Object oriented java script

Object Oriented JavaScript
VIVEK.P.S
http://vivekcek.wordpress.com

Agenda
• Objects in JavaScript
• Functions: Declaration, Expression and
Invocation
• this keyword
• Scopes and Closures
• Object oriented programming

Objects in JavaScript
• From one side, an object is an associative
array (called hash in some languages).
• It stores key-value pairs

Creating objects
• An empty object (you may also read as empty
associative array) is created with one of two
syntaxes:
• 1. o = new Object()
• 2. o = { } // the same

It is also possible to create nested
objects

Non-existing properties, undefined
• But if the property does not exist,
then undefined is returned

Object variables are references
• A variable which is assigned to object actually
keeps reference to it. That is, a variable stores
kind-of pointer to real data.
• The variable is a reference, not a value

Properties and methods
• You can store anything in object. Not just
simple values, but also functions.

Calling methods
• Note the this keyword
inside askName and sayHi. When a function is
called from the object, this becomes a
reference to this object.

The constructor function, “new”
• An object can be created, using
obj = { ..} syntax.
• Another way of creating an object in
JavaScript is to construct it by calling a
function with new directive.

A simple example
• A function takes the following steps:
• Create this = {}.
• The function then runs and may change this,
add properties, methods etc.
• The resulting this is returned.
• So, the function constructs an object by
modifying this.

Summary
• Objects are associative arrays with additional features.
– Assign keys with obj[key] = value or obj.name = value
– Remove keys with delete obj.name
– Iterate over keys with for(key in obj), remember iteration order for
string keys is always in definition order, for numeric keys it may
change.
• Properties, which are functions, can be called as obj.method(). They
can refer to the object as this.
• Properties can be assigned and removed any time.
• A function can create new objects when run in constructor mode
as new Func(params).It takes this, which is initially an empty object,
and assigns properties to it. The result is returned (unless the
function has explicit return anotherObject call).

Functions: declarations, expressions
and Invocation

The syntax
• function f(arg1, arg2, ...) {
... code ...
}

Returning a value
• use return statement
If a function does not return anything, it’s result is
considered to be a special value, undefined
• function getNothing() {
// no return
}

Local variables
• variables, defined by var. Such variables are
called local and are only visible inside the
function

Function Declaration
• Function Declarations are parsed at pre-
execution stage, when the browser prepares
to execute the code.
• why the function declared this way can be
called both after and before the definition

Function Expression
• Function Expressions are created when the
execution flow reaches them. As a
consequence, Function Expressions can be
used only after they are executed.

Function is a value
• A function in JavaScript is a regular value. We
could even output it
• a function can be assigned, passed as a
parameter for another function and so on.

Running at place
• Running in place is mostly used when we want
to do the job involving local variables. We
don’t want our local variables to become
global, so wrap the code into a function.
• After the execution, the global namespace is
still clean. That’s a good practice.

Named function expressions
• The syntax is called named function expression
• the name is visible inside the function only
• NFEs exist to allow recursive calls from
anonymous functions

Summary
• Functions in JavaScript are regular values.
They can be assigned, passed around and
called when needed.
• A function which returns nothing actually
returns special value: undefined.
• Use verbs to name functions. Short names are
allowable in two edge cases: a name is used in
the nearest code only, or it is extremely widely
used.

Introduction
• The value of this is dynamic in JavaScript
• It is determined when function is called, not
when it is declared
• Any function may use this. It doesn’t matter if
the function is assigned to the object or not
• The real value of this is evaluated in the call
time anyway, and there are 4 possible cases

First, when called as a method
• If a function is called from the object (either
dot or square bracket will do), this refers to
this object.
• In the example above, func is initially apart
from the object. But when
called john.sayHi() sets this to the object
before dot: john.

Second, when called as a function
• If a function uses this, then it is meant to be
called as a method. A simple func() call is
usually a bug

Third, in new
• When a new function is called, this is
initialized as a new object

Fourth, explicit this
• A function can be called with
explicit this value. This is done out by one of
two methods: call or apply

apply
• The func.apply is same as func.call, but it
accepts an array of arguments instead of a list
• The following two lines are same:
• func.call(john, 'firstName', 'surname')

Initialization of functions and variables
• In JavaScript, all local variables and functions
are properties of the special internal object,
called LexicalEnvironment
• The top-level LexicalEnvironment in browser
is window. It is also called a global object.

Instantiation of top-level variables
• When the script is going to be executed, there
is a pre-processing stage called variables
instantiation.
• The top-level LexicalEnvironment in browser
is window. It is also called a global object.

• the browser finds function f, creates the
function and stores it as window.f
• Function Declarations are initialized before
the code is executed.
• As a side effect, f can be called before it is
declared (Hoisting)

• Second, the interpreter scans
for var declarations and
creates window properties. Assignments are
not executed at this stage. All variables start
as undefined.

• FunctionDeclarations become ready-to-use
functions. That allows to call a function before
it’s declaration.
• Variables start as undefined.
• All assignments happen later, when the
execution reaches them

Function variables
• When the interpreter is preparing to start
function code execution, before the first line is
run, an empty LexicalEnvironment is created
and populated with arguments, local variables
and nested functions.

• Then the function code runs, eventually
assignments are executed.A variable
assignment internally means that the
corresponding property of
theLexicalEnvironment gets a new value.

Closures
• So variable is a property of
the LexicalEnvironment object
• Here we discuss access to outer variables and
nested functions

Nested functions
• Functions can be nested one inside another,
forming a chain of LexicalEnvironments which
can also be called a scope chain

Closures
• Nested function may continue to live after the
outer function has finished:

Closure
• The inner function keeps a reference to the
outer LexicalEnvironment.
• The inner function may access variables from it
any time even if the outer function is finished.
• The browser keeps the LexicalEnvironment and
all it’s properties(variables) in memory until there
is an inner function which references it.
• This is called a closure.

[[Scope]] for new Function
• There is an exception to general scope binding
rule. When you create a function using new
Function, it’s[[Scope]] points to window, not
to current LexicalEnvironment.

Summary
• How variables are handled in JavaScript.
• How scopes work.
• What is a closure and how to use it.
• Possible pitfalls and subtles in working with
closures.
• In JavaScript, a variable can be declared after it
has been used (Hoisting).
• In other words; a variable can be used before it
has been declared (Hoisting).

Prototypal inheritance
• In JavaScript, the inheritance is prototype-
based. That means that there are no classes.
Instead, an object inherits from another
object

Object.create, Object.getPrototypeOf
• The __proto__ is a non-standard property,
provided by Firefox/Chrome. In other
browsers the property still exists internally,
but it is hidden

The prototype
• There is a good and crossbrowser way of
setting __proto__. It requires the use of
constructor functions.

hasOwnProperty
• All objects have hasOwnProperty method
which allows to check if a property belongs to
the object or its prototype.

Summary
• The inheritance is implemented through a
special property __proto__ (named
[[Prototype]] in the specification).
• When a property is accessed, and the
interpreter can’t find it in the object, it follows
the __proto__link and searches it there.
• The value of this for function properties is set
to the object, not its prototype.
• Assignment obj.prop = val and deletion delete
obj.prop

Object oriented java script

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