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Constructor paradigm

Some got around this problem by defining the object methods outside of the factory functions and then

pointing to them:

function showColor() {

alert(this.color);

}

function createCar(sColor, iDoors, iMpg) {

var oTempCar = new Object;

oTempCar.color = sColor;

oTempCar.doors = iDoors;

oTempCar.mpg = iMpg;

oTempCar.showColor = showColor;

return oTempCar;

}

var oCar1 = createCar(“red”, 4, 23);

var oCar2 = createCar(“blue”, 3, 25);

oCar1.showColor(); //outputs “red”

oCar2.showColor(); //outputs “blue”

In this rewritten code, the

showColor()

function is defined before the

createCar()

function. Inside

createCar()

, the object is assigned a pointer to the already existing

showColor()

function. Functionally,

this solves the problem of creating duplicate function objects; but semantically, the function doesn’t look

like it is a method of an object.

All these problems led to the creation of developer-defined constructors.

Constructor paradigm

Creating a constructor is just as easy as defining a factory function, if not easier. The first step is selection

of a class name, which becomes the name of the constructor. Traditionally, this name begins with a capi-

tal letter to differentiate it from variable names, which typically begin with lowercase letters. Other than

this difference, a constructor looks a lot like a factory function. Consider the following example:

function Car(sColor, iDoors, iMpg) {

this.color = sColor;

this.doors = iDoors;

this.mpg = iMpg;

this.showColor = function () {

alert(this.color)

};

}

var oCar1 = new Car(“red”, 4, 23);

var oCar2 = new Car(“blue”, 3, 25);

The first difference you may notice is that no object is created inside the constructor; instead, the

this

keyword is used. When a constructor is called with the

new

operator, an object is created before the first

line of the constructor is executed; that object is accessible (at that point) only by using

this

. It is then

possible to assign properties directly to

this

that are returned as the function value by default (no need

to explicitly use the

return

operator).

Chapter 3

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