let and var difference

1var is function scoped and let is block scoped. Let's say you have:
2function understanding_var() {
3	if (1 == 1) {
4    	var x = 5;
5        console.log('the value of x inside the if statement is ' + x);
6    }
7    console.log(x);
8} 
9//output: the value of x inside the if statement is 5
10		  5
11
12function understanding_let() {
13	if (1 == 1) {
14    	let x = 5;
15        console.log('the value of x inside the if statement is ' + x);
16    }
17    console.log(x);
18} 
19//output: the value of x inside the if statement is 5
20		  Uncaught ReferenceError: x is not defined
21          
22var is defined throughout the entire function, even if it's inside the if 
23statement, but the scope of let is always within the curly braces, not outside
24it, even if the conditional statement is inside the function.

1let a = 'hello'; // globally scoped
2var b = 'world'; // globally scoped
3console.log(window.a); // undefined
4console.log(window.b); // 'world'
5var a = 'hello';
6var a = 'world'; // No problem, 'hello' is replaced.
7let b = 'hello';
8let b = 'world'; // SyntaxError: Identifier 'b' has already been declared

1function run() {
2  var foo = "Foo";
3  let bar = "Bar";
4
5  console.log(foo, bar);
6
7  {
8    let baz = "Bazz";
9    console.log(baz);
10  }
11
12  console.log(baz); // ReferenceError
13}
14
15run();

1// var is function-scoped, so redeclaring it in a block will cause its value outside the block to change as well:
2
3var one = 'one: declared outside the block';
4
5if (true === true) {
6  var one = 'one: declared inside the block'; // notice: we redeclare 'one' here
7  console.log(one); // prints 'one: declared inside the block'
8}
9
10console.log(one); // also prints 'one: declared inside the block', because the variable was redeclared in the 'if' block. The outer 'var' variable was therefore destroyed and replaced by inner var variable.
11
12// 'let' is block-scoped, so redeclaring a 'let' variable inside of a block creates a different 'let' variable with the same name whose scope is inside the block:
13
14let two = 'two: declared outside the block';
15
16if (true === true) {
17  let two = 'two: declared inside the block';
18  console.log(two); // prints 'two: declared inside the block'
19}
20
21console.log(two); // prints 'two: declared outside the block', because two declared inside the block is a separate variable. The 'let' variables are unrelated and therefore are unaffected by each other.

1/* DIFFERENCE BETWEEN LET AND VAR */
2
3//LET EXAMPLE
4{
5  let a = 123;
6};
7
8console.log(a); // undefined
9
10//VAR EXAMPLE
11{
12  var a = 123;
13};
14
15console.log(a); // 123

1/*
2
3Therefore, 
4  
5  var:
6    -can be declared outside any function to be used inside any function
7    -can be declared inside any function or any other {} which are of only if or if-else or switch etc. and can be used anywhere inside the function
8    -can be changed again and again anywhere
9  let:
10    -can be declared outside any function to be used inside any function
11    -if declared inside any function or any other {} which are of only if or if-else or switch etc. and can't be used anywhere inside the function and can be only used inside statement
12    - can be changed again and again only inside the statement in which they are made in
13    const:
14    -can be declared outside any function to be used inside any function
15    -can be declared inside any function or any other {} which are of only if or if-else or switch etc. and can be used anywhere inside the function
16    -cannot be changed again and agan anywhere, if tried to, will result in an error
17
18*/
19// var has function scope. (this variable can be accssed from anywhere inside function)
20// let & const has block scope.(this variable can not be accessed out of block. means outside of if else but inside of a function it can not be accessed. only inside that block we can access this variable)
21// https://www.youtube.com/watch?v=--Vh17ocC_s
22function func(){
23  if(true){
24    var A = 1;
25    let B = 2;
26  }
27  A++; // 2 --> ok, inside function scope
28  B++; // B is not defined --> not ok, outside of block scope
29  return A + B; // NaN --> B is not defined
30}
31//example 2
32var variable1 // declared using var
33const variable2 // declared using const
34
35myFunction1();
36
37function myFunction1() {
38  variable1 = "hello!";
39  console.log(variable1);
40  // "hello!"
41}
42
43myFunction2();
44
45function myFunction2() {
46  variable2 = "hello!";
47  // error: variable2 is a constant and can not be redifined
48}
49
50myFunction3(true);
51
52myFunction3(codition) {
53  if(condition) {
54    let variable3 = "helo!" // declared using const
55  }
56  variable3 = "hello!";
57  // error: variable3 is out of scope
58}