Checkbox object in HTML has a property checked as boolean - it has true or false, so javascript can check its status easily.
<script> function myFunction() { // Get the checkbox var checkBox = document.getElementById("myCheck"); // Get the output text var text = document.getElementById("text"); // If the checkbox is checked, display the output text if (checkBox.checked == true){ text.style.display = "block"; } else { text.style.display = "none"; } } </script> Checkbox: <input type="checkbox" id="myCheck" onclick="myFunction()"> <p id="text" style="display:none">Checkbox is CHECKED!</p>