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HTML

              
                <input id="form" type="text" value="小数を入力してください" />
<p id="result" />
              
            
!

CSS

              
                
              
            
!

JS

              
                /*入力フォームの要素*/
var form=document.getElementById("form");
/*結果出力用の要素*/
var result=document.getElementById("result");

/*小数点以下の桁数を返す関数*/
function numDigitsUnder1(num){
  /*一旦数値を文字列に変換し、小数点で分割、2つめの長さを求める*/
   return num.toString().split('.')[1].length;
}
/*桁数を返す関数*/
function numDigits(num){
  /*一旦数値を文字列に変換し、長さを求める*/
   return num.toString().length;
}

/*フォーム入力のイベントハンドラ*/
form.addEventListener("input", (e) => {
  /*数値かチェック*/
  if (isNaN(form.value)) {
    /*パターンにマッチした場合*/
    result.textContent = "数値ではありません";
  } else if (form.value.toString().split('.').length != 2) {
    /*小数ではありません場合*/
    result.textContent = "小数ではありません";
  } else {
    /*パターンにマッチしない場合*/
    result.textContent = '小数点以下'+numDigitsUnder1(form.value) + "桁";
  }
})
              
            
!
999px

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