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<!DOCTYPE html> <html> <head> <script> function validateForm() { var x = document.forms["myForm"]["fname"].value; if (x == "" || x == null) { alert("Name must be filled out"); return false; } } </script> </head> <body> <h1>JavaScript 验证空输入框</h1> <p>请尝试在不输入任何文本的情况下提交表单。</p> <form name="myForm" action="/demo/action_page.php" onsubmit="return validateForm()" method="post" required> Name: <input type="text" name="fname"> <input type="submit" value="Submit"> </form> </body> </html>