http://killdream.github.com/blog/2011/10/understanding-javascript-oop/index.html(Thanks original writer for this )
1. Objects
Everything you can manipulate in JavaScript is an object. This includes Strings, Arrays,Numbers, Functions, and, obviously, the so-called Object — there are primitives, but they're converted to an object when you need to operate upon them. An object in the language is simply a collection of key/value pairs (and some internal magic sometimes).
There are no concepts of classes anywhere, though. That is, an object with propertiesname: Linda, age: 21 is not an instance of any class, the Object class. Both Object andLinda are instances of themselves. They define their own behaviour, directly. There are no layers of meta-data (i.e.: classes) to dictate what given object must look like.(js 没有类的概念)
You might ask: "how?"; more so if you come from a highly classically Object Orientated language (like Java or C#). "Wouldn't having each object defining their own behaviour, instead of a common class mean that if I have 100 objects, I will have 100 different methods? Also, isn't it dangerous? How would one know if an object is really an Array, for example?"
Well, to answer all those questions, we'll first need to unlearn everything about the classical OO approach and start from the ground up. But, trust me, it's worth it.
The prototypical OO model brings in some new ways of solving old problems, in an more dynamic and expressive way. It also presents new and more powerful models for extensibility and code-reuse, which is what most people are interested about when they talk about Object Orientation. It does not, however, give you contracts. Thus, there are no static guarantees that an object X will always have a given set of properties, but to understand the trade-offs here, we'll need to know what we're talking about first.
1.1. What are objects?
As mentioned previously, objects are simple pairs of unique keys that correspond to a value — we'll call this pair a property. So, suppose you'd want to describe a few aspects of an old friend (say Mikhail), like age, name and gender:
Objects are created in JavaScript using the Object.create function. It takes a parent and an optional set of property descriptors and makes a brand new instance. We'll not worry much about the parameters now.
An empty object is an object with no parent, and no properties. The syntax to create such object in JavaScript is the following:
var mikhail = Object.create(null)
1.2. Creating properties
So, now we have an object, but no properties — we've got to fix that if we want to describe Mikhail.
Properties in JavaScript are dynamic. That means that they can be created or removed at any time. Properties are also unique, in the sense that a property key inside an object correspond to exactly one value.
Creating new properties is done through the Object.defineProperty function, which takes a reference to an object, the name of the property to create and a descriptor that defines the semantics of the property.
Object.defineProperty(mikhail, 'name', { value: 'Mikhail'
, writable: true
, configurable: true
, enumerable: true })
Object.defineProperty(mikhail, 'age', { value: 19
, writable: true
, configurable: true
, enumerable: true })
Object.defineProperty(mikhail, 'gender', { value: 'Male'
, writable: true
, configurable: true
, enumerable: true })
Object.defineProperty will create a new property if a property with the given key does not exist in the object, otherwise it'll update the semantics and value of the existing property.
- By the way
-
You can also use the
Object.definePropertieswhen you need to add more than one property to an object:
Object.defineProperties(mikhail, { name: { value: 'Mikhail'
, writable: true
, configurable: true
, enumerable: true }
, age: { value: 19
, writable: true
, configurable: true
, enumerable: true }
, gender: { value: 'Male'
, writable: true
, configurable: true
, enumerable: true }})
Obviously, both calls are overtly verbose — albeit also quite configurable —, thus not really meant for end-user code. It's better to create an abstraction layer on top of them.
1.3. Descriptors
The little objects that carry the semantics of a property are called descriptors (we used them in the previous Object.defineProperty calls). Descriptors can be one of two types - data descriptors or accessor descriptors.
Both types of descriptor contain flags, which define how a property is treated in the language. If a flag is not set, it's assumed to be false — unfortunately this is usually not a good default value for them, which adds to the verbosity of these descriptors.
- writable
-
Whether the concrete value of the property may be changed. Only applies to data descriptors.
- configurable
-
Whether the type of descriptor may be changed, or if the property can be removed.
- enumerable
-
Whether the property is listed in a loop through the properties of the object.
Data descriptors are those that hold concrete values, and therefore have an additionalvalue parameter, describing the concrete data bound to the property:
- value
-
The value of a property.
Accessor descriptors, on the other hand, proxy access to the concrete value through getter and setter functions. When not set, they'll default to undefined.
- get ()
-
A function called with no arguments when the property value is requested.
- set (new_value)
-
A function called with the new value for the property when the user tries to modify the value of the property.
1.4. Ditching the verbosity
Luckily, property descriptors are not the only way of working with properties in JavaScript, they can also be handled in a sane and concise way.
JavaScript also understands references to a property using what we call bracket notation. The general rule is:
<bracket-access> ::= <identifier> "[" <expression> "]"
Where identifier is the variable that holds the object containing the properties we want to access, and expression is any valid JavaScript expression that defines the name of the property. There are no constraints in which name a property can have1, everything is fair game.
Thus, we could just as well rewrite our previous example as:
mikhail['name'] = 'Mikhail' mikhail['age'] = 19 mikhail['gender'] = 'Male'
- Note
-
All property names are ultimately converted to a String, such that
object[1],object[[1]],object['1']andobject[variable](when the variable resolves to1) are all equivalent.
There is another way of referring to a property called dot notation, which usually looks less cluttered and is easier to read than the bracket alternative. However, it only works when the property name is a valid JavaScript IdentifierName2, and doesn't allow for arbitrary expressions (so, variables here are a no-go).
The rule for dot notation is:
<dot-access> ::= <identifier> "." <identifier-name>
This would give us an even sweeter way of defining properties:
mikhail.name = 'Mikhail' mikhail.age = 19 mikhail.gender = 'Male'
Both of these syntaxes are equivalent to creating a data property, with all semantic flags set to true.
1.5. Accessing properties
Retrieving the values stored in a given property is as easy as creating new ones, and the syntax is mostly similar as well — the only difference being there isn't an assignment.
So, if we want to check on Mikhail's age:
mikhail['age'] // => 19
Trying to access a property that does not exist in the object simply returns undefined
mikhail['address'] // => undefined
1.6. Removing properties
To remove entire properties from an object, JavaScript provides the delete operator. So, if you wanted to remove the gender property from the mikhail object:
delete mikhail['gender'] // => true mikhail['gender'] // => undefined
The delete operator returns true if the property was removed, false otherwise. I won't delve into details of the workings of this operator, since @kangax has already written a most awesome article on how delete works.
1.7. Getters and setters
Getters and setters are usually used in classical object oriented languages to provide encapsulation. They are not much needed in JavaScript, though, given how dynamic the language is — and my bias against the feature.
At any rate, they allow you to proxy the requests for reading a property value or setting it, and decide how to handle each situation. So, suppose we had separate slots for our object's first and last name, but wanted a simple interface for reading and setting it.
First, let's set the first and last names of our friend, as concrete data properties:
bject.defineProperty(mikhail, 'first_name', { value: 'Mikhail'
, writable: true })
Object.defineProperty(mikhail, 'last_name', { value: 'Weiß'
, writable: true })
Then we can define a common way of accessing and setting both of those values at the same time — let's call it name:
// () → String
// Returns the full name of object.
function get_full_name() {
return this.first_name + ' ' + this.last_name
}
// (new_name:String) → undefined
// Sets the name components of the object, from a full name.
function set_full_name(new_name) { var names
names = new_name.trim().split(/\s+/)
this.first_name = names['0'] || ''
this.last_name = names['1'] || ''
}
Object.defineProperty(mikhail, 'name', { get: get_full_name
, set: set_full_name
, configurable: true
, enumerable: true })
Now, every-time we try to access the value of Mikhail's name property, it'll execute theget_full_name getter.
mikhail.name // => 'Mikhail Weiß' mikhail.first_name // => 'Mikhail' mikhail.last_name // => 'Weiß' mikhail.last_name = 'White' mikhail.name // => 'Mikhail White'
We can also set the name of the object, by assigning a value to the property, this will then execute set_full_name to do the dirty work.
mikhail.name = 'Michael White' mikhail.name // => 'Michael White' mikhail.first_name // => 'Michael' mikhail.last_name // => 'White'
Of course, getters and setters make property access and modification fairly slower. They do have some use-cases, but while browsers don't optimise them better, methods seem to be the way to go.
Also, it should be noted that while getters and setters are usually used for encapsulation in other languages, in ECMAScript 5 you still can't have such if you need the information to be stored in the object itself. All properties in an object are public.
1.8. Listing properties
Since properties are dynamic, JavaScript provides a way of checking out which properties an object defines. There are two ways of listing the properties of an object, depending on what kind of properties one is interested into.
The first one is done through a call to Object.getOwnPropertyNames, which returns anArray containing the names of all properties set directly in the object — we call these kind of property own, by the way.
If we check now what we know about Mikhail:
Object.getOwnPropertyNames(mikhail) // => [ 'name', 'age', 'gender', 'first_name', 'last_name' ]
1.9. Object literals
An even easier way of defining objects is to use the object literal (also called object initialiser) syntax that JavaScript provides. An object literal denotes a fresh object, that has its parent as the Object.prototype object. We'll talk more about parents when we visit inheritance, later on.
At any rate, the object literal syntax allows you to define simple objects and initialise it with properties at the same time. So, we could rewrite our Mikhail object to the following:
var mikhail = { first_name: 'Mikhail'
, last_name: 'Weiß'
, age: 19
, gender: 'Male'
// () → String
// Returns the full name of object.
, get name() {
return this.first_name + ' ' + this.last_name }
// (new_name:String) → undefined
// Sets the name components of the object,
// from a full name.
, set name(new_name) { var names
names = new_name.trim().split(/\s+/)
this.first_name = names['0'] || ''
this.last_name = names['1'] || '' }
}
Property names that are not valid identifiers must be quoted. Also note that the getter/setter notation for object literals strictly defines a new anonymous function. If you want to assign a previously declared function to a getter/setter, you need to use theObject.defineProperty function.
The rules for object literal can be described as the following:
<object-literal> ::= "{" <property-list> "}"
;
<property-list> ::= <property> ["," <property>]*
;
<property> ::= <data-property>
| <getter-property>
| <setter-property>
;
<data-property> ::= <property-name> ":" <expression>
;
<getter-property> ::= "get" <identifier>
: <function-parameters>
: <function-block>
;
<setter-property> ::= "set" <identifier>
: <function-parameters>
: <function-block>
;
<property-name> ::= <identifier>
| <quoted-identifier>
;
Object literals can only appear inside expressions in JavaScript. Since the syntax is ambiguous to block statements in the language, new-comers usually confound the two:
// This is a block statement, with a label:
{ foo: 'bar' }
// => 'bar'
// This is a syntax error (labels can't be quoted):
{ "foo": 'bar' }
// => SyntaxError: Invalid label
// This is an object literal (note the parenthesis to force
// parsing the contents as an expression):
({ "foo": 'bar' })
// => { foo: 'bar' }
// Where the parser is already expecting expressions,
// object literals don't need to be forced. E.g.:
var x = { foo: 'bar' }
fn({foo: 'bar'})
return { foo: 'bar' }
1, { foo: 'bar' }
( ... )
2. Methods
Up until now, the Mikhail object only defined slots of concrete data — with the exception of the name getter/setter. Defining actions that may be performed on a certain object in JavaScript is just as simple.
This is because JavaScript does not differentiate how you can manipulate a Function, aNumber or an Object. Everything is treated the same way (i.e.: functions in JavaScript are first-class).
As such, to define an action for a given object, you just assign a function object reference to a property. Let's say we wanted a way for Mikhail to greet someone:
// (person:String) → String
// Greets a random person
mikhail.greet = function(person) {
return this.name + ': Why, hello there, ' + person + '.'
}
After setting the property, we can use it the same way we used the concrete data that were assigned to the object. That is, accessing the property will return a reference to the function object stored there, so we can just call.
mikhail.greet('you')
// => 'Michael White: Why, hello there, you.'
mikhail.greet('Kristin')
// => 'Michael White: Why, hello there, Kristin.'
2.1. Dynamic this
One thing that you must have noticed both in the greet function, and the functions we've used for the name's getter/setter, is that they use a magical variable called this.
It holds a reference to the object that the function is being applied to. This doesn't necessarily means that this will equal the object where the function is stored. No, JavaScript is not so selfish.
Functions are generics. That is, in JavaScript, what this refers to is decided dynamically, at the time the function is called, and depending only on how such a function is called.
Having this dynamically resolved is an incredible powerful mechanism for the dynamism of JavaScript's object orientation and lack of strictly enforced structures (i.e.: classes), this means one can apply a function to any object that meets the requirements of the actions it performs, regardless of how the object has been constructed — hack in some custom multiple dispatcher and you have CLOS.
2.2. How this is resolved
There are four different ways of resolving the this variable in a function, depending on how a function is called: directly; as a method; explicitly applied; as a constructor. We'll dive in the first three for now, and come back at constructors later on.
For the following examples, we'll take these definitions into account:
// (other:Number[, yet_another:Number]) → Number
// Returns the sum of the object's value with the given Number
function add(other, yet_another) {
return this.value + other + (yet_another || 0)
}
var one = { value: 1, add: add }
var two = { value: 2, add: add }
2.2.1. Called as a method
If a function is called as an object's method, then this inside the function will refer to the object. That is, when we explicitly state that an object is carrying an action, then that object will be our this inside the function.
This is what happened when we called mikhail.greet(). The property access at the time of the call tells JavaScript that we want to apply whatever actions the greet function defines to the mikhail object.
one.add(two.value) // this === one // => 3 two.add(3) // this === two // => 5 one['add'](two.value) // brackets are cool too // => 3
2.2.2. Called directly
When a function is called directly, this will be resolved to the global object in the engine (e.g.: window in browsers, global in Node.js)
add(two.value) // this === global // => NaN // The global object still has no `value' property, let's fix that. value = 2 add(two.value) // this === global // => 4
2.2.3. Explicitly applied
Finally, a function may be explicitly applied to any object, regardless of whether the object has the function stored as a property or not. These applications are done through a either the call or apply method of a function object.
The difference between these two methods is the way they take in the parameters that will be passed to the function, and the performance — apply being up to 55x slower than a direct call, whereas call is usually not as bad. This might vary greatly depending on the engine though, so it's always better to do a Perf test rather than being scared of using the functionality — don't optimise early!
Anyways, call expects the object that the function will be applied to as the first parameter, and the parameters to apply to the function as positional arguments:
add.call(two, 2, 2) // this === two // => 6 add.call(window, 4) // this === global // => 6 add.call(one, one.value) // this === one // => 2
On the other hand, apply lets you pass an array of parameters as the second parameter of the function. The array will be passed as positional arguments to the target function:
dd.apply(two, [2, 2]) // equivalent to two.add(2, 2) // => 6 add.apply(window, [ 4 ]) // equivalent to add(4) // => 6 add.apply(one, [one.value]) // equivalent to one.add(one.value) // => 2
Note
What this resolves to when applying a function to null or undefined depends on the semantics used by the engine. Usually, it would be the same as explicitly applying the function to the global object. But if the engine is running on strict mode, then thiswill be resolved as expected — to the exact thing it was applied to:
window.value = 2
add.call(undefined, 1) // this === window
// => 3
void function() {
"use strict"
add.call(undefined, 1) // this === undefined
// => NaN
// Since primitives can't hold properties.
}()
2.3. Bound methods
Aside from the dynamic nature of functions in JavaScript, there is also a way of making a function bound to an specific object, such that this inside that function will always resolve to the given object, regardless of whether it's called as that object's method or directly.
The function that provides such functionality is bind. It takes an object, and additional parameters (in the same manner as call), and returns a new function that will apply those parameters to the original function when called:
var one_add = add.bind(one) one_add(2) // this === one // => 3 two.one_adder = one_add two.one_adder(2) // this === one // => 3 one_add.call(two) // this === one // => 3
3. Inheritance
Up to this point we have seen how objects can define their own behaviours, and how we can reuse (by explicit application) actions in other objects, however, this still doesn't give us a nice way for code reuse and extensibility.
That's where inheritance comes into play. Inheritance allows for a greater separation of concerns, where objects define specialised behaviours by building upon the behaviours of other objects.
The prototypical model goes further than that, though, and allows for selective extensibility, behaviour sharing and other interesting patterns we'll explore in a bit. Sad thing is: the specific model of prototypical OO implemented by JavaScript is a bit limited, so circumventing these limitations to accommodate these patterns will bring in a bit of overhead sometimes.
3.1. Prototypes
Inheritance in JavaScript revolves around cloning the behaviours of an object and extending it with specialised behaviours. The object that has it's behaviours cloned is called Prototype (not to be confounded with the prototype property of functions).
A prototype is just a plain object, that happens to share it's behaviours with another object — it acts as the object's parent.
Now, the concepts of this behaviour cloning does not imply that you'll have two different copies of the same function, or data. In fact, JavaScript implements inheritance by delegation, all properties are kept in the parent, and access to them is just extended for the child.
As mentioned previously, the parent (or [[Prototype]]) of an object is defined by making a call to Object.create, and passing a reference of the object to use as parent in the first parameter.
This would come well in our example up until now. For example, the greeting and name actions can be well defined in a separate object and shared with other objects that need them.
Which takes us to the following model:
We can implement this in JavaScript with the following definitions:
var person = Object.create(null)
// Here we are reusing the previous getter/setter functions
Object.defineProperty(person, 'name', { get: get_full_name
, set: set_full_name
, configurable: true
, enumerable: true })
// And adding the `greet' function
person.greet = function (person) {
return this.name + ': Why, hello there, ' + person + '.'
}
// Then we can share those behaviours with Mikhail
// By creating a new object that has it's [[Prototype]] property
// pointing to `person'.
var mikhail = Object.create(person)
mikhail.first_name = 'Mikhail'
mikhail.last_name = 'Weiß'
mikhail.age = 19
mikhail.gender = 'Male'
// And we can test whether things are actually working.
// First, `name' should be looked on `person'
mikhail.name
// => 'Mikhail Weiß'
// Setting `name' should trigger the setter
mikhail.name = 'Michael White'
// Such that `first_name' and `last_name' now reflect the
// previously name setting.
mikhail.first_name
// => 'Michael'
mikhail.last_name
// => 'White'
// `greet' is also inherited from `person'.
mikhail.greet('you')
// => 'Michael White: Why, hello there, you.'
// And just to be sure, we can check which properties actually
// belong to `mikhail'
Object.keys(mikhail)
// => [ 'first_name', 'last_name', 'age', 'gender' ]
3.2. How [[Prototype]] works
As you could see from the previous example, none of the properties defined in Personhave flown to the Mikhail object, and yet we could access them just fine. This happens because JavaScript implements delegated property access, that is, a property is searched through all parents of an object.
This parent chain is defined by a hidden slot in every object, called [[Prototype]]. You can't change this slot directly4, so the only way of setting it is when you're creating a fresh object.
When a property is requested from the object, the engine first tries to retrieve the property from the target object. If the property isn't there, the search continue through the immediate parent of that object, and the parent of that parent, and so on.
This means that we can change the behaviours of a prototype at run time, and have it reflected in all objects that inherit from it. For example, let's suppose we wanted a different default greeting:
// (person:String) → String
// Greets the given person
person.greet = function(person) {
return this.name + ': Harro, ' + person + '.'
}
mikhail.greet('you')
// => 'Michael White: Harro, you.'
3.3. Overriding properties
So, prototypes (that is, inheritance) are used for sharing data with other objects, and it does such in a pretty fast and memory-effective manner too, since you'll always have only one instance of a given piece of data lying around.
Now what if we want to add specialised behaviours, that build upon the data that was shared with the object? Well, we have seen before that objects define their own behaviours by means of properties, so specialised behaviours follow the same principle — you just assign a value to the relevant property.
To better demonstrate it, suppose Person implements only a general greeting, and everyone inheriting from Person define their own specialised and unique greetings. Also, let's add a new person to our case scenario, so to outline better how objects are extended:
Note that both mikhail and kristin define their own version of greet. In this case, whenever we call the greet method on them they'll use their own version of that behaviour, instead of the one that was shared from person.
// Here we set up the greeting for a generic person
// (person:String) → String
// Greets the given person, formally
person.greet = function(person) {
return this.name + ': Hello, ' + (person || 'you')
}
// And a greeting for our protagonist, Mikhail
// (person:String) → String
// Greets the given person, like a bro
mikhail.greet = function(person) {
return this.name + ': \'sup, ' + (person || 'dude')
}
// And define our new protagonist, Kristin
var kristin = Object.create(person)
kristin.first_name = 'Kristin'
kristin.last_name = 'Weiß'
kristin.age = 19
kristin.gender = 'Female'
// Alongside with her specific greeting manners
// (person:String) → String
// Greets the given person, sweetly
kristin.greet = function(person) {
return this.name + ': \'ello, ' + (person || 'sweetie')
}
// Finally, we test if everything works according to the expected
mikhail.greet(kristin.first_name)
// => 'Michael White: \'sup, Kristin'
mikhail.greet()
// => 'Michael White: \'sup, dude'
kristin.greet(mikhail.first_name)
// => 'Kristin Weiß: \'ello, Michael'
// And just so we check how cool this [[Prototype]] thing is,
// let's get Kristin back to the generic behaviour
delete kristin.greet
// => true
kristin.greet(mikhail.first_name)
// => 'Kristin Weiß: Hello, Michael'
3.4. Mixins
Prototypes allow for behaviour sharing in JavaScript, and although they are undeniably powerful, they aren't quite as powerful as they could be. For one, prototypes only allow that one object inherit from another single object, while extending those behaviours as they see fit.
However, this approach quickly kills interesting things like behaviour composition, where we could mix-and-match several objects into one, with all the advantages highlighted in the prototypical inheritance.
Multiple inheritance would also allow the usage of data-parents — objects that provide an example state that fulfils the requirements for a given behaviour. Default properties, if you will.
Luckily, since we can define behaviours directly on an object in JavaScript, we can work-around these issues by using mixins — and adding a little overhead at object's creation time.
So, what are mixins anyways? Well, they are parent-less objects. That is, they fully define their own behaviour, and are mostly designed to be incorporated in other objects (although you could use their methods directly).
Continuing with our little protagonists' scenario, let's extend it to add some capabilities to them. Let's say that every person can also be a pianist or a singer. A given person can have no such abilities, be just a pianist, just a singer or both. This is the kind of case where JavaScript's model of prototypical inheritance falls short, so we're going to cheat a little bit.
For mixins to work, we first need to have a way of combining different objects into a single one. JavaScript doesn't provide this out-of-the box, but we can easily make one by copying all own property descriptors, the ones defined directly in the object, rather than inherited, from one object to another.
// Aliases for the rather verbose methods on ES5
var descriptor = Object.getOwnPropertyDescriptor
, properties = Object.getOwnPropertyNames
, define_prop = Object.defineProperty
// (target:Object, source:Object) → Object
// Copies properties from `source' to `target'
function extend(target, source) {
properties(source).forEach(function(key) {
define_prop(target, key, descriptor(source, key)) })
return target
}
Basically, what extend does here is taking two objects — a source and a target, — iterating over all properties present on the source object, and copying the property descriptors over to target. Note that this is a destructive method, meaning that targetwill be modified in-place. It's the cheapest way, though, and usually not a problem.
Now that we have a method for copying properties over, we can start assigning multiple abilities to our objects (mikhail e kristin):
// A pianist is someone who can `play' the piano
var pianist = Object.create(null)
pianist.play = function() {
return this.name + ' starts playing the piano.'
}
// A singer is someone who can `sing'
var singer = Object.create(null)
singer.sing = function() {
return this.name + ' starts singing.'
}
// Then we can move on to adding those abilities to
// our main objects:
extend(mikhail, pianist)
mikhail.play()
// => 'Michael White starts playing the piano.'
// We can see that all that ends up as an own property of
// mikhail. It is not shared.
Object.keys(mikhail)
['first_name', 'last_name', 'age', 'gender', 'greet', 'play']
// Then we can define kristin as a singer
extend(kristin, singer)
kristin.sing()
// => 'Kristin Weiß starts singing.'
// Mikhail can't sing yet though
mikhail.sing()
// => TypeError: Object #<Object> has no method 'sing'
// But mikhail will inherit the `sing' method if we
// extend the Person prototype with it:
extend(person, singer)
mikhail.sing()
// => 'Michael White starts singing.'
3.5. Accessing overwritten properties
Now that we're able to inherit properties from other objects and extend the specialised objects to define their own behaviours, we have a little problem: what if we actually wanted to access the parent behaviours that we just overwrote?
JavaScript provides the Object.getPrototypeOf function, that returns the[[Prototype]] of an object. This way, we have access to all properties defined within the prototype chain of an object. So, accessing a property in the parent of an object is quite simple:
Object.getPrototypeOf(mikhail).name // same as `person.name' // => 'undefined undefined' // We can assert it's really being called on `person' by // giving `person' a `first_name' and `last_name' person.first_name = 'Random' person.last_name = 'Person' Object.getPrototypeOf(mikhail).name // => 'Random Person'
So, a naïve solution for applying a method stored in the [[Prototype]] of an object to the current one, would then follow, quite naturally, by looking the property on the[[Prototype]] of this:
var proto = Object.getPrototypeOf
// (name:String) → String
// Greets someone intimately if we know them, otherwise use
// the generic greeting
mikhail.greet = function(name) {
return name == 'Kristin Weiß'? this.name + ': Heya, Kristty'
: proto(this).greet.call(this, name)/* we dunno this guy */
}
mikhail.greet(kristin.name)
// => 'Michael White: Heya, Kristty'
mikhail.greet('Margareth')
// => 'Michael White: Hello, Margareth'
This looks all good and well, but there's a little catch: it will enter in endless recursion if you try to apply this approach to more than one parent. This happens because the methods are always applied in the context of the message's first target, making the[[Prototype]] lookup resolve always to the same object:
The simple solution to this, then, is to make all parent look-ups static, by passing the object where the current function is stored, rather than the object that the function was applied to.
So, the last example becomes:
var proto = Object.getPrototypeOf
// (name:String) → String
// Greets someone intimately if we know them, otherwise use
// the generic greeting.
//
// Note that now we explicitly state that the lookup should take
// the parent of `mikhail', so we can be assured the cyclic parent
// resolution above won't happen.
mikhail.greet = function(name) {
return name == 'Kristin Weiß'? this.name + ': Heya, Kristty'
: /* we dunno this guy */ proto(mikhail).greet.call(this, name)
}
mikhail.greet(kristin.name)
// => 'Michael White: Heya, Kristty'
mikhail.greet('Margareth')
// => 'Michael White: Hello, Margareth'
Still, this has quite some short-commings. First, since the object is hard-coded in the function, we can't just assign the function to any object and have it just work, as we did up 'till now. The function would always resolve to the parent of mikhail, not of the object where it's stored.
Likewise, we can't just apply the function to any object. The function is not generic anymore. Unfortunately, though, making the parent resolution dynamic would require us to pass an additional parameter to every function call, which is something that can't be achieved short of ugly hacks.
The approach proposed for the next version of JavaScript only solves the first problem, which is the easiest. Here, we'll do the same, by introducing a new way of defining methods. Yes, methods, not generic functions.
Functions that need to access the properties in the [[Prototype]] will require an additional information: the object where they are stored. This makes the lookup static, but solves our cyclic lookup problem.
We do this by introducing a new function — make_method — which creates a function that passes this information to the target function.
// (object:Object, fun:Function) → Function
// Creates a method
function make_method(object, fun) {
return function() { var args
args = slice.call(arguments)
args.unshift(object) // insert `object' as first parameter
fn.apply(this, args) }
}
// Now, all functions that are expected to be used as a method
// should remember to reserve the first parameter to the object
// where they're stored.
//
// Note that, however, this is a magical parameter introduced
// by the method function, so any function calling the method
// should pass only the usual arguments.
function message(self, message) { var parent
parent = Object.getPrototypeOf(self)
if (parent && parent.log)
parent.log.call(this, message)
console.log('-- At ' + self.name)
console.log(this.name + ': ' + message)
}
// Here we define a prototype chain C -> B -> A
var A = Object.create(null)
A.name = 'A'
A.log = make_method(A, message)
var B = Object.create(A)
B.name = 'B'
B.log = make_method(B, message)
var C = Object.create(B)
C.name = 'C'
C.log = make_method(C, message)
// And we can test if it works by calling the methods:
A.log('foo')
// => '-- At A'
// => 'A: foo'
B.log('foo')
// => '-- At A'
// => 'B: foo'
// => '-- At B'
// => 'B: foo'
C.log('foo')
// => '-- At A'
// => 'C: foo'
// => '-- At B'
// => 'C: foo'
// => '-- At C'
// => 'C: foo'
4. Constructors
Constructor functions are the old pattern for creating objects in JavaScript, which couple inheritance with initialisation in an imperative manner.
Constructor functions are not, however, a special construct in the language. Any simple function can be used as a constructor function; just like this, it all depends on how the function is called.
So, what's it about constructor functions, really? Well, every function object in JavaScript automatically gets a prototype property, that is a simple object with a constructorproperty pointing back to the constructor function. And this object is used to determine the [[Prototype]] of instances created with that constructor function.
The following diagram shows the objects for the constructor functionfunction Person(first_name, last_name):
4.1. The new magic
The prototype per se is not a special property, however it gains special meaning when a constructor function is used in conjunction with the new statement. As I said before, in this case the prototype property of the constructor function is used to provide the[[Prototype]] of the instance.
// Constructs a new Person
function Person(first_name, last_name) {
// If the function is called with `new', as we expect
// `this' here will be the freshly created object
// with the [[Prototype]] set to Person.prototype
//
// Of course, if someone omits new when calling the
// function, the usual resolution of `this' — as
// explained before — will take place.
this.first_name = first_name
this.last_name = last_name
}
// (person:String) → String
// Greets the given person
Person.prototype.greet = function(person) {
return this.name + ': Harro, ' + person + '.'
}
var person = new Person('Mikhail', 'Weiß')
// We could de-sugar the constructor pattern in the following
// Taking into account that `Person' here means the `prototype'
// property of the `Person' constructor.
var Person = Object.create(Object.prototype)
// (person:String) → String
// Greets the given person
Person.greet = function(person) {
return this.name + ': Harro, ' + person + '.'
}
// Here's what the constructor does when called with `new'
var person = Object.create(Person)
person.first_name = 'Mikhail'
person.last_name = 'Weiß'
When a function is called with the new statement, the following magic happens:
- Create a fresh
Object, inheriting fromObject.prototype, say{ } - Set the
[[Prototype]]internal property of the new object to point to the constructor'sprototypeproperty, so it inherits those behaviours. - Call the constructor in the context of this fresh object, such that
thisinside the constructor will be the fresh object, and pass any parameters given to the function. - If the function returns an
Object, make that be the return value of the function. - Otherwise, return the fresh object.
This means that the resulting value of calling a function with the new operator is not necessarily the object that was created. A function is free to return any other Objectvalue as it sees fit. This is an interesting and — to a certain extent — powerful behaviour, but also a confusing one for many newcomers:
function Foo() {
this.foo = 'bar'
}
new Foo()
// => { foo: 'bar' }
function Foo() {
this.foo = 'bar'
return Foo
}
new Foo()
// => [Function: Foo]
See also: http://www.phpied.com/3-ways-to-define-a-javascript-class/
http://stackoverflow.com/questions/387707/whats-the-best-way-to-define-a-class-in-javascript