name value pairs: A name may have been defined more than once, but can have only one value at any given context.(ie the section of code that is currently running)

In JS: object is a collection of name value pairs.

JS engine gives us two things whenever JS is executed on is "this" and other is "globa object"

so every tab you open will have its own window object

At a global level this == window(global in case of nodejs)

Code NOT inside a function == global

js is executed in 2 phases

(creation phase)in first phase js engine sets aside memory for variable and functions to be executed --- till this functions and their definitions have been hoisted but for variable since js does not know what value will be for the variable in the end there only the declarations are hoisted. Therefore we get undefined when we

(execuation phase)code is read line by line

function invocation as soon as js is executed a global execution phase is wrapped around

when function is called it stacks over the global execution... this stack is called execution stack which contains all the function execution contexts and a global execution context.

for every function invocation there will be a creation phase and execution phase ie they will have their own execution contexts

The lexical context (physical position)wont matter, functions will be stacked according to their invocations... (in execution stack) --- we are only talking about stacking here-- during the global execution phase all function have already been hoisted but for each invocation a new execution context for each function is created and pushed on the stack(both creation phase and execution phase will happen for each invocation)

Remember undefined is a value in js.

function b() { var myVar; console.log(myVar); }

function a() { var myVar = 2; console.log(myVar); b(); }

var myVar = 1; console.log(myVar); a(); console.log(myVar);

output: //1 //2 //undefined //1

eg 2 for global scope chain function b() {

console.log(myVar);

}

function a() { var myVar = 2; console.log(myVar); b(); }

var myVar = 1; console.log(myVar); a(); console.log(myVar);

1 2 1 1

//The scope chain function b() {

console.log(myVar);

}

function a() { var myVar = 2;

b();

}

var myVar = 1;

a(); function b is (physically)(lexically)present on top ie in global therefore we are getting the global value(because it could not find the value within func b) of myVar because the outer env physically or lexically is global env Lexically the function b is sitting in the global env right now

Asynchronous: simply means running more than one at a time

but js is synchronous

Whatever is happing inside JS Engine is synchronous So how do we are able to use async stuff in browser

That is because browsers does not "only" contains js engine

It alse include redering engine, HTTP req/res elements... the engine is sort just having hooks attached to these

eg for events that are outside scope of js engine but part of somewhere in the browser.. these events are stored in event queue which wait for the stack to empty to put the methods called by those events to be put back on stack,

JS objects and JSON

javascript obj and json are different things dont confuse them with each other earlier we used to send/receive data in form of xml, but now, inspired from js object literals data is sent in form of json

{ "property":"Value" }

note that in json properties are also wrapped in double quotes.

To work work with json and objects there are some predefined methods which make converting json to object and objects to json easier like

var obj = {name:"Dheeraj"}

console.log(JSON.stringify(obj));// this will conver object to JSON string ie {"name":"Dheeraj"}

To convert JSON to obj we can use var jsonValue = JSON.parse('{"firstname":"Mary", "isAprogrammer":true}');

Functions are objects

By value and By reference

By value

By refernce