How to change the value of a global variable inside of a function using JavaScript ?

Pre-requisite: Global and Local variables in JavaScript

Local Scope: Variables which are declared inside a function is called local variables and these are accessed only inside the function. Local variables are deleted when the function is completed.

Global Scope: Global variables can be accessed from inside and outside the function. They are deleted when the browser window is closed but is available to other pages loaded on the same window. There are two ways to declare a variable globally:

Declare a variable outside the functions.
Assign value to a variable inside a function without declaring it using “var” keyword.

<!DOCTYPE html> 
<html> 
  
<head> 
    <title> 
        How to change the value of a global 
        variable inside of a function using 
        JavaScript? 
    </title> 
      
    <script> 
          
        // Declare global variables 
        var globalFirstNum1 = 9; 
        var globalSecondNum1 = 8; 
      
        function add() { 
      
            // Access and change globalFirstNum1 and globalSecondNum1 
            globalFirstNum1 = Number(document.getElementById("fNum").value); 
            globalSecondNum1 = Number(document.getElementById("sNum").value); 
                  
            // Add local variables 
            var result = globalFirstNum1 + globalSecondNum1; 
              
            var output = "Sum of 2 numbers is " + result; 
              
            // Display result 
            document.getElementById("result").innerHTML = output; 
    } 
            // Declare global variables 
            globalFirstNum2 = 8; 
            globalSecondNum2 = 9; 
      
             
        function subtract() { 
               // Access and change globalFirstNum2 
            // and globalSecondNum2 
  
                globalFirstNum2 = Number(document.getElementById("fNum").value); 
                globalSecondNum2 = Number(document.getElementById("sNum").value); 
              
      
            // Use global variables to subtract numbers 
            var result = globalFirstNum2-globalSecondNum2; 
              
            var output = "Difference of 2 numbers is " + result; 
            document.getElementById("result").innerHTML = output; 
        } 
    </script> 
</head> 
  
<body style="text-align:center;"> 
      
    <h1 style="color:green"> 
        GeeksForGeeks 
    </h1>  
      
    <b>Enter first number :- </b>  
    <input type="number" id="fNum">  
      
    <br><br>  
      
    <b>Enter second number :- </b>  
    <input type="number" id="sNum">  
      
    <br><br>  
      
    <button onclick="add()">Add</button>  
    <button onclick="subtract()">Subtract</button> 
          
    <p id="result" style = "color:green; font-weight:bold;"> 
    </p> 
</body> 
  
</html> 

Output:

Before clicking the button:
After clicking add button:
After clicking subtract button:

My Personal Notes

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