JavaScript Comment
The JavaScript comments are meaningful way to deliver message. It is used to add information about the code, Warnings or Suggestions.

So that end user can easily interpret(understand) the code.
The Java Script comment is ignored by the JavaScript engine i.e. embedded in the browser.

Advantages of JavaScript Comments.

There are mainly two advantages of JS comments.

1. To make Code easy to understand.
2. To avoid the unnecessary code.

JS variables

Variable can store some information, we can use that information later
We can change that information later.

Rules for naming the variables:
Name must start with a letter (a to z ), underscore or dollar sign
After first letter we can use digit (0 to 9)
JavaScript variable case sensitive
Cannot start with numbers :

Eg. 1value (invalid)

We can only use dollar symbol or (under score) to define the variable

E.g. _value  OR $value (both valid)

Js local variable : it is declared inside block or function .it is accessible within function or block only.

Js global variable :it is accessible from any function. A variable that declared outside the function or declared with window object is known as global variable.

*var : The var is the oldest keyword to declare a variable in *
JavaScript.
Scope: Global scoped or function scoped. The scope of the var keyword is the global or function scope. It means variables defined outside the function can be accessed globally, and variables defined inside a particular function can be accessed within the function.

Example 1: Variable ‘a’ is declared globally. So, the scope of the variable ‘a’ is global, and it can be accessible everywhere in the program. The output shown is in the console.

<script>
    var a = 10
        function f(){
            console.log(a)
        }
    f();
    console.log(a);
</script>
Output: 

10
10

**Example 2: **The variable ‘a’ is declared inside the function. If the user tries to access it outside the function, it will display the error. Users can declare the 2 variables with the same name using the var keyword. Also, the user can reassign the value into the var variable. The output shown in the console.

<script>
    function f() {
         // It can be accessible any
        // where within this function
        var a = 10;
        console.log(a)
    }
    f(); 
    // A cannot be accessible outside of function
    console.log(a);
</script>
Output:

10
ReferenceError: a is not defined

Example 3: we can re-declare variable using var and we can update var variable. The output is shown in the console.

<script> 
    var a = 10
       // User can re-declare
    // variable using var
    var a = 8
     // User can update var variable
    a = 7
</script>
Output:
7

**example 4: **If users use the var variable before the declaration, it initializes with the undefined value. The output is shown in the console.

<script>
    console.log(a);
    var a = 10; 
<script>g 

============================================================
let keyword in JavaScript: The let keyword is an improved version of the var keyword.

Scope: block scoped: The scope of a let variable is only block scoped. It can’t be accessible outside the particular block ({block}). Let’s see the below example.

Example 1: The output is shown in the console.

<script>
    let a = 10;
    function f() {
        let b = 9
        console.log(b);
        console.log(a);
    }
    f();
</script>
Output:

9
10
///////////////////////////////////////////////////////////////////Var 
    {// this is a block 
       var a = 10;
        console.log(a)
    }
   console.log(a)/////
> 10
> 10
/////////////////////////////////////////////////////00

Example 2: The code returns an error because we are accessing the let variable outside the function block. The output is shown in the console.

<script>
    let a = 10;
    function f() {
        if (true) {
            let b = 9

            // It prints 9
            console.log(b);
        } 
        // It gives error as it
        // defined in if block
        console.log(b);
    }
    f()
    console.log(a)    // It prints 10
</script>
Output:
9
ReferenceError: b is not defined

Example 3: we cannot re-declare the variable defined with the let keyword but can update it.

<script> 
    let a = 10 
    // It is not allowed
    let a = 10

    // It is allowed
    a = 4
</script>
Output:

Uncaught SyntaxError: Identifier 'a' has already been declared

Example 4: we can declare the variable with the same name in different blocks using the let keyword.

<script>
  let a = 10
  if (true) {
    let a=9
    console.log(a) // It prints 9
  }
  console.log(a) // It prints 10t5re
</script>
Output:

9 
10

Example 5: If we use the let variable before the declaration, it does not initialize with undefined just like a var variable and return an error.

<script>
    console.log(a);
    let a = 10;
</script>
Output:

Uncaught ReferenceError: Cannot access 'a' before initialization

const keyword in JavaScript: The const keyword has all the properties that are the same as the let keyword, except the user cannot update it.

Scope: block scoped: When users declare a const variable, they need to initialize it, otherwise, it returns an error. The user cannot update the const variable once it is declared.

Example 1: We are changing the value of the const variable so that it returns an error. The output is shown in the console.

<script>
    const a = 10;
    function f() {
        a = 9
[]7
        console.log(a)
    }
    f();
</script>
Output:
a=9
TypeError:Assignment to constant variable.

////////////////////
  const a = 10;
    function f() {
       const a = 9
        console.log(a)
    }
    f();//>9
///////////////////////////////////

const a = 10;
    function f() {
       var/let a = 9
        console.log(a)
    }
    f();//9
////////////////////////////////////////

Example 2: Users cannot change the properties of the const object, but they can change the value of properties of the const object.

<script>
    const a = {
        prop1: 10,
        prop2: 9
    }

    // It is allowed
    a.prop1 = 3

    // It is not allowed
    a = {
        b: 10,
        prop2: 9
    }
</script>
Output:

Uncaught SyntaxError:Unexpected identifier