SOURCE

const domInput = document.querySelector('input');
const domBtn = document.querySelector('button');
const domError = document.querySelector('.error');

const domSquared = document.querySelector('.squared');
const domCubed = document.querySelector('.cubed');
const domFactorial = document.querySelector('.factorial');

domBtn.addEventListener('click', result);
function result() {
    const num = domInput.value.trim();
    if (num === '' || isNaN(num)) {
        domError.textContent = '请输入数字！';
        return;
    }
    domError.textContent = '';

    domSquared.textContent = squared(num);
    domCubed.textContent = cubed(num);
    domFactorial.textContent = factorial(num);
}

// 平方
function squared(num) {
    const res = num ** 2;
    return `${num} 的平方等于 ${res}`;
}

// 立方
function cubed(num) {
    const res = num ** 3;
    return `${num} 的立方等于 ${res}`;
}

// 阶乘
function factorial(num) {
    let res = 1;
    for(i = num; i > 1; i--) {
        res *= i
    }
    return `${num} 的阶乘等于 ${res}`;
}

<p>请输入数字：</p>
<input type="text">
<button>确认</button>

<p class="error"></p>

<p class="squared"></p>
<p class="cubed"></p>
<p class="factorial"></p>

.error {
    color: red;
}