what is the difference between let and const in javascript

1//let is used to declare the variable or initialize a variable..
2//let variable can be updated in future.
3//Its block scope and we cannot redeclare it.
4
5let a; // Initializing the variable.
6let b = 4; // Initializing and declaring variable.
7let b = 5; //Error:- cannot redeclare a value but we can update the value.
8
9//var is just like let but var is used to declare a variable value at top of
10//the program, its hoisted.
11
12var a; // Initializing the variable.
13var b = 4; // Initializing and declaring variable.
14var b = 5; //b will be redeclared and we can update its value.
15
16//const variable should be declared and intialized at same time.
17//const variable cannot be updated in future.
18//we cannot even redeclare the variable in future.
19
20const a = 10; //Initilizing and declaring a variable at a time.
21const b; //Error:- const should be declared and intialized.
22const a = 11 // Error:- cannot redeclare the variable again.

1The difference is that with const you can only only assign a value to a variable
2once, but with let it allows you to reassign after it has been assigned.

1//functional scope 
2 var a; // declaration
3 a=10; // initialization; 
4//global scope
5// re-initialization possible
6 let a;//only blocked scope & re-initialization possible
7 a=10;
8let a =20;
9if(true){
10  let b =30;
11}
12console.log(b); // b is not defined
13const // const also blocked scope,Re-initialization and re-declaration not possible
14const a; // throws error {when we declaring the value we should assign the value.
15const a =20;
16if(true){
17  const b =30;
18}
19console.log(b); // b is not defined
20console.log(a); // no output here because code execution break at leve b.
21