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