javascript in ten minutes javascript)

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avascript in Ten Minutes (Javascript)

Javascript(ECMAScript)

Javascript in Ten Minut es

Breakdown...

Basic Types

var intValue = 1;

var floatValue = 3.0;

var stringValue = "This is a string\n";

var sqString = 'This is also a string';

Javascript is a dynamically typed language. Variables are declared with thekeyword var. Common simple types are supported.

Arrays

var emptyList = [];

var homogenousList = [1, 2, 3];

var heterogenousList = ["one", 2, 3.0];

Javascript has built-in collection objects. The Array object is a dynamically typedsequence of Javascript values. They are created with the bracket notation [] or

with the new operator on the Array object (e.g. new Array(5)).

Propert y Maps

var emptyMap = {};

var homogenousMap = {"one": 1, "two": 2, "three": 3};

var heterogenousMap = {"one": 1,

"two": "two",

"three": 3.0};

Along with Arrays are the Object objects. They act as property maps with strings

serving as keys to dynamically typed data.

Access

// Dot notation property access

window.alert("Homogenous map property \"one\" "

+ homogenousMap.one);

// Subscript notation property access

window.alert("Homogenous map property \"two\" "

+ homogenousMap["two"]);

Assignment

homogenousMap["one"] = 10;

homogenousMap.two = 20;

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Removal

delete homogenousMap["one"];

delete homogenousMap.two;

I terat ion

for (var key in heterogenousMap) {

window.alert("Heterogenous map property \""

+ key

+ "\" = "

+ heterogenousMap[key]);

}

Functions

var callable = function (message) { //


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initialized) in this example with the dot notation.

The object constructor, MyObject, is an object constructor not in how it's defined,

which looks like any other Javascript function, but in how it's ''invoked''.

var my = new MyObject("foo", 5);

The new operator before the function invokes the function with a newly construced

object as this and returns that the initialized object.

Object Protot ype

Part of what makes a language object oriented is that data not only has propertiesbut also ''behaviors''. Also known as: member functions; methods; and objectmessages. To implement a member function in Javascript one would be temptedto write something like what's below based on the member initialization exampledabove.

function BadObject(data) {

this.data = data

this.memberFunction = function () {// ...functions on data...

}

}

While the code above will work without error, it does create a new closure for eachmember function for each new instance of the object. What's really required is aclass level function that works on instance data. But remember, Javascript objectsaren't class based but prototype based. So how do we implement "class" levelmember functions? (Skip to Implementation) Better yet, how do we implement

"class" level members functions in general?

Enter the prototype member.

The internal object member, prototype, has language defined significance in that

it is used for resolving property names if the property isn't found in the currentproperty map. It's considered internal because, while the instance's prototype

member is ''inherited'' from the ''constructor's'' prototype member, it cannot be

accessed directly from the object instance itself. The defined prototype member

is a property map itself which holds members for property name resolution.Consider the example below:

var parentPropertyMap = {"bar": "I'm the bar"};

// Define the constructor with inheritable properties

function ChildObject(foo) {

this.foo = foo;

}

ChildObject.prototype = parentPropertyMap;

childPropertyMap1 = new ChildObject("I'm the foo1");


// Prints "childPropertyMap1.foo = I'm the foo1"

http://javascript.infogami.com/#Member_Function_Implementationhttp://javascript.infogami.com/#Member_Function_Implementation


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window.alert("childPropertyMap1.foo = " + childPropertyMap1.foo);

// Prints "childPropertyMap2.foo = I'm the foo2"

window.alert("childPropertyMap2.foo = " + childPropertyMap2.foo);

// Prints "childPropertyMap1.bar = I'm the bar"

window.alert("childPropertyMap1.bar = " + childPropertyMap1.bar);

// Prints "childPropertyMap2.bar = I'm the bar"

window.alert("childPropertyMap2.bar = " + childPropertyMap2.bar);

The member foo is an instance member added to the instance's property map

during construction:

function ChildObject(foo) {

this.foo = foo;

}

while bar is in the constructor's prototype:

var parentPropertyMap = {"bar": "I'm the bar"};

...

ChildObject.prototype = parentPropertyMap;

which is ''inherited'' during the new operation:



In other words, the member, bar, is shared across all instances ofChildObject.

Therefore, by implementing the prototype member of the constructor function,

we can think of the constructor function itself as the "class" object. Complete withstatic class functions:

function ClassObject() {}

ClassObject.staticClassFunction = function(x) {

return x * 2;

}

static class variables:


ClassObject.staticClassVariable = 5;

shared member variables:


ClassObject.prototype.sharedMember = 5;

and of course, shared member functions:

function ClassObject(x) {

this.x = x;



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}

ClassObject.prototype.memberFunction = function(x) {

return x * this.x;

}

Member Function I mplementat ion

function Message(message) {

this.message = message;

}

Message.prototype.show = function() {

window.alert("Message.show() with message = "

+ this.message);

}

(More on Classes and Objects)

Example Code

//////////////////////////////////////// Basic Types

var intValue = 1;

var floatValue = 3.0;

var stringValue = "This is a string\n";

///////////////////////////////////////

// Array

var emptyList = [];

var homogenousList = [1, 2, 3];

var heterogenousList = ["one", 2, 3.0];

///////////////////////////////////////// Property Map

//

var emptyMap = {};

var homogenousMap = {"one": 1, "two": 2, "three": 3};

var heterogenousMap = {"one": 1,

"two": "two",

"three": 3.0};

///////////////////////////////////////

// Functions as values

//

var callable = function (message) { //


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}

///////////////////////////////////////

// Functions as arguments

//

function callCallable(f, x) {

f(x);

}

function composeCallables(f, g, x) {

f(g(x));

}

///////////////////////////////////////

// Objects

//

function MyObject(name, value) {

this.name = name;

this.value = value;

}

///////////////////////////////////////

// Objects with Member Functions

//

function Message(message) {

this.message = message;

}

Message.prototype.show = function() {

window.alert("Message.show() with message = "

+ this.message);

}

///////////////////////////////////////// Demo Utilities

//

function quote(message) {

return "\"" + message + "\"";

}

///////////////////////////////////////

// HTML Invoked demonstration

//

//

function main() {

window.alert("Integer = " + intValue);window.alert("Float = " + floatValue);

window.alert("String = " + stringValue);

for (var item in emptyList) {

window.alert("Empty list item = " + item);

}

// Script style index iteration

for (var i in homogenousList) {

window.alert("Homogenous list item = "

+ homogenousList[i]);



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}

// C style index iteration

for (var i=0; i < heterogenousList.length; ++i) {

window.alert("Heterogenous list item = "

+ heterogenousList[i]);

}

// Dot notation property access

window.alert("Homogenous map property \"one\" "

+ homogenousMap.one);

// Subscript notation property access

window.alert("Homogenous map property \"two\" "

+ homogenousMap["two"]);

for (var key in heterogenousMap) {

window.alert("Heterogenous map property \""

+ key

+ "\" = "

+ heterogenousMap[key]);

}

callable("(Function value invoked)");

closure();

closure();

callCallable(closure);

composeCallables(callable, quote, "My Message");

var my = new MyObject("foo", 5);

window.alert("MyObject my.name = " + my.name);

window.alert("MyObject my[\"value\"] = " + my["value"]);

var msg = new Message("bar");for (var key in Message.prototype) {

window.alert("Message prototype member \""

+ key

+ "\" = "

+ Message.prototype[key]);

}

window.alert("Message msg.message = " + msg.message);

msg.show();

}

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javascript in ten minutes javascript)

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