Var, Let , Const - Javascript basics

Var, let & Const are used to declare variables, objects and constants in JavaScript.

Var

Var keyword has been used for a longtime for variable declaration.
It is Function Scoped, it makes variables accessible only within the function boundary where it is declared.
Take a look at this snippet

function getX(){
    var x = 1;
}
getX();
console.log(x)

//ReferenceError: x is not defined

You can re-declare same variable in same scope with Var.

if (true){
    var x = 2;
    var x = 3;
}
console.log(x)

// 3

Let

Let was introduced in ES6ECMAScript2015.
It is Block Scoped, any code written within {} is said to be in a Block. So, that's the restriction that ECMA wanted to implement with let, making variables inaccessible outside the block.

function exampleLet() {
  if (true) {
    let y = 20;
  }
  console.log(y); // Error: y is not defined
}

Also, you cannot re-declare same variable within the same scope.

if (true){
    let x = 2;
    let x = 3;
}
// SyntaxError: Identifier 'x' has already been declared

Const

Const as the name suggests is used to declare Constants.
Declaration with consts are *Block Scoped *
Constant needs to be initialized at the time of declaration, else it throws Syntax error
Re-assigning or re-declaration is not allowed, it throws Syntax or Type Error.
Check out the snippet below

if (true){
    const x;
}
// SyntaxError: Missing initializer in const declaration

if (true){
    const x = 1;
    x = 2; // TypeError: Assignment to constant variable.
}

if (true){
    const x = 1;
    const x = 2;
} 
// SyntaxError: Identifier 'x' has already been declared

const x = 2;
if (true){
    const x = 1;
}
// Output : x = 2

Strict Mode

A feature introduced in ES5, it enforces stricter parsing and error handling.

To enable strict mode, you need to put _use strict;_ at the beginning of code or function block.

// Global variable
var globalVar = "I'm a global variable";

function baz() {
  var localVar = "I'm a local variable";
  console.log(localVar); // Accessible inside the function

  if (true) {
    let blockVar = "I'm a block-scoped variable";
    console.log(blockVar); // Accessible inside the block
  }
  // console.log(blockVar); // Error: blockVar is not defined (not accessible outside the block)
}