Change the value of a global variable inside of a function using JavaScript

Last Updated : 03 Nov, 2025

Global variables can be accessed from inside and outside the function. They are deleted when the browser window is closed but are 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 a value to a variable inside a function without declaring it using the "var" keyword.

JavaScript

let count = 10;
function updateCount() {
    
  // modifies the global variable
  count = 20; 
}
updateCount();
console.log(count);

This example demonstrates how to change and use global variables inside functions in JavaScript.

HTML

<body>
    <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>
    <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>
</body>