<!DOCTYPE html>
<html>
<head>
  <meta charset="utf-8">
  <title>Prevent return key should not submit form</title>
  <script type="application/javascript">
  function init() {
    let input = document.getElementById("input");
    input.addEventListener("keydown", function(aEvent) {
      if (aEvent.keyCode == 13) { // return key
        alert("Hello!");
        aEvent.preventDefault();
        return false;
      }
      return true;
    }, {once: true});

    let form = document.getElementById("form");
    form.addEventListener("submit", function() {
      let result = document.getElementById("result");
      result.innerHTML = "submitted";
    }, {once: true});
  }
  </script>
</head>

<body onload="init()">
  <form id="form">
    <input type="text" id="input">
    <button type="submit" id="submitBtn">Submit</button>
  </form>
  <p id="result">not submitted</p>
</body>
</html>
