Object-oriented JavaScript
constructors and member access

In JavaScript, rather than defining a class to be instantiated by objects, one writes a constructor of the objects. Variables and functions defined within the constructor body become the new member data (properties) and member functions (methods) of the object (in addition to any inherited properties and methods).

In an object’s public code and constructor, the variable this is defined to refer to the object.

Members defined in the constructor but not qualified by this are private to the objects’ code, meaning there is no direct access to them from the outside. To make a member of the object public, qualify it by this:

	this.member_name

To access a public member of an object from code outside the object’s code, one connects the object variable name with a dot “.” to the member name.

	object_variable.member_name

The keyword new indicates to JavaScript that a function call is an object constructor, and creates a new object using constructor supplied to it.

example: constructor, properties, creation and access

All JavaScript objects inherit from the the built-in type Object. This type has one property: constructor which is the object’s constructor function. Any function in turn has a property prototype. One of the Object type methods is toString(), which is called automatically to convert the object to a string. We use these members in the code that is executed in the Inspect button, and will discuss them more in the other pages.

Note that a member variable declared with the keyword var is accessible only within the code block where it was declared, and so is called local. Without the var keyword, the variable could accidentally refer to another variable defined outside the code block. It is good practice in JavaScript (especially in object-oriented code) to

always declare variables using the var keyword

unless you really mean the variable to be global (in which case, you should prefix its name with global_ or g_ to make this obvious).

ownership of object members

A member of an object really belongs to its object: there is a distinct copy of the member for each object.

Note that the member prop2 of tc and tc2 vary independently.

adding members on the fly

In JavaScript, after an object is created, public members may be added to it at will, by simply assigning it a value. Members can also be removed by operator delete.

Clicking the first button gives tc2 a member prop3 that tc does not have. This is sometimes called dynamic polymorphism.

Programmers familiar with C, C++ and Java will find dynamic polymorphism very strange. It is not something special to JavaScript, however: it appears in Perl, Python and SmallTalk.

See

JavaScript Guide: Working with objects