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<!DOCTYPE html> <html> <body> <form id="myForm"> Checkbox: <input type="checkbox" id="myCheck"> </form> <p>Click the "Try it" button to display the id of the form the checkbox belongs to.</p> <button onclick="myFunction()">Try it</button> <p id="demo"></p> <script> function myFunction() { var x = document.getElementById("myCheck").form.id; document.getElementById("demo").innerHTML = x; } </script> </body> </html>