即使包裹在Number（）中，JavaScript函数也会返回NaN吗？

Question

公平警告，我是JS初学者：

尝试进行初学者项目（技巧计算器）。我一直在努力使这项工作太久了。

我试图获取输入/选择的值，做一个方程式，并将结果显示在底部。

但是该函数立即显示，而不是在单击按钮时显示。。。它显示的是NaN而不是我想要的值。我想念什么？

const billInput = document.getElementById('billInput');
const serviceInput = document.getElementById('serviceInput');
const peopleInput = document.getElementById('peopleInput');
    
const tipBtn = document.getElementById('tipBtn');
    
const tipOutput = document.getElementById('tipOutput');
    
tipBtn.addEventListener('click', tipCalculation());
    
function tipCalculation() {
    let cost = Number(billInput.value);
    let service = Number(serviceInput.value);
    let people = Number(peopleInput.value);
    
    document.getElementById('tipOutput').innerHTML = cost * service / people;
}

<div class="main-box">
    <h2 class="title-top">What Should I Tip?</h2>
    <div class="input-group">
        <div class="pre">$</div>
        <input type="number" id="billInput" placeholder="How much was the bill?">
    </div>

    <p class="titles service-title"></p>
    <select required name="Service-lvl" id="serviceInput">
        <option disabled selected>How was the service?</option>
        <option value=".35">Amazing!</option>
        <option value=".25">Solid</option>
        <option value=".15">Meh...</option>
    </select>

    <div class="input-group bottom-input">
        <input type="number" id="peopleInput" placeholder="How many are you splitting with?">
    </div>
    
    <button class="tipBtn" id="tipBtn">How Much Are You Tipping?</button>
    
    <h3 id="tipOutput">Tip Ammount</h3>
</div>