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HTML

              
                <!DOCTYPE html>
<html>

<head>
  <meta charset="utf-8">
  <meta name="viewport" content="width=device-width">
  <title>曜日を表示する</title>
</head>

<body>
  <p>inputにyyyy/mm/dd形式で日付を入力してください</p>
  <p>その日の曜日をアラートで表示します。</p>
  <input type="text" name="date_str" onblur="blurFunction()"></input>
  <p>フォーカスアウトした際に過去日かの判定を行います。</p>

</body>

</html>
              
            
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CSS

              
                
              
            
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JS

              
                function blurFunction() {
  // テキストボックスの日時をDate型にし、曜日を取得する

  // 文字列から年月日を抜き出しDate型に
  let date_str = document.getElementsByName("date_str");
  let date_str_value = date_str[0].value;
  if (date_str_value != "") {
    let year = parseInt(date_str_value.substr(0, 4), 10);
    let month = parseInt(date_str_value.substr(5, 2), 10) - 1;
    let day = parseInt(date_str_value.substr(8, 2), 10);
    let valueDate = new Date(year, month, day);

    
    // 曜日の配列を作成
    let dayOfTheWeekStr = ["日", "月", "火", "水", "木", "金", "土"];

    // 入力された日付の曜日を取得(数値)
    let dayOfTheWeekNum = valueDate.getDay();

    // 配列の曜日の文字列から曜日の数値の配列を表示するアラート
    alert(dayOfTheWeekStr[dayOfTheWeekNum] + "曜日です。");
  }
}

              
            
!
999px

Console