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JavaScript ist nach einem einfachen objekt-basierten Muster entworfen. Ein Objekt ist eine Sammlung von Eigenschaften und eine Eigenschaft ist eine Verbindung von einem Namen und einem Wert. Der Wert einer Eigenschaft kann eine Funktion sein. In diesem Fall wird die Eigenschaft Methode genannt. Zusätzlich zu Objekten, die vom Browser vordefiniert sind, können auch eigene Objekte definiert werden. Dieses Kapitel beschreibt, wie Objekte, Eigenschaften, Funktionen und Methoden benutzt und eingene Objekte erzeugt werden können.
Objekte Übersicht
Objekte in JavaScript, wie in vielen anderen Programmiersprachen, können mit Objekten aus dem realen Leben verglichen werden. Das Konzept der Objekte in JavaScript kann mit greifbaren Objekten aus der realen Welt verstanden werden.
In JavaScript ist ein Objekt eine für sich stehende Einheit mit Eigenschaften und Typ. Vergleichbar mit einer Tasse zum Beispiel. Eine Tasse ist ein Objekt mit Eigenschaften. Eine Tasse hat eine Farbe, ein Design, ein Gewicht, ein Material, aus dem sie erstellt wurde, usw. Ebenso können JavaScript Objekte Eigenschaften besitzen, die Ihre Besonderheiten definieren.
Objekte und Eigenschaften
Ein JavaScript Objekt besitzt mit ihm verbundene Eigenschaften. Eine Eigenschaft eines Objekts kann als Variable erklärt werden, die dem Objekt angeheftet ist. Objekteigenschaften sind grundsätzlich das Gleiche wie übliche Variablen ausser der Verknüpfung mit Objekten. Die Eigenschaften eines Objekts definieren seine Charakteristik. Auf die Eigenschaften eines Objekt kann mit einer einfachen Punkt-Notation zugegriffen werden:
objectName.propertyName
Wie bei allen JavaScript-Variablen sind Objektname (eine normale Variable) und Attribute groß-, und kleinschreibungsabhängig. Ein Attribut wird definiert, indem es einen Wert zugewiesen bekommt. Zum Beispiel: Ein Objekt namens myCar
mit Attributen make
, model
, und year
:
var myCar = new Object(); myCar.make = "Ford"; myCar.model = "Mustang"; myCar.year = 1969;
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 variable 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);
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) { 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
Object everything
In JavaScript, almost everything is an object. All primitive types except null
and undefined
are treated as objects. They can be assigned properties (assigned properties of some types are not persistent), and they have all characteristics of objects.
Enumerating all properties of an object
Starting with ECMAScript 5, there are three native ways to list/traverse object properties:
for...in
loops
This method traverses all enumerable properties of an object and its prototype chainObject.keys(o)
This method returns an array with all the own (not in the prototype chain) enumerable properties' names ("keys") of an objecto
.Object.getOwnPropertyNames(o)
This method returns an array containing all own properties' names (enumerable or not) of an objecto
.
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.
Creating new objects
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 using that function and the new
operator.
Using object initializers
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
. The 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 = {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.
Using a constructor function
Alternatively, you can create an object with these two steps:
- Define the object type by writing a constructor function. There is a strong convention, with good reason, to use a capital initial letter.
- Create an instance of the object with
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
. For example,
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
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.
Using the Object.create
method
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
Inheritance
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 Inheritance and the prototype chain for more information.
Indexing object properties
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 can subsequently refer to the property only as myCar[5]
.
The exception to this rule is objects reflected from HTML, such as the forms
array. You can always refer to objects in these arrays by either their ordinal number (based on where they appear in the document) or their name (if defined). For example, if the second
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.myForm
.
Defining properties for an object type
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";
See the prototype
property of the Function
object in the JavaScript reference for more information.
Defining methods
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 = function_name; var myObj = { myMethod: function(params) { // ...do something } };
where objectName
is an existing object, methodname
is the name you are assigning to the method, and function_name
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. For example, 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();
Using 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 a function called validate
that validates an object's value
property, given the object and the high and low values:
function validate(obj, lowval, hival) { if ((obj.value < lowval) || (obj.value > hival)) { alert("Invalid Value!"); } }
Then, you could call validate
in each form element's onchange
event handler, using this
to pass it the element, as in the following example:
In general, this
refers to the calling object in a method.
When combined with the form
property, this
can refer to the current object's parent form. In the following example, the form myForm
contains a Text
object and a button. When the user clicks the button, the value of the Text
object is set to the form's name. The button's onclick
event handler uses this.form
to refer to the parent form, myForm
.
Defining getters and setters
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. The syntax for defining getters and setters uses the object literal syntax.
The following illustrates how getters and setters could work for a user-defined object o
.
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 o.c = 50; console.log(o.a); // 25
The o
object's properties are:
o.a
— a numbero.b
— a getter that returnso.a
plus 1o.c
— a setter that sets the value ofo.a
to half of the valueo.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. To name a function in a getter or setter using the "[gs]et property()" syntax, define an explicitly named function programmatically using Object.defineProperty
(or the Object.prototype.__defineGetter__
legacy fallback).
The following code illustrates how getters and setters can extend the Date
prototype to add a year
property to all instances of the predefined Date
class. It uses the Date
class's existing getFullYear
and setFullYear
methods to support the year
property's getter and setter.
These statements define a getter and setter for the year property:
var d = Date.prototype; Object.defineProperty(d, "year", { get: function() { return this.getFullYear() }, set: function(y) { this.setFullYear(y) } });
These statements use the getter and setter in a Date
object:
var now = new Date(); console.log(now.year); // 2000 now.year = 2001; // 987617605170 console.log(now); // Wed Apr 18 11:13:25 GMT-0700 (Pacific Daylight Time) 2001
In principle, getters and setters can be either
- defined using object initializers, or
- added later to any object at any time using a getter or setter adding method.
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; } };
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.
Deleting properties
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) // yields "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;
Comparing Objects
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 distict 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
For more information about comparison operators, see Comparison operators.
See also
- To dive deeper, read about the details of javaScript's objects model.
- To learn about ECMAScript6 classes (a new way to create objects), read the JavaScript classes chapter.