Core JavaScript 1.5 Reference:Operators:Special Operators:new Operator

From MDC

Contents 1 Summary 2 Syntax 3 Parameters 4 Description 5 Examples 5.1 Example: Object type and object instance 5.2 Example: Object property that is itself another object

[edit] Summary

The new operator creates an instance of a user-defined object type or of one of the built-in object types that has a constructor function.

Operator
Implemented in:	JavaScript 1.0
ECMA Version:	ECMA-262

[edit] Syntax

objectName = new objectType(param1 [,param2] ...[,paramN])

[edit] Parameters

objectName 

Name of the new object instance.

objectType 

Object type. It must be a function that defines an object type.

param1...paramN 

Property values for the object. These properties are parameters defined for the objectType function.

[edit] Description

Creating a user-defined object type requires two steps:

1. Define the object type by writing a function.
2. 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. An object can have a property that is itself another object. See the examples below.

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 must add the property to the definition of the car object type.

You can add a shared property to a previously defined object type by using the Function.prototype property. This defines a property that is shared by all objects created with that function, rather than by just one instance of the object type. The following code adds a color property with value null to all objects of type car, and then overwrites that value with the string "black" only in the instance object car1. For more information, see prototype.

 function Car() {}
 car1 = new Car()
 
 alert(car1.color)           // undefined
 
 Car.prototype.color = null
 alert(car1.color)           // null
 
 car1.color = "black"
 alert(car1.color)           // black

[edit] Examples

[edit] Example: Object type and object instance

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;
}

Now you can create an object called mycar as follows:

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:

kenscar = new car("Nissan", "300ZX", 1992);

[edit] Example: Object property that is itself another object

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:

rand = new person("Rand McNally", 33, "M");
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:

car1 = new car("Eagle", "Talon TSi", 1993, rand);
car2 = new car("Nissan", "300ZX", 1992, ken);

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 parameters for the owners. To find out the name of the owner of car2, you can access the following property:

car2.owner.name