In this article, we'll look at fundamental JavaScript object syntax, and revisit some JavaScript features that we've already seen earlier in the course, reiterating the fact that many of the features you've already dealt with are objects.
| Prerequisites: | Basic computer literacy, a basic understanding of HTML and CSS, familiarity with JavaScript basics (see 第一步 and Building blocks ). |
|---|---|
| Objective: | To understand the basics of working with objects in JavaScript: creating objects, accessing and modifying object properties, and using constructors. |
An object is a collection of related data and/or functionality. These usually consist of several variables and functions (which are called properties and methods when they are inside objects). Let's work through an example to understand what they look like.
To begin with, make a local copy of our
oojs.html
file. This contains very little — a
<script>
element for us to write our source code into. We'll use this as a basis for exploring basic object syntax. While working with this example you should have your
developer tools JavaScript console
open and ready to type in some commands.
As with many things in JavaScript, creating an object often begins with defining and initializing a variable. Try entering the following line below the JavaScript code that's already in your file, then saving and refreshing:
const person = {};
Now open your browser's
JavaScript console
, enter
person
into it, and press
Enter
/
返回
. You should get a result similar to one of the below lines:
[object Object]
Object { }
{ }
Congratulations, you've just created your first object. Job done! But this is an empty object, so we can't really do much with it. Let's update the JavaScript object in our file to look like this:
const person = {
name: ['Bob', 'Smith'],
age: 32,
bio: function() {
console.log(`${this.name[0]} ${this.name[1]} is ${this.age} years old.`);
},
introduceSelf: function() {
console.log(`Hi! I'm ${this.name[0]}.`);
}
};
After saving and refreshing, try entering some of the following into the JavaScript console on your browser devtools:
person.name
person.name[0]
person.age
person.bio()
person.introduceSelf()
You have now got some data and functionality inside your object, and are now able to access them with some nice simple syntax!
So what is going on here? Well, an object is made up of multiple members, each of which has a name (e.g.
名称
and
age
above), and a value (e.g.
['Bob', 'Smith']
and
32
). Each name/value pair must be separated by a comma, and the name and value in each case are separated by a colon. The syntax always follows this pattern:
const objectName = {
member1Name: member1Value,
member2Name: member2Value,
member3Name: member3Value
};
The value of an object member can be pretty much anything — in our person object we've got a number, an array, and two functions. The first four items are data items, and are referred to as the object's properties . The last two items are functions that allow the object to do something with that data, and are referred to as the object's 方法 .
When the object's members are functions there's a simpler syntax. Instead of
bio: function()
we can write
bio()
. Like this:
const person = {
name: ['Bob', 'Smith'],
age: 32,
bio() {
console.log(`${this.name[0]} ${this.name[1]} is ${this.age} years old.`);
},
introduceSelf() {
console.log(`Hi! I'm ${this.name[0]}.`);
}
};
From now on, we'll use this shorter syntax.
An object like this is referred to as an object literal — we've literally written out the object contents as we've come to create it. This is in contrast to objects instantiated from classes, which we'll look at later on.
It is very common to create an object using an object literal when you want to transfer a series of structured, related data items in some manner, for example sending a request to the server to be put into a database. Sending a single object is much more efficient than sending several items individually, and it is easier to work with than an array, when you want to identify individual items by name.
Above, you accessed the object's properties and methods using 点表示法 . The object name (person) acts as the namespace — it must be entered first to access anything inside the object. Next you write a dot, then the item you want to access — this can be the name of a simple property, an item of an array property, or a call to one of the object's methods, for example:
person.age
person.bio()
An object property can itself be an object. For example, try changing the
名称
member from
name: ['Bob', 'Smith'],
to
name : {
first: 'Bob',
last: 'Smith'
},
To access these items you just need to chain the extra step onto the end with another dot. Try these in the JS console:
person.name.first
person.name.last
If you do this, you'll also need to go through your method code and change any instances of
name[0]
name[1]
to
name.first
name.last
Otherwise your methods will no longer work.
There is another way to access object properties — using bracket notation. Instead of using these:
person.age
person.name.first
可以使用
person['age']
person['name']['first']
This looks very similar to how you access the items in an array, and it is basically the same thing — instead of using an index number to select an item, you are using the name associated with each member's value. It is no wonder that objects are sometimes called associative arrays — they map strings to values in the same way that arrays map numbers to values.
So far we've only looked at retrieving (or getting ) object members — you can also set (update) the value of object members by declaring the member you want to set (using dot or bracket notation), like this:
person.age = 45;
person['name']['last'] = 'Cratchit';
Try entering the above lines, and then getting the members again to see how they've changed, like so:
person.age
person['name']['last']
Setting members doesn't just stop at updating the values of existing properties and methods; you can also create completely new members. Try these in the JS console:
person['eyes'] = 'hazel';
person.farewell = function() {
console.log('Bye everybody!');
}
You can now test out your new members:
person['eyes']
person.farewell()
One useful aspect of bracket notation is that it can be used to set not only member values dynamically, but member names too. Let's say we wanted users to be able to store custom value types in their people data, by typing the member name and value into two text inputs. We could get those values like this:
const myDataName = nameInput.value;
const myDataValue = nameValue.value;
We could then add this new member name and value to the
person
object like this:
person[myDataName] = myDataValue;
To test this, try adding the following lines into your code, just below the closing curly brace of the
person
对象：
const myDataName = 'height';
const myDataValue = '1.75m';
person[myDataName] = myDataValue;
Now try saving and refreshing, and entering the following into your text input:
person.height
Adding a property to an object using the method above isn't possible with dot notation, which can only accept a literal member name, not a variable value pointing to a name.
You may have noticed something slightly strange in our methods. Look at this one for example:
introduceSelf() {
console.log(`Hi! I'm ${this.name[0]}.`);
}
You are probably wondering what "this" is. The
this
keyword refers to the current object the code is being written inside — so in this case
this
相当于
person
. So why not just write
person
instead?
Well, when you only have to create a single object literal, it's not so useful. But if you create more than one,
this
enables you to use the same method definition for every object you create.
Let's illustrate what we mean with a simplified pair of person objects:
const person1 = {
name: 'Chris',
introduceSelf() {
console.log(`Hi! I'm ${this.name}.`);
}
}
const person2 = {
name: 'Deepti',
introduceSelf() {
console.log(`Hi! I'm ${this.name}.`);
}
}
在此情况下，
person1.introduceSelf()
outputs "Hi! I'm Chris.";
person2.introduceSelf()
on the other hand outputs "Hi! I'm Deepti.", even though the method's code is exactly the same in each case. This isn't hugely useful when you are writing out object literals by hand, but it will be essential when we start using
constructors
to create more than one object from a single object definition, and that's the subject of the next section.
Using object literals is fine when you only need to create one object, but if you have to create more than one, as in the previous section, they're seriously inadequate. We have to write out the same code for every object we create, and if we want to change some properties of the object - like adding a
height
property - then we have to remember to update every object.
We would like a way to define the "shape" of an object — the set of methods and the properties it can have — and then create as many objects as we like, just updating the values for the properties that are different.
The first version of this is just a function:
function createPerson(name) {
const obj = {};
obj.name = name;
obj.introduceSelf = function() {
console.log(`Hi! I'm ${this.name}.`);
}
return obj;
}
This function creates and returns a new object each time we call it. The object will have two members:
名称
introduceSelf()
.
注意，
createPerson()
takes a parameter
名称
to set the value of the
名称
property, but the value of the
introduceSelf()
method will be the same for all objects created using this function. This is a very common pattern for creating objects. You can see here how being able to use
this
in the definition of
introduceSelf()
enables us to use the same code for every object we create.
Now we can create as many objects as we like, reusing the definition:
const salva = createPerson('Salva');
salva.name;
salva.introduceSelf();
const frankie = createPerson('Frankie');
frankie.name;
frankie.introduceSelf();
This works fine but is a bit long-winded: we have to create an empty object, initialize it, and return it. A better way is to use a
构造函数
. A constructor is just a function called using the
new
keyword. When you call a constructor, it will:
this
to the new object, so you can refer to
this
in your constructor code
Constructors, by convention, start with a capital letter and are named for the type of object they create. So we could rewrite our example like this:
function Person(name) {
this.name = name;
this.introduceSelf = function() {
console.log(`Hi! I'm ${this.name}.`);
}
}
To call
Person()
as a constructor, we use
new
:
const salva = new Person('Salva');
salva.name;
salva.introduceSelf();
const frankie = new Person('Frankie');
frankie.name;
frankie.introduceSelf();
As you've been going through these examples, you have probably been thinking that the dot notation you've been using is very familiar. That's because you've been using it throughout the course! Every time we've been working through an example that uses a built-in browser API or JavaScript object, we've been using objects, because such features are built using exactly the same kind of object structures that we've been looking at here, albeit more complex ones than in our own basic custom examples.
So when you used string methods like:
myString.split(',');
You were using a method available on a
字符串
object. Every time you create a string in your code, that string is automatically created as an instance of
字符串
, and therefore has several common methods and properties available on it.
When you accessed the document object model using lines like this:
const myDiv = document.createElement('div');
const myVideo = document.querySelector('video');
You were using methods available on a
Document
object. For each webpage loaded, an instance of
Document
is created, called
document
, which represents the entire page's structure, content, and other features such as its URL. Again, this means that it has several common methods and properties available on it.
The same is true of pretty much any other built-in object or API you've been using —
数组
,
数学
, and so on.
Note that built in objects and APIs don't always create object instances automatically. As an example, the Notifications API — which allows modern browsers to fire system notifications — requires you to instantiate a new object instance using the constructor for each notification you want to fire. Try entering the following into your JavaScript console:
const myNotification = new Notification('Hello!');
You've reached the end of this article, but can you remember the most important information? You can find some further tests to verify that you've retained this information before you move on — see Test your skills: Object basics .
Congratulations, you've reached the end of our first JS objects article — you should now have a good idea of how to work with objects in JavaScript — including creating your own simple objects. You should also appreciate that objects are very useful as structures for storing related data and functionality — if you tried to keep track of all the properties and methods in our
person
object as separate variables and functions, it would be inefficient and frustrating, and we'd run the risk of clashing with other variables and functions that have the same names. Objects let us keep the information safely locked away in their own package, out of harm's way.
In the next article we'll look at prototypes , which is the fundamental way that JavaScript lets an object inherit properties from other objects.
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