theory.txt

Introduction
Hoisting
Types
Primitive Types
functions
Objects
arrays
Closures
Bind call apply
this
Prototypes
Class
Inheritance
Polymorphism
Error Handling
Singleton
Module Pattern


JavaScript (22/09/2020)
-----------
Javascript is a programming language that allows us to implement complex
features on web pages

HTML - is the markup language that we use to structure and give meaning to our 
       web content

CSS - is a language of style rules that we used to apply styling to our HTMl Cotent

Javsscript - is scripting language that enables you to create dynamically 
             updating content


Syntax Parser
-------------
A program that reads your code and determines what it does and if its grammar is valid

Lexical Environment
-------------------
Where something sits physically in the code we write

Execution Context
-----------------
A wrapper to help manage the code that is running

lexical 1  -
lexical 2
lexical 3

for e.g There are lots of lexical environments which one is currently 
        running is managed by execution context

Name/Value pairs
------------------
A name which mapes to a unique value (name: 'uma')

Object
-------
A collection of these name value pairs

Global environment and Global Object
------------------------------------
When ever code is running in javascript it runs inside an execution context 

Execution Context (Global)   
-------------------------

Global Object   // javascript execution engine creating these two things for us
this 

this - window ( this browser window)
Global Object - window

Global -  means not inside the function

Hoisting
---------
When you declare variables and functions in javascript, the execution 
engine sets up a memory space for variables and functions.

1. If it is a a variable it will keep that variable in memory and assign 
   a default value called undefined

2. If it is a function it will copy entire function and keeps in memory 



1. find out all the variables and functions in the code file

a = undefined
function b() {
    console.log('called b')
}

2. executing the code 

This phenomenon is called hoisting


undefined (23/09/2020)
----------
Special value in javascript which takes up memory space

Warning : Never ever declare  a variable undefined, because 
          it is difficult to find value is 'undefined' because
          we set it or javascript execution engine set it up


Types 
--------
Dynamic Typing
--------------
you don't tell the engine what type of data a variabe holds,
it figures out while your code running.

Strict Typing
-------------
C# or java follows script typing

Prmitive Types
-----------------
A type of data that represents a  single value is called primitive data type

1. undefined - this represents lack of existance ( we should never use this)
2. null - this represents lack of existance ( we can set a variable to this)
3. Boolean - treu or false
4. number - integer/floating type(decimal)
5. string - a sequence oc characters
6. Symbol - used in ES6

Anythiing which is not primitive is an Object in javascript

object 
function
array
Date
RegularExpression

Functions
--------------

Function is a block of code wrapped around flower brackets and invocated later

function functionname(){

}

1. Regular Functions
2. Anonymous Functions
3. Immediately Invoked functions ( IIFs)

First Class Functions
-------------------
1. can be Invoked
2. assign to a variable
3. passed as a parameter to another variable

Advantages with IIFs (24/09/2020)
--------------------
1. will not create unnecessary gloabal variables and functions
2. functions and variables defined don not conflict


Objects
--------
An Object is a collection of name value pairs

1. using New Object()
2. using Object Literal {}
3. using Object.create()
4. Construtor function

Object.create() method creates a new object using an existing 
object or null as the protoype for newly created Object

var person = Object.create(null) // create an empty/null object
var person = Object.create({}) // create a normal function


JSON
-------
JavaScript Object Notation

common usage of 

Inspired by the object literal notation {}

<data>
    <firstname>uma</firstname>
    <lastName>Mahesh</lastName>
</data>

JSON
------
common usage of JSON is to exchange data to/from web servers
When sending data to a webserver that data has to be string 

{   
    "firstname": "uma",
    "lastName": "Mahesh"
}

Object
---------
{   
    firstname: "uma",
    lastName: "Mahesh"
}

poperties have to be wrapped in quotes for JSON

Anything which is JSON valid is valid in object literal


JSON.stringify - converts a javascript value into seriazlied string 

JSOn.parser - deserialzes a JSON String to a JSON Object

arrays
-------
An array is an object that can store a collection of items

Useful when you want to store large amount of data


var students = ["Shanthanu", "Sunitha" , "Surekha", "Prdeep"]

var people = new Array("Uma", "Swathi", "Jagrav", "Manasvi)

In javascript arrays are object types with special constructor

var array = new Array(3) - length 
var array  = new Array(100000) - length will not create any memory

array index starts from zero

Closure (28/09/2020)
----------
Inner functions having access to the variables of outer functions

Bind vs call vs apply
-----------------------

bind - sets `this` keyword and creates a copy of that function

call -  sets `this` keyword and execute the function immediately and 
        doesn't create a new copy of the function, it takes comma seperated values

apply - sets `this` keyword, similar to call but the difference is apply accepts
        an array of arguments instead comma seperated values


The call, bind and apply methods can be used to set `this` keyword 
independent of how function is called

Class (29/09/2020)
------------------
Javascript is a prototype-based language which contains no class statement 
such as is found in C# or Java

Javascript uses function as a class

e.g. 

function Student(){

}

instance
--------
To create instance of the class

function Person(){

}

var person1 = new Person();

var person2 = new Person();

constructor
-----------

function Order(){
    console.log('Order is placed')
}

var order = new Order(); // Order() is called constructor

Properties
----------
function Customer(gender){
    this.gender = gender
}


var customer1 = new Customer('Female')
var customer2 = new Customer('Male')


Prototype
----------

The prototype is an object that is associated with 
every functions and objects by default in javascript

This is special type of obejct to which additional properties and methods can be attached.
Which will shared acorss all the instnaces of its contsructor function

Usages
------
1. extend properties and methods of an object
2. To implement inheritance

Object 'prototype'
-----------------
Every Object which is created using object literal syntax or constructor syntax 
with new keyword includes __proto__ property that points to prortotype object of
a function that created this Object

Inheritance (1/10/2020)
-----------------------
One Object get access to the properties and methods of another object.

In Javascript inherirtance is supported by using prototype object. 
This is also called prototypal inheritance.

Polymorphism 
-------------

Means function existing in many forms. 

In programming, the ability for objects to present the same interface 
for different behaviour

Error Handling
---------------
Helps us to handle the errors (runtime), for accessing undefined variables 
or undefined functions

try- catch-finally block

try {
    // code that may or may not throws error
}
catch {
    // code to be executed if an error occurs
}
finally {
    // code to be executed regardless of an error occurs or not
}

Singleton Design Pattern
------------------------

Limits the number of instnaces of a perticular object to just one. This single instance
is called the Singleton.

Singletons are useful in stuations where system wide actions need to be coorodinated 
from a single Central place

E.g  Database Connection pool

The pool that manages the creation, destruction and lifetime of all database connections
for the entire application


Module Pattern
---------------
Deals with code encapsulation. It is used to create private publci properties


code encapsulation
-------------------

var person = function(){
    var name =  'uma'
}