JavaScript　割り勘電卓

Math.floorは切り捨て、Math.ceilは切り上げ、Math.absは絶対値という仕組みを上手く利用します。正規関数/^[1-9][0-9]*$/も使用します。

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<!DOCTYPE html>
<html lang="ja">
  <head>
    <meta charset="utf-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>割り勘電卓</title>
    <link rel="stylesheet" href="styles.css">
    <style>
      body {
        font-size:16px;
        text-align: center;
        font-family: Arial, sans-serif;
      }
      h1 {
        font-size: 24px;
      }
      input[type="text"]{
        padding: 7px;
        border: 1px solid #ddd;
        border-radius: 3px;
        width: 100px;
        font-weight: bold;
        font-size: 18px;
        text-align: right;
      }
      #btn {
        margin: 30px auto;
        width: 180px;
        border-radius: 5px;
        box-shadow: 0 4px 0 #e91b0c;
        background: #f44336;
        color: #fff;
        cursor: pointer;
        padding: 7px;
      }
      #btn:hover{
        opacity: 0.8;
      }
    </style>
  </head>
  <body>
      <h1>割り勘電卓</h1>
      <p>金額 <input type="text" id="price" value="0">
      <p>人数 <input type="text" id="num" value="0">
      <div id="btn">計算する</div>
      <p id="result"></p>
      <script>
      (function(){
          'use strict';
 
          var priceForm = document.getElementById('price');
          var numForm = document.getElementById('num');
          var btn = document.getElementById('btn');
          var result = document.getElementById('result');
 
          priceForm.addEventListener('click', function(){
              this.select();
          });
          numForm.addEventListener('click', function(){
              this.select();
          });
 
          btn.addEventListener('click', function(){
              var price = priceForm.value;
              var num = numForm.value;
              var x1, x2, y1, y2;
              var unit = 100;
 
              if (price.match(/^[1-9][0-9]*$/) && num.match(/^[1-9][0-9]*$/)){
                  if (price % num === 0){
                    result.innerHTML = '一人' +(price / num)+'円丁度です！';
                  } else {
                    x1 = Math.floor(price / num / unit) * unit;
                    y1 = price - (x1 * num);
                    x2 = Math.ceil(price / num / unit) * unit;
                    y2 = Math.abs(price - (x2 * num));
                    result.innerHTML =
                    '一人' + x1 + '円だと'　+ y1 + '円足りません。<br>' +
                    '一人' + x2 + '円だと' + y2 + '円余ります。';
                  }
              } else {
                result.innerHTML = '入力された値に誤りがあります。'
              }
          });
      })();
      </script>
  </body>
</html>

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