2.js

// Varaibles: variables in javascript are like containers which stores data in itself.

// there are 3 ways to declare a variable: Var, Let, Const.

// Var:

// var name = "rahman";
// console.log(name);

// var name = "Rahman";
// console.log(name);

// var name = "abdul rahman";
// console.log(name);

// var name = "ab";
// console.log(name);

// var name = "Sar";
// console.log(name);


// Let:

// let name = "rahman";
// console.log(name);

// let course = "ece";
// console.log(course);

// let year = 4;
// console.log(year);

// let isPass = true;
// console.log(isPass);

// let study = "CFI";
// console.log(study);

//  Const:

// const name = "syed abdul rahman";
// console.log(name);

// const rollno = 49;
// console.log(rollno);

// const isPass = true;
// console.log(isPass);

// const isFail = false;
// console.log(isFail);

// const Course = "CFI-Ful stack web dev";
// console.log(Course);

// rules for declaring a variable:
/*
--> you can declare a variable using characters from A-Z, numbers, special characters like $ and _
--> a variable name cannot be started with a number but $ and _ can be used to start a variable name
*/

// basically there is another way of decaring a variable which is a fourth method.
// a varaible can be declared automatically, witout using var, let or const. simply just write variable name and
// give it a value. the js compiler will automatically considers it as a variable declared using var

// lets practice again all the 4 methods used to declare a variable:

// 1.Var: a variable declared using var can be redclared and its value can also be updated

// var name = "rahman";
// // console.log(name);

// var name = "syed abdul rahman";
// console.log(name); //the console will print syed abdul rahman and not just rahman.

// var course = "Python + DA & ML";
// // console.log(course);
// var course = "Full Stack Web Devolopment";
// console.log(course);

// var engineering = "CSE";
// // console.log(engineering);
// var engineering = "ECE";
// console.log(engineering);

// var rollno = 326;
// // console.log(rollno);
// var rollno = 49;
// console.log(rollno);

// var isPass = false;
// // console.log(isPass);
// var isPass = true;
// console.log(isPass);


//  2.let : let is also used t declare variables. let and const keywords was itroduced in 2015 js es6 version
//       a variable declared using let cannot be redclared but its value can be updated.

// let standard = 8 + "th";
// // console.log(standard);
// standard = 9 + "th";
// console.log(standard);

// let course = "aiml";
// // console.log(course);
// course = "ECE";
// console.log(course);

// let age = 20;
// // console.log(age);
// age = 21;
// console.log(age);

// let grade = "fail";
// // console.log(grade);
// grade = "Pass";
// console.log(grade);

// let floor = 5 + "th";
// // console.log(floor);
// floor = 5 + "th";
// console.log(floor);


// 3.Const: it was also introduced in js es6 version in 2015. as its name suggests, a variable declared using 
//        const cannot be redclared or updated.

// const name = "syed";
// console.log(name);

// const rollno = 49;
// console.log(rollno);

// const Course = "CFI Full Stack";
// console.log(Course);

// const week = 8;
// console.log(week);

// 4. default: we can also declare a variable without using var, let or const but by default, the js compiler
//             understands it as a variable declared using var.

// here are some examples:

// name = "rahman"; //here before name variable, the js compiler takes it as a variable declared using var
// console.log(name);

// course = "ece";
// console.log(course);

// isPass = true;
// console.log(isPass);

// isFail = false;
// console.log(isFail);

// score = 99;
// console.log(score);