note.txt

Hoisting and scoping
Objects and JSON
Arrays and Array Methods.

Hoisting & scoping:-
=> If we a print a variable without declaration it leads to Hoisting
=> In var it will print undefined
=> In let and const it will throw error
=> It is a developer mistake
=> Let and const block variables
=> var global variables

Example for hositing:- 
console.log(a);
const/let/var a =10;

Example for scoping:-
function test() {
  let a=10;
  if (true) {
    let b = 20;
    console.log(a);
   }
  console.log(b);
}
test();

Array :-
=>Collection of datatypes
=>It will be in sequence manner.
=>Enclosed within []
=>Index always starts with 0
=>To print the values inside the array we have use
arrayname[indexvalue]
=>To find the length dynamically we have to use
arrayname.length
=>To find the last index in an array we have to use
arrayname.length-1

Example:- 
var a =[1,2,3,"abc",true]
for(var i=0;i<a.length;i=i+1){
    console.log(a[i]);
}

Objects:- 
=>Objects are structure with key value pair
=> Objects do not have index
syntax:- 
var/let/const objectname = {
    keyname:value
}
=> Accessing the element inside the object we have 2 methods
1.Dot method
2.Box method
=>Dot method
syntax:-
Objectname.Keyname
=>Box method
synatx:-
Objectname["Keyname"]

=>Example
Objects example using dot method

const person = {
  name: "rupan",
  age: 15,
  gender: "male",
  phonenumber: 9876543210,
  skills: ["js", "html", "css", "react", "node", "mongodb"],
}
console.log(person);
console.log(person.name);
console.log(person.age);
console.log(person.gender);
console.log(person.phonenumber);
for(let i=0;i<person.skills.length;i=i+1){
    console.log(person.skills[i]);
}
Objects example using box method
const person = {
  name: "rupan",
  age: 15,
  gender: "male",
  phonenumber: 9876543210,
  skills: ["js", "html", "css", "react", "node", "mongodb"], 
}
console.log(person);
console.log(person["name"]);
console.log(person["age"]);
console.log(person["gender"]);
console.log(person["phonenumber"]);
for(let i =0;i<=person["skills"].length-1;i=i+1){
    console.log(person["skills"][i]);
}

For in loop:- loop over the keys 
syntax:-
for(let key in objectname){
  /block code
}
example:-
let arr = ["a","b","c"]
for(let key in arr){
    console.log(key);
}
let str = "strn"
for(let key in str){
    console.log(key);
}
let obj = {
    a:1,
    b:2,
    c:3
}
for(let key in obj){
    console.log(key);
}
const person = {
    name: "rupan",
    age: 15,
    gender: "male",
    phonenumber: 9876543210,
    skills: ["js", "html", "css", "react", "node", "mongodb"],
  }
for(let key in person){
    console.log(key);
}

For of loop:- Loop over the values
syntax:-
for(let variable of iterable){
    //block of code
}

For each loop :- It used to print the values
example:- 
const person = {
    name: "rupan",
    age: 15,
    gender: "male",
    phonenumber: 9876543210,
    skills: ["js", "html", "css", "react", "node", "mongodb"],
  }
for(let key in person){
    console.log(person[key]);
}
person.skills.forEach(ele=>{console.log(ele)})

// let arr = ["a","b","c"]
// arr.forEach(ele=>{console.log(ele)})




=>Adding,updating,Deleting the elements in objects
Adding a new element in object
syntax:- Objectname.keyname = value
Updating the existing element in object
syntax:- Objectname.keyname = value
Deleting the element in objectname
syntax:- delete objectname.keyname

Examples:- 
const person = {
    name: "rupan",
    age: 15,
    gender: "male",
    phonenumber: 9876543210,
    skills: ["js", "html", "css", "react", "node", "mongodb"],
  }
  console.log(person)
for(let key in person){
    console.log(person[key]);
}
person.skills.forEach(ele=>{console.log(ele)})

Adding element in object
person.mailid="abc@gmail.com";
Updating the element in object
person.age=25;
Deleting the element in object
delete person.phonenumber;


JSON:- 
=>Javascript Object Notation
syntax:
var/let/const objectname = {
    "keyname":"value"
}


//Array Of Objects
//[{},{},{}]
const arr = [
    {
        "name":"John",
        "age":"15"
    },
    {
        "name":"Doe",
        "age":"15"
    },
    {
        "name":"Jack",
        "age":"15"
    },
    {
        "name":"Adam",
        "age":"15"
    }
]
for(let i=0;i<arr.length;i=i+1){
    console.log(arr[i].name,arr[i].age);
    
}