Live Demo: Set and Get Cookies in JavaScript

<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>Set and Get Cookies in JavaScript</title>
</head>
<body>
    <script>
    // A custom function to set cookies
    function setCookie(name, value, daysToLive) {
        // Encode value in order to escape semicolons, commas, and whitespace
        let cookie = name + "=" + encodeURIComponent(value);
    
        if(typeof daysToLive === "number") {
            /* Sets the max-age attribute so that the cookie expires
            after the specified number of days */
            cookie += "; max-age=" + (daysToLive*24*60*60);
    
            document.cookie = cookie;
        }
    }
    
    // A custom function to get cookies
    function getCookie(name) {
        // Split cookie string and get all individual name=value pairs in an array
        let cookieArr = document.cookie.split(";");
    
        // Loop through the array elements
        for(let i = 0; i < cookieArr.length; i++) {
            let cookiePair = cookieArr[i].split("=");
    
            /* Removing whitespace at the beginning of the cookie name
            and compare it with the given string */
            if(name == cookiePair[0].trim()) {
                // Decode the cookie value and return
                return decodeURIComponent(cookiePair[1]);
            }
        }
    
        // Return null if not found
        return null;
    }
    
    // A custom function to check cookies
    function checkCookie() {
        // Get cookie using our custom function
        let firstName = getCookie("firstName");
    
        if(firstName != null) {
            alert("Welcome again, " + firstName);
        } else {
            firstName = prompt("Please enter your first name:");
            if(firstName != "" && firstName != null) {
                // Set cookie using our custom function
                setCookie("firstName", firstName, 1);
            }
        }
    }
    
    // Check the cookie on page load
    window.onload = checkCookie;

// Uncomment the following line to delete this cookie
	// setCookie("firstName", "", 0);
    </script>
    <p><strong>Note:</strong> Now if you press the "Show Output" button or refresh the page, you will see a greeting message, if you've entered your first name in the prompt dialog box. See the last line of the JavaScript code to know how to delete this <b>firstName</b> cookie.</p>
</body>
</html>