var vs let vs const typescript

1var: 
2	- hoisted (always declared at top of scope, global if none)
3    - function scope
4let:
5    - block scope
6    - not redeclarable
7const: 
8    - block scope
9    - not reassignable
10    - not redeclarable
11    
12Note: Although it may seem like these hold only semantic meaning, using the
13appropriate keywords helps the JS engines' compiler to decide on what to optimize.

1// ⚡ Initialization
2var a; //✅
3let b; //✅
4const c; //❌ const must have an initial value
5
6// ⚡ Re-Assignment
7var d = 1; 
8d = 10; //✅
9
10let e = 2; 
11e = 20; //✅
12
13const f = 3; 
14f = 30 //❌ const value is always constant and cannot be reassigned
15
16// ⚡ Re-Declaration
17var g = 1; 
18var g = 10; //✅
19
20let h = 2; 
21let h = 20; //❌ must use variable name only to reassign, cannot be reassigned 
22
23const i = 3; 
24const i = 30 //❌ cannot be reassigned
25
26// ⚡ Scope [Global, Function or local and block scope]
27// Global Scope ==> Any variable that is declared outside any of the function
28// Function Scope(var) ==> Can be accessed inside the function it was declared and its child
29// Block Scope(let and const) ==> Con be accessed inside curly brackets where it was declared and its child
30
31function myFunction(){
32    var myName1 = 'Asmita';
33    let myName2 = 'Dikshit';
34    const myName3 = 'Sizen';
35    console.log(myName1);
36    console.log(myName2);
37    console.log(myName3);
38
39    if(true){
40        console.log(myName1);
41        console.log(myName2);
42        console.log(myName3);
43
44        var myAge1 = 13;
45        let myAge2 = 15;
46        const myAge3 = 14;
47        console.log(myAge1);
48        console.log(myAge2);
49        console.log(myAge3);
50    }
51
52    console.log(myName1);
53    console.log(myName2);
54    console.log(myName3);
55
56    console.log(myAge1);
57    console.log(myAge2);
58    console.log(myAge3);
59};
60myFunction();

1let num1:number = 1; 
2    
3function letDeclaration() { 
4    let num2:number = 2; 
5
6    if (num2 > num1) { 
7        let num3: number = 3;
8        num3++; 
9    } 
10
11    while(num1 < num2) { 
12        let num4: number = 4;
13        num1++;
14    }
15
16    console.log(num1); //OK
17    console.log(num2); //OK 
18    console.log(num3); //Compiler Error: Cannot find name 'num3'
19    console.log(num4); //Compiler Error: Cannot find name 'num4'
20}
21
22letDeclaration();
23