Anda perlu mengulangi julat setiap .price元素，而不是每个.percentage，因为你试图执行的每个单独的计算都是基于一个单独的<div class="price">.

Adakah semua pengiraan di dalam gelung, iaitu salah satu daripada priceElement（它是你的<div class="price">elemen semasa).

Kini anda boleh menggunakan priceElement.querySelector，而不是document.querySelector.

document.querySelector在整个文档中找到第一个匹配的元素，而priceElement.querySelector将在该范围内找到第一个匹配的元素：priceElement.

Jadi fungsinya kelihatan lebih seperti ini:

const priceDifference = () => {
  const price = document.querySelectorAll(".price");

  for (i = 0; i < price.length; i++) {
    const priceElement = price[i];

    const regPrice = priceElement.querySelector(".old").innerText;
    const salePrice = priceElement.querySelector(".new").innerText;
    const percentage = priceElement.querySelector(".percentage");

    const priceNumReg = parseFloat(regPrice.substring(1));
    const priceNumSale = parseFloat(salePrice.substring(1));

    const finalPrice = priceNumReg - priceNumSale;
    const percent = (finalPrice / priceNumReg) * 100;
    
    if (percentage.textContent.trim() === '') {
      percentage.textContent += ' (' + percent.toFixed(0) + '% OFF)';
    }
  }
};

Kod ini sah, tetapi masih ada beberapa kerja yang perlu dilakukan, seperti tiada i的声明和innerText vs. textContent, tetapi kod ini juga boleh dipendekkan seperti ini:

const parseCurrencyValue = ({ textContent }) => Number.parseFloat(textContent.replace(/^[^-+0-9.]*/u, "")),
  priceDifference = () => Array.from(document.querySelectorAll(".price"))
  .forEach((priceElement) => {
    const [
        regularPrice,
        salePrice
      ] = [ ".old", ".new" ]
        .map((selector) => parseCurrencyValue(priceElement.querySelector(selector))),
      discount = 1 - (salePrice / regularPrice),
      percentageElement = priceElement.querySelector(".percentage");

    if (percentageElement.textContent.trim() === "") {
      percentageElement.insertAdjacentText("beforeend", ` (${Math.round(discount * 100)} % off)`);
    }
  });

priceDifference();

.percentage {
  text-transform: uppercase;
}

<div class="price">
  <div class="old">.00</div>
  <div class="new">.00</div>
  <div class="percentage"></div>
</div>

<div class="price">
  <div class="old">.00</div>
  <div class="new">.00</div>
  <div class="percentage"></div>
</div>

<div class="price">
  <div class="old">.00</div>
  <div class="new">.00</div>
  <div class="percentage"></div>
</div>