首先，我querySelector改为querySelectorAll. 然后，我遍历页面上的所有卡片并附加单击事件处理程序，就像您最初所做的那样。

var cards = document.querySelectorAll('.card');

cards.forEach(card => {
  card.addEventListener('click', function() {
    card.classList.toggle('is-flipped');
  });
})
.flip-box {
  background-color: transparent;
  width: 300px;
  height: 200px;
  border: 1px solid #f1f1f1;
}

.card {
  position: relative;
  width: 100%;
  height: 100%;
  text-align: center;
  transition: transform 1s;
  transform-style: preserve-3d;
}

.flip-box-front,
.flip-box-back {
  position: absolute;
  width: 100%;
  height: 100%;
  -webkit-backface-visibility: hidden;
  backface-visibility: hidden;
}

.flip-box-front {
  background-color: #bbb;
  color: black;
}

.flip-box-back {
  background-color: #333;
  color: white;
  transform: rotateY(180deg);
}

.card.is-flipped {
  transform: rotateY(180deg);
}
<div class="flip-box">
  <div class="card">
    <div class="flip-box-front">
      <h2>Front Side</h2>
      <p> click to fillp </p>
      <p> click to fillp </p>
      <p> click to fillp </p>
    </div>

    <div class="flip-box-back">
      <h2>Back Side</h2>
      <p> click to fillp </p>
      <p> click to fillp </p>
      <p> click to fillp </p>

    </div>
  </div>

  <div class="card">
    <div class="flip-box-front">
      <h2>Front Side</h2>
      <p> click to fillp </p>
      <p> click to fillp </p>
      <p> click to fillp </p>
    </div>

    <div class="flip-box-back">
      <h2>Back Side</h2>
      <p> click to fillp </p>
      <p> click to fillp </p>
      <p> click to fillp </p>

    </div>
  </div>
</div>