SOURCE

/**
     * 银用卡分期利率计算
     * @param qi
     * @param fee
     */
function getTrueRate() {
  qi = document.getElementById("qi").value;
  fee = document.getElementById("fee").value;
  trueRate = 0.0;
  maxRate = 200.0;
  minRate = 0.0;
  count = 0;
  do {
    var money, tmoney = 100000;
    money = tmoney;
    blanceMonthly = money / qi;
    earn = 0.0;
    for (i = 0; i < qi; i++) {
      earn += money * trueRate / 12;
      money -= blanceMonthly;
    }
    pay = fee * tmoney * qi;
    if (Math.abs(earn - pay) < 0.0001 || (maxRate - minRate < 0.0000001)) {
      document.getElementById("result").innerHTML = ("年化利率：" + (trueRate  ) + "%,<br/> <br/>");
      return;
    } else {
      if ((earn - pay) < 0) {
        minRate = trueRate;
      } else {
        maxRate = trueRate;
      }
      trueRate = (maxRate + minRate) / 2;
      count++;
    }
  } while ( true );

}

/**
     * 分期套现 投资收益计算
     * @param money
     * @param qi
     * @param rate
     * @param fee
     */
function caculator(money, qi, rate, fee) {
  tmoney = money;
  blanceMonthly = money / qi;
  earn = 0.0;
  for (i = 0; i < qi; i++) {
    earn += money * rate / 12;
    money -= blanceMonthly;
  }
  pay = fee * tmoney * qi;

  result = "钱：" + tmoney + ", " + qi + "期,手续费：" + fee + "%，投资收益：" + rate + "%";
  result += "手续费共计：" + pay;
  result += "资赚取：" + earn;
  result += "赚：" + (earn - pay);
  result += "===================";
  document.getElementById("result").innerHTML = result;
}

<div class="box">
  <h2>信用卡分期，实际年化利率计算器</h2>
  <div>
    <label>
      分期数
      <input type="input" value="6" id="qi" />
    </label>
    
    <label>
      费率
      <input type="input" value="0.7" id="fee" /> %
    </label>
  </div>
  <div id="result">
    点击计算按钮进行计算<br/><br/>
  </div>
  <button onclick="getTrueRate()">
    计算
  </button>
</div>

.box {
  width: 400px;
  margin: auto;
  background: #003942;
  color: #FFF;
  padding: 15px;
  border-radius: 10px;
}

input {
  width: 60px;
}

button{     
  outline: none;
  margin: auto;
  width:100%;
  height:40px;
  background: #007588;
  border: none;
  border-radius: 30px;
  color: #FFF;
}
button:hover{
  background:#0098b1;
}
#result {
  margin: 10px -15px;
  background: #2f2f2f;
  padding: 10px;
  font-size:12px;
}