Object prototypes
Prototypes are the mechanism by which JavaScript objects inherit features from one another. In this article, we explain what a prototype is, how prototype chains work, and how a prototype for an object can be set.
| Prerequisites: | Understanding JavaScript functions, familiarity with JavaScript basics (see First steps and Building blocks ), and OOJS basics (see Introduction to objects ). |
|---|---|
| Objective: | To understand JavaScript object prototypes, how prototype chains work, and how to set the prototype of an object. |
The prototype chain
In the browser's console, try creating an object literal:
const
myObject =
{
city
:
"Madrid"
,
greet
(
)
{
console.
log
(
`
Greetings from
${
this
.
city}
`
)
;
}
,
}
;
myObject.
greet
(
)
;
// Greetings from Madrid
This is an object with one data property, city
, and one method, greet()
. If you type the object's name followed by a period
into the console, like myObject.
, then the console will pop up a list of all the properties available to this object. You'll see that as well as city
and greet
, there are lots of other properties!
__defineGetter__ __defineSetter__ __lookupGetter__ __lookupSetter__ __proto__ city constructor greet hasOwnProperty isPrototypeOf propertyIsEnumerable toLocaleString toString valueOf
Try accessing one of them:
myObject.
toString
(
)
;
// "[object Object]"
It works (even if it's not obvious what toString()
does).
What are these extra properties, and where do they come from?
Every object in JavaScript has a built-in property, which is called its prototype
. The prototype is itself an object, so the prototype will have its own prototype, making what's called a prototype chain
. The chain ends when we reach a prototype that has null
for its own prototype.
Note:
The property of an object that points to its prototype is not
called prototype
. Its name is not standard, but in practice all browsers use
__proto__
. The standard way to access an object's prototype is the
Object.getPrototypeOf()
method.
When you try to access a property of an object: if the property can't be found in the object itself, the prototype is searched for the property. If the property still can't be found, then the prototype's prototype is searched, and so on until either the property is found, or the end of the chain is reached, in which case undefined
is returned.
So when we call myObject.toString()
, the browser:
-
looks for
toStringinmyObject -
can't find it there, so looks in the prototype object of
myObjectfortoString - finds it there, and calls it.
What is the prototype for myObject
? To find out, we can use the function Object.getPrototypeOf()
:
Object.
getPrototypeOf
(
myObject)
;
// Object { }
This is an object called Object.prototype
, and it is the most basic prototype, that all objects have by default. The prototype of Object.prototype
is null
, so it's at the end of the prototype chain:
The prototype of an object is not always Object.prototype
. Try this:
const
myDate =
new
Date
(
)
;
let
object =
myDate;
do
{
object =
Object.
getPrototypeOf
(
object)
;
console.
log
(
object)
;
}
while
(
object)
;
// Date.prototype
// Object { }
// null
This code creates a Date
object, then walks up the prototype chain, logging the prototypes. It shows us that the prototype of myDate
is a Date.prototype
object, and the prototype of that
is Object.prototype
.
In fact, when you call familiar methods, like myDate2.getMonth()
,
you are calling a method that's defined on Date.prototype
.
Shadowing properties
What happens if you define a property in an object, when a property with the same name is defined in the object's prototype? Let's see:
const
myDate =
new
Date
(
1995
,
11
,
17
)
;
console.
log
(
myDate.
getYear
(
)
)
;
// 95
myDate.
getYear
=
function
(
)
{
console.
log
(
"something else!"
)
;
}
;
myDate.
getYear
(
)
;
// 'something else!'
This should be predictable, given the description of the prototype chain. When we call getYear()
the browser first looks in myDate
for a property with that name, and only checks the prototype if myDate
does not define it. So when we add getYear()
to myDate
, then the version in myDate
is called.
This is called "shadowing" the property.
Setting a prototype
There are various ways of setting an object's prototype in JavaScript, and here we'll describe two: Object.create()
and constructors.
Using Object.create
The Object.create()
method creates a new object and allows you to specify an object that will be used as the new object's prototype.
Here's an example:
const
personPrototype =
{
greet
(
)
{
console.
log
(
"hello!"
)
;
}
,
}
;
const
carl =
Object.
create
(
personPrototype)
;
carl.
greet
(
)
;
// hello!
Here we create an object personPrototype
, which has a greet()
method. We then use Object.create()
to create a new object with personPrototype
as its prototype. Now we can call greet()
on the new object, and the prototype provides its implementation.
Using a constructor
In JavaScript, all functions have a property named prototype
. When you call a function as a constructor, this property is set as the prototype of the newly constructed object (by convention, in the property named __proto__
).
So if we set the prototype
of a constructor, we can ensure that all objects created with that constructor are given that prototype:
const
personPrototype =
{
greet
(
)
{
console.
log
(
`
hello, my name is
${
this
.
name}
!
`
)
;
}
,
}
;
function
Person
(
name
)
{
this
.
name =
name;
}
Object.
assign
(
Person
.
prototype,
personPrototype)
;
// or
// Person.prototype.greet = personPrototype.greet;
Here we create:
-
an object
personPrototype, which has agreet()method -
a
Person()constructor function which initializes the name of the person to create.
We then put the methods defined in personPrototype
onto the Person
function's prototype
property using Object.assign
.
After this code, objects created using Person()
will get Person.prototype
as their prototype, which automatically contains the greet
method.
const
reuben =
new
Person
(
"Reuben"
)
;
reuben.
greet
(
)
;
// hello, my name is Reuben!
This also explains why we said earlier that the prototype of myDate
is called Date.prototype
: it's the prototype
property of the Date
constructor.
Own properties
The objects we create using the Person
constructor above have two properties:
-
a
nameproperty, which is set in the constructor, so it appears directly onPersonobjects -
a
greet()method, which is set in the prototype.
It's common to see this pattern, in which methods are defined on the prototype, but data properties are defined in the constructor. That's because methods are usually the same for every object we create, while we often want each object to have its own value for its data properties (just as here where every person has a different name).
Properties that are defined directly in the object, like name
here, are called own properties
, and you can check whether a property is an own property using the static
Object.hasOwn()
method:
const
irma =
new
Person
(
"Irma"
)
;
console.
log
(
Object.
hasOwn
(
irma,
"name"
)
)
;
// true
console.
log
(
Object.
hasOwn
(
irma,
"greet"
)
)
;
// false
Note:
You can also use the non-static
Object.hasOwnProperty()
method here, but we recommend that you use Object.hasOwn()
if you can.
Prototypes and inheritance
Prototypes are a powerful and very flexible feature of JavaScript, making it possible to reuse code and combine objects.
In particular they support a version of inheritance . Inheritance is a feature of object-oriented programming languages that lets programmers express the idea that some objects in a system are more specialized versions of other objects.
For example, if we're modeling a school, we might have professors and students : they are both people , so have some features in common (for example, they both have names), but each might add extra features (for example, professors have a subject that they teach), or might implement the same feature in different ways. In an OOP system we might say that professors and students both inherit from people.
You can see how in JavaScript, if Professor
and Student
objects can have Person
prototypes, then they can inherit the common properties, while adding and redefining those properties which need to differ.
In the next article we'll discuss inheritance along with the other main features of object-oriented programming languages, and see how JavaScript supports them.
Summary
This article has covered JavaScript object prototypes, including how prototype object chains allow objects to inherit features from one another, the prototype property and how it can be used to add methods to constructors, and other related topics.
In the next article we'll look at the concepts underlying object-oriented programming.