<form name="dummy" action="dummy" onsubmit="return formCheck()">
  <input type="text" id="text1" />
  <input type="submit" value="送信" />
</form>
function formCheck() {
  /*ここにチェックロジック*/
  // テキストボックスの値取得
  var text1 = document.getElementById("text1").value;
  
  // 未入力の場合alert表示&submit止める
  if (text1 == "") {
    alert("未入力です");
    return false;
  } else {
    return true;
  }
}

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