.
name)
;
// John
user =
new
User
(
""
)
;
// Name is too short.
Technically, such class declaration works by creating getters and setters in User.prototype
.
Computed names […]
Here’s an example with a computed method name using brackets [...]
:
class
User
{
[
'say'
+
'Hi'
]
(
)
{
alert
(
"Hello"
)
;
}
}
new
User
(
)
.
sayHi
(
)
;
Such features are easy to remember, as they resemble that of literal objects.
Class fields
Class fields are a recent addition to the language.
Previously, our classes only had methods.
“Class fields” is a syntax that allows to add any properties.
For instance, let’s add name
property to class User
:
class
User
{
name =
"John"
;
sayHi
(
)
{
alert
(
`
Hello,
${
this
.
name}
!
`
)
;
}
}
new
User
(
)
.
sayHi
(
)
;
// Hello, John!
So, we just write “
The important difference of class fields is that they are set on individual objects, not User.prototype
:
class
User
{
name =
"John"
;
}
let
user =
new
User
(
)
;
alert
(
user.
name)
;
// John
alert
(
User
.
prototype.
name)
;
// undefined
We can also assign values using more complex expressions and function calls:
class
User
{
name =
prompt
(
"Name, please?"
,
"John"
)
;
}
let
user =
new
User
(
)
;
alert
(
user.
name)
;
// John
Making bound methods with class fields
As demonstrated in the chapter Function binding functions in JavaScript have a dynamic this
. It depends on the context of the call.
So if an object method is passed around and called in another context, this
won’t be a reference to its object any more.
For instance, this code will show undefined
:
class
Button
{
constructor
(
value
)
{
this
.
value =
value;
}
click
(
)
{
alert
(
this
.
value)
;
}
}
let
button =
new
Button
(
"hello"
)
;
setTimeout
(
button.
click,
1000
)
;
// undefined
The problem is called “losing this
”.
There are two approaches to fixing it, as discussed in the chapter Function binding:
- Pass a wrapper-function, such as
setTimeout(() => button.click(), 1000)
. - Bind the method to object, e.g. in the constructor.
Class fields provide another, quite elegant syntax:
class
Button
{
constructor
(
value
)
{
this
.
value =
value;
}
click
=
(
)
=>
{
alert
(
this
.
value)
;
}
}
let
button =
new
Button
(
"hello"
)
;
setTimeout
(
button.
click,
1000
)
;
// hello
The class field click = () => {...}
is created on a per-object basis, there’s a separate function for each Button
object, with this
inside it referencing that object. We can pass button.click
around anywhere, and the value of this
will always be correct.
That’s especially useful in browser environment, for event listeners.
Summary
The basic class syntax looks like this:
class
MyClass
{
prop =
value;
// property
constructor
(
...
)
{
// constructor
// ...
}
method
(
...
)
{
}
// method
get
something
(
...
)
{
}
// getter method
set
something
(
...
)
{
}
// setter method
[
Symbol.
iterator]
(
)
{
}
// method with computed name (symbol here)
// ...
}
MyClass
is technically a function (the one that we provide as constructor
), while methods, getters and setters are written to MyClass.prototype
.
In the next chapters we’ll learn more about classes, including inheritance and other features.
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