JSFiddle - React, Tailwind, and code Playground

HTML

<meta name="viewport" content="width=device-width, initial-scale=1.0">

<h2>利息差計算</h2>

<div class="card">
  <label>本金</label>
  <input type="number" id="principal" inputmode="numeric" placeholder="例如 200000">

  <label>天數</label>
  <div class="days">
    <button onclick="setDays(30)" id="d30">30 天</button>
    <button onclick="setDays(31)" id="d31">31 天</button>
  </div>

  <button class="main-btn" onclick="calculate()">計算</button>
</div>

<div class="card">
  <h3>A 利息 (1.5%)</h3>
  <div id="detailA"></div>
  <div class="big-number" id="resultA">0</div>
</div>

<div class="card">
  <h3>B 利息</h3>
  <div id="detailB"></div>
  <div class="big-number" id="resultB">0</div>
</div>

<div class="card highlight">
  <h3>差額 B - A</h3>
  <div class="big-number" id="diff">0</div>
</div>

CSS

body {
  font-family: -apple-system, BlinkMacSystemFont, sans-serif;
  margin: 0;
  padding: 15px;
  background: #f2f4f7;
}

h2 {
  text-align: center;
  margin-bottom: 15px;
}

.card {
  background: white;
  padding: 15px;
  margin-bottom: 12px;
  border-radius: 12px;
  box-shadow: 0 2px 6px rgba(0,0,0,0.08);
}

.highlight {
  background: #eaf7ff;
}

label {
  display: block;
  margin-top: 10px;
  margin-bottom: 5px;
  font-weight: bold;
}

input {
  width: 100%;
  padding: 10px;
  font-size: 16px;
  border-radius: 8px;
  border: 1px solid #ccc;
  box-sizing: border-box;
}

/* 天數按鈕 */
.days {
  display: flex;
  gap: 10px;
}

.days button {
  flex: 1;
  padding: 10px;
  font-size: 16px;
  border-radius: 8px;
  border: 1px solid #ccc;
  background: #fff;
}

.days button.active {
  background: #007bff;
  color: white;
  border: none;
}

/* 主按鈕 */
.main-btn {
  width: 100%;
  margin-top: 15px;
  padding: 12px;
  font-size: 18px;
  border: none;
  border-radius: 10px;
  background: #28a745;
  color: white;
}

/* 大數字 */
.big-number {
  font-size: 28px;
  font-weight: bold;
  text-align: right;
  margin-top: 10px;
}

/* 計算式 */
.formula {
  font-size: 13px;
  color: #555;
  margin: 4px 0;
  word-break: break-all;
}

JavaScript

let selectedDays = 30;

function setDays(days) {
  selectedDays = days;

  document.getElementById("d30").classList.remove("active");
  document.getElementById("d31").classList.remove("active");

  document.getElementById("d" + days).classList.add("active");
}

function format(num) {
  return num.toLocaleString();
}

function calculate() {
  const principal = Number(document.getElementById("principal").value);
  const days = selectedDays;

  if (!principal || principal <= 0) {
    alert("請輸入本金");
    return;
  }

  // ===== A =====
  let rawA = principal * 0.015 * days / 365;
  let interestA = Math.round(rawA);

  document.getElementById("detailA").innerHTML = `
    <div class="formula">${format(principal)} × 1.5% × ${days} / 365 = ${rawA.toFixed(2)}</div>
    <div class="formula">→ ${interestA}</div>
  `;

  // ===== B =====
  let remaining = principal;
  let totalB = 0;
  let html = "";

  let tier1 = Math.min(remaining, 150000);
  let calc1 = tier1 * 0.00875 * days / 365;
  totalB += calc1;
  remaining -= tier1;

  if (tier1 > 0) {
    html += `<div class="formula">${format(tier1)} × 0.875% × ${days}/365 = ${calc1.toFixed(2)}</div>`;
  }

  if (remaining > 0) {
    let tier2 = Math.min(remaining, 150000);
    let calc2 = tier2 * 0.02 * days / 365;
    totalB += calc2;
    remaining -= tier2;

    html += `<div class="formula">${format(tier2)} × 2% × ${days}/365 = ${calc2.toFixed(2)}</div>`;
  }

  if (remaining > 0) {
    let calc3 = remaining * 0.016 * days / 365;
    totalB += calc3;

    html += `<div class="formula">${format(remaining)} × 1.6% × ${days}/365 = ${calc3.toFixed(2)}</div>`;
  }

  let interestB = Math.round(totalB);

  html += `<div class="formula">合計 ${totalB.toFixed(2)} → ${interestB}</div>`;

  document.getElementById("detailB").innerHTML = html;

  // ===== 差額 =====
  let diff = interestB - interestA;

  document.getElementById("resultA").innerText =...