HTML Code

<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="utf-8">
  <title>beforeinput block digits</title>
</head>
<body>
  <label>
    Name (letters only):
    <input id="name" placeholder="No digits" />
  </label>
  <pre id="out">Try typing letters and numbers</pre>
  <script>
    var field = document.getElementById("name");
    var out = document.getElementById("out");
    field.addEventListener("beforeinput", function (event) {
      if (event.data && /\d/.test(event.data)) {
        event.preventDefault();
        out.textContent = "Blocked digit: " + event.data;
      } else if (event.data) {
        out.textContent = "Allowed: " + JSON.stringify(event.data);
      }
    });
  </script>
</body>
</html>

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Element beforeinput — block digits (Try It)