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Author: Michael Barry | Website: http://www.iol.ie/~michaelbarry/public_html

Prototypes and inheritance.

When you create a function you also create a 'prototype' of the function. Even if the prototype is
not defined explicitly it exists as a 'template' for the function and any new instances of that function.
When you want to add properties to the new instances of the function you can explicitly define these
properties using 'prototype'. e.g. myFunction.prototype.y = "v";
We now have defined a property, y, with a value "v".
This property and value can be inherited by any new instances of the function:

//define a function;
function myFunction(a){
this.a=a;
}

//create an instance of this function with the 'new' operator;
var myNewFun = new myFunction();

//define the prototype and create a properties 'x', 'y' and 'z' in the prototype;
myFunction.prototype.x = "Create";
myFunction.prototype.y = "a";
myFunction.prototype.z = "sentence";

Now access these properties through the new myFunction, namely 'myNewFun'.

var output = myNewFun.x+" "+myNewFun.y+" "+myNewFun.z;

output = "Create a sentence"

What happens here is that Flash searches 'myNewFun' for the property 'x'. It does not find it because
we have not explicitly stated it in that instance, so it goes to the original function "myFunction" and
does not find it there so it goes to the prototype and finds it there and adds its value to output.
The same process happens for y and z and their values are added to 'output'. If a value is found in the
new instance it takes precedance over values in the function itself or its prototype. This is called 'shadowing'
where a value shadows another value deeper in the chain.

This process gives rise to the notion of 'order of precedence' with regard to properties in each section of
the function chain. This order is;
myNewFun; -- original myFunction; -- myFunction.prototype.

For example, suppose we were to define our function chain as follows;

myFunction.prototype.x = "Create";
myFunction.prototype.y = "a";
myFunction.prototype.z = "sentence";

function myFunction(){
this.x = "Make";
this.y = "I'm shadowed by y in 'myNewFun'";
}

var myNewFun = new myFunction();
myNewFun.y = "a shadow";

var output = myNewFun.x+" "+myNewFun.y+" "+myNewFun.z;

output = "Make a shadow sentence";

This is how the sentence is created:
x is not found in myNewFun.
x is found in myFunction so this value ("Make") is inserted in output and the x value in the prototype ("Create") is shadowed.
y is found in myNewFun so "a shadow" is chosen instead of the y value in myFunction or the y value in its prototype.
z is not found in myNewFun or myFunction but is found in myFunction.prototype so "sentence" is appended to 'output'.

» Level Intermediate

Added: 2001-06-22
Rating: 8 Votes: 30
(10 being the highest)
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