Is there any way to reset Global variable in JavaScript

Based on your comment:

I am defining a local variable named FunctionName with className:MethodName as format. If I can define a variable with className, then I can use that instead of placing complete className in every method, just trying avoid retyping and spelling mistakes mostly.

...I think you want to just use className without modifying it:

var a = function() {
    var functionName = className + "a";
    // ...use functionName...
};

But if you really want to modify it for some reason, without using a local, then you can remember its old value and restore it:

var a = function() {
    var savedClassName = className + "a";
    try {
        className = className + "a";
        // ...use className...
    } finally {
        className = savedClassName;
    }
};

No matter how you exit the code in the try block, the finally block will be executed.

But there are very few use cases where that's the best approach.

If you really want to use a local variable with the same name, that is possible:

var className = "My Class is."; // define global var

let a = function() {
  let className = window.className; // define local var and copy global var value to local var
  className = className + "a"; // modify local var
  console.log(className); // print local var
};

console.log(className); // print global var value
a(); // does not change global var value
console.log(className) // print unchanged global var value

But it would be much clearer to just have different names for your local and global variables:

var className = "My Class is.";

let a = function() {
  let localClassName = className;
  localClassName = localClassName + "a";
  console.log(localClassName)
};

console.log(className);
a();
console.log(className)

Two ways:

1. (Better) Don't add the letter in the first place

var a = function() { console.log(className + "a"); }

or

var a = function() {
  let temp = className + "a" ;
  console.log(temp);
}    

2. Store the original value

`

 var className = ...;
 var backup = className;

 var a = function() {
   className = className + "a" ;
   console.log(className);
 }    
 console.log(className);
 a();
 className = backup;
 console.log(className);

`