JavaScript is designed on a simple object-based paradigm. An object is a collection of properties, and a property is an association between a name (or key ) and a value. A property's value can be a function, in which case the property is known as a method. In addition to objects that are predefined in the browser, you can define your own objects. This chapter describes how to use objects, properties, functions, and methods, and how to create your own objects.
Objects in JavaScript, just as in many other programming languages, can be compared to objects in real life. The concept of objects in JavaScript can be understood with real life, tangible objects.
In JavaScript, an object is a standalone entity, with properties and type. Compare it with a cup, for example. A cup is an object, with properties. A cup has a color, a design, weight, a material it is made of, etc. The same way, JavaScript objects can have properties, which define their characteristics.
A JavaScript object has properties associated with it. A property of an object can be explained as a variable that is attached to the object. Object properties are basically the same as ordinary JavaScript variables, except for the attachment to objects. The properties of an object define the characteristics of the object. You access the properties of an object with a simple dot-notation:
objectName.propertyName
Like all JavaScript variables, both the object name (which could be a normal variable) and property name are case sensitive. You can define a property by assigning it a value. For example, let's create an object named
myCar
and give it properties named
make
,
model
，和
year
as follows:
var myCar = new Object(); myCar.make = 'Ford'; myCar.model = 'Mustang'; myCar.year = 1969;
The above example could also be written using an
object initializer
, which is a comma-delimited list of zero or more pairs of property names and associated values of an object, enclosed in curly braces (
{}
):
var myCar = {
make: 'Ford',
model: 'Mustang',
year: 1969
};
Unassigned properties of an object are
undefined
(and not
null
).
myCar.color; // undefined
Properties of JavaScript objects can also be accessed or set using a bracket notation (for more details see
property accessors
). Objects are sometimes called
associative arrays
, since each property is associated with a string value that can be used to access it. So, for example, you could access the properties of the
myCar
object as follows:
myCar['make'] = 'Ford'; myCar['model'] = 'Mustang'; myCar['year'] = 1969;
An object property name can be any valid JavaScript string, or anything that can be converted to a string, including the empty string. However, any property name that is not a valid JavaScript identifier (for example, a property name that has a space or a hyphen, or that starts with a number) can only be accessed using the square bracket notation. This notation is also very useful when property names are to be dynamically determined (when the property name is not determined until runtime). Examples are as follows:
// four variables are created and assigned in a single go,
// separated by commas
var myObj = new Object(),
str = 'myString',
rand = Math.random(),
obj = new Object();
myObj.type = 'Dot syntax';
myObj['date created'] = 'String with space';
myObj[str] = 'String value';
myObj[rand] = 'Random Number';
myObj[obj] = 'Object';
myObj[''] = 'Even an empty string';
console.log(myObj);
Please note that all keys in the square bracket notation are converted to string unless they're Symbols, since JavaScript object property names (keys) can only be strings or Symbols (at some point, private names will also be added as the
class fields proposal
progresses, but you won't use them with
[]
form). For example, in the above code, when the key
obj
is added to the
myObj
, JavaScript will call the
obj.toString()
method, and use this result string as the new key.
You can also access properties by using a string value that is stored in a variable:
var propertyName = 'make'; myCar[propertyName] = 'Ford'; propertyName = 'model'; myCar[propertyName] = 'Mustang';
You can use the bracket notation with
for...in
to iterate over all the enumerable properties of an object. To illustrate how this works, the following function displays the properties of the object when you pass the object and the object's name as arguments to the function:
function showProps(obj, objName) {
var result = ``;
for (var i in obj) {
// obj.hasOwnProperty() is used to filter out properties from the object's prototype chain
if (obj.hasOwnProperty(i)) {
result += `${objName}.${i} = ${obj[i]}\n`;
}
}
return result;
}
So, the function call
showProps(myCar, "myCar")
would return the following:
myCar.make = Ford myCar.model = Mustang myCar.year = 1969
Starting with ECMAScript 5 , there are three native ways to list/traverse object properties:
for...in
loops
Object.keys(o)
o
.
Object.getOwnPropertyNames(o)
o
.
Before ECMAScript 5, there was no native way to list all properties of an object. However, this can be achieved with the following function:
function listAllProperties(o) {
var objectToInspect;
var result = [];
for(objectToInspect = o; objectToInspect !== null;
objectToInspect = Object.getPrototypeOf(objectToInspect)) {
result = result.concat(
Object.getOwnPropertyNames(objectToInspect)
);
}
return result;
}
This can be useful to reveal "hidden" properties (properties in the prototype chain which are not accessible through the object, because another property has the same name earlier in the prototype chain). Listing accessible properties only can easily be done by removing duplicates in the array.
JavaScript has a number of predefined objects. In addition, you can create your own objects. You can create an object using an
object initializer
. Alternatively, you can first create a constructor function and then instantiate an object invoking that function in conjunction with the
new
operator.
In addition to creating objects using a constructor function, you can create objects using an object initializer . Using object initializers is sometimes referred to as creating objects with literal notation. "Object initializer" is consistent with the terminology used by C++.
The syntax for an object using an object initializer is:
var obj = { property_1: value_1, // property_# may be an identifier...
2: value_2, // or a number...
// ...,
'property n': value_n }; // or a string
where
obj
is the name of the new object, each
property_
i
is an identifier (either a name, a number, or a string literal), and each
value_
i
is an expression whose value is assigned to the
property_
i
。
obj
and assignment is optional; if you do not need to refer to this object elsewhere, you do not need to assign it to a variable. (Note that you may need to wrap the object literal in parentheses if the object appears where a statement is expected, so as not to have the literal be confused with a block statement.)
Object initializers are expressions, and each object initializer results in a new object being created whenever the statement in which it appears is executed. Identical object initializers create distinct objects that will not compare to each other as equal. Objects are created as if a call to
new Object()
were made; that is, objects made from object literal expressions are instances of
Object
.
The following statement creates an object and assigns it to the variable
x
if and only if the expression
cond
is true:
if (cond) var x = {greeting: 'hi there'};
The following example creates
myHonda
with three properties. Note that the
engine
property is also an object with its own properties.
var myHonda = {color: 'red', wheels: 4, engine: {cylinders: 4, size: 2.2}};
You can also use object initializers to create arrays. See array literals .
Alternatively, you can create an object with these two steps:
new
.
To define an object type, create a function for the object type that specifies its name, properties, and methods. For example, suppose you want to create an object type for cars. You want this type of object to be called
Car
, and you want it to have properties for make, model, and year. To do this, you would write the following function:
function Car(make, model, year) {
this.make = make;
this.model = model;
this.year = year;
}
Notice the use of
this
to assign values to the object's properties based on the values passed to the function.
Now you can create an object called
mycar
as follows:
var mycar = new Car('Eagle', 'Talon TSi', 1993);
This statement creates
mycar
and assigns it the specified values for its properties. Then the value of
mycar.make
is the string "Eagle",
mycar.year
is the integer 1993, and so on.
You can create any number of
Car
objects by calls to
new
。例如，
var kenscar = new Car('Nissan', '300ZX', 1992);
var vpgscar = new Car('Mazda', 'Miata', 1990);
An object can have a property that is itself another object. For example, suppose you define an object called
person
as follows:
function Person(name, age, sex) {
this.name = name;
this.age = age;
this.sex = sex;
}
and then instantiate two new
person
objects as follows:
var rand = new Person('Rand McKinnon', 33, 'M');
var ken = new Person('Ken Jones', 39, 'M');
Then, you can rewrite the definition of
Car
to include an
owner
property that takes a
person
object, as follows:
function Car(make, model, year, owner) {
this.make = make;
this.model = model;
this.year = year;
this.owner = owner;
}
To instantiate the new objects, you then use the following:
var car1 = new Car('Eagle', 'Talon TSi', 1993, rand);
var car2 = new Car('Nissan', '300ZX', 1992, ken);
Notice that instead of passing a literal string or integer value when creating the new objects, the above statements pass the objects
rand
and
ken
as the arguments for the owners. Then if you want to find out the name of the owner of car2, you can access the following property:
car2.owner.name
Note that you can always add a property to a previously defined object. For example, the statement
car1.color = 'black';
adds a property
color
to car1, and assigns it a value of "black." However, this does not affect any other objects. To add the new property to all objects of the same type, you have to add the property to the definition of the
Car
object type.
Object.create
方法
Objects can also be created using the
Object.create()
method. This method can be very useful, because it allows you to choose the prototype object for the object you want to create, without having to define a constructor function.
// Animal properties and method encapsulation
var Animal = {
type: 'Invertebrates', // Default value of properties
displayType: function() { // Method which will display type of Animal
console.log(this.type);
}
};
// Create new animal type called animal1
var animal1 = Object.create(Animal);
animal1.displayType(); // Output:Invertebrates
// Create new animal type called Fishes
var fish = Object.create(Animal);
fish.type = 'Fishes';
fish.displayType(); // Output:Fishes
All objects in JavaScript inherit from at least one other object. The object being inherited from is known as the prototype, and the inherited properties can be found in the
prototype
object of the constructor. See
继承和原型链
了解更多信息。
You can refer to a property of an object either by its property name or by its ordinal index. If you initially define a property by its name, you must always refer to it by its name, and if you initially define a property by an index, you must always refer to it by its index.
This restriction applies when you create an object and its properties with a constructor function (as we did previously with the
Car
object type) and when you define individual properties explicitly (for example,
myCar.color = "red"
). If you initially define an object property with an index, such as
myCar[5] = "25 mpg"
, you subsequently refer to the property only as
myCar[5]
.
The exception to this rule is array-like object reflected from HTML, such as the
forms
array-like object. You can always refer to objects in these array-like objects by either their ordinal number (based on where they appear in the document) or their name (if defined). For example, if the second
<FORM>
tag in a document has a
NAME
attribute of "myForm", you can refer to the form as
document.forms[1]
or
document.forms["myForm"]
or
document.forms.myForm
.
You can add a property to a previously defined object type by using the
prototype
property. This defines a property that is shared by all objects of the specified type, rather than by just one instance of the object. The following code adds a
color
property to all objects of type
Car
, and then assigns a value to the
color
property of the object
car1
.
Car.prototype.color = null; car1.color = 'black';
见
prototype
property
of the
Function
object in the
JavaScript 参考
了解更多信息。
A method is a function associated with an object, or, simply put, a method is a property of an object that is a function. Methods are defined the way normal functions are defined, except that they have to be assigned as the property of an object. See also method definitions for more details. An example is:
objectName.methodname = functionName;
var myObj = {
myMethod: function(params) {
// ...do something
}
// OR THIS WORKS TOO
myOtherMethod(params) {
// ...do something else
}
};
where
objectName
is an existing object,
methodname
is the name you are assigning to the method, and
functionName
is the name of the function.
You can then call the method in the context of the object as follows:
object.methodname(params);
You can define methods for an object type by including a method definition in the object constructor function. You could define a function that would format and display the properties of the previously-defined
Car
objects; for example,
function displayCar() {
var result = `A Beautiful ${this.year} ${this.make} ${this.model}`;
pretty_print(result);
}
where
pretty_print
is a function to display a horizontal rule and a string. Notice the use of
this
to refer to the object to which the method belongs.
You can make this function a method of
Car
by adding the statement
this.displayCar = displayCar;
to the object definition. So, the full definition of
Car
would now look like
function Car(make, model, year, owner) {
this.make = make;
this.model = model;
this.year = year;
this.owner = owner;
this.displayCar = displayCar;
}
Then you can call the
displayCar
method for each of the objects as follows:
car1.displayCar(); car2.displayCar();
this
for object references
JavaScript has a special keyword,
this
, that you can use within a method to refer to the current object. For example, suppose you have 2 objects,
Manager
and
Intern
. Each object have their own
name
,
age
and
job
. In the function
sayHi()
, notice there is
this.name
. When added to the 2 objects they can be called and returns the
'Hello, My name is'
then adds the
name
value from that specific object. As shown below.
const Manager = {
name: "John",
age: 27,
job: "Software Engineer"
}
const Intern= {
name: "Ben",
age: 21,
job: "Software Engineer Intern"
}
function sayHi() {
console.log('Hello, my name is', this.name)
}
// add sayHi function to both objects
Manager.sayHi = sayHi;
Intern.sayHi = sayHi;
Manager.sayHi() // Hello, my name is John'
Intern.sayHi() // Hello, my name is Ben'
this
refers to the object that it is in. You can create a new function called
howOldAmI()
which logs a sentence saying how old the person is.
function howOldAmI (){
console.log('I am ' + this.age + ' years old.')
}
Manager.howOldAmI = howOldAmI;
Manager.howOldAmI() // I am 27 years old.
A getter is a method that gets the value of a specific property. A setter is a method that sets the value of a specific property. You can define getters and setters on any predefined core object or user-defined object that supports the addition of new properties.
Getters and setters can be either
When defining getters and setters using
object initializers
all you need to do is to prefix a getter method with
get
and a setter method with
set
. Of course, the getter method must not expect a parameter, while the setter method expects exactly one parameter (the new value to set). For instance:
var o = {
a: 7,
get b() {
return this.a + 1;
},
set c(x) {
this.a = x / 2;
}
};
console.log(o.a); // 7
console.log(o.b); // 8 <-- At this point the get b() method is initiated.
o.c = 50; // <-- At this point the set c(x) method is initiated
console.log(o.a); // 25
o
object's properties are:
o.a
— a number
o.b
— a getter that returns
o.a
plus 1
o.c
— a setter that sets the value of
o.a
to half of the value
o.c
is being set to
Please note that function names of getters and setters defined in an object literal using "[gs]et
property
()" (as opposed to
__define[GS]etter__
) are not the names of the getters themselves, even though the
[gs]et
propertyName
(){ }
syntax may mislead you to think otherwise.
Getters and setters can also be added to an object at any time after creation using the
Object.defineProperties
method. This method's first parameter is the object on which you want to define the getter or setter. The second parameter is an object whose property names are the getter or setter names, and whose property values are objects for defining the getter or setter functions. Here's an example that defines the same getter and setter used in the previous example:
var o = { a: 0 };
Object.defineProperties(o, {
'b': { get: function() { return this.a + 1; } },
'c': { set: function(x) { this.a = x / 2; } }
});
o.c = 10; // Runs the setter, which assigns 10 / 2 (5) to the 'a' property
console.log(o.b); // Runs the getter, which yields a + 1 or 6
Which of the two forms to choose depends on your programming style and task at hand. If you already go for the object initializer when defining a prototype you will probably most of the time choose the first form. This form is more compact and natural. However, if you need to add getters and setters later — because you did not write the prototype or particular object — then the second form is the only possible form. The second form probably best represents the dynamic nature of JavaScript — but it can make the code hard to read and understand.
You can remove a non-inherited property by using the
delete
operator. The following code shows how to remove a property.
// Creates a new object, myobj, with two properties, a and b.
var myobj = new Object;
myobj.a = 5;
myobj.b = 12;
// Removes the a property, leaving myobj with only the b property.
delete myobj.a;
console.log ('a' in myobj); // output: "false"
You can also use
delete
to delete a global variable if the
var
keyword was not used to declare the variable:
g = 17; delete g;
In JavaScript, objects are a reference type. Two distinct objects are never equal, even if they have the same properties. Only comparing the same object reference with itself yields true.
// Two variables, two distinct objects with the same properties
var fruit = {name: 'apple'};
var fruitbear = {name: 'apple'};
fruit == fruitbear; // return false
fruit === fruitbear; // return false
// Two variables, a single object
var fruit = {name: 'apple'};
var fruitbear = fruit; // Assign fruit object reference to fruitbear
// Here fruit and fruitbear are pointing to same object
fruit == fruitbear; // return true
fruit === fruitbear; // return true
fruit.name = 'grape';
console.log(fruitbear); // output: { name: "grape" }, instead of { name: "apple" }
For more information about comparison operators, see Comparison operators .