<div calss="wrap">
  <div class="box">1</div>
  <div class="box">2</div>
  <div class="box">3</div>
  <div class="box">4</div>
  <div class="box">5</div>
  <div class="box">6</div>
  <div class="box">7</div>
  <div class="box">8</div>
  <div class="box">9</div>
  <div class="box">10</div>
</div>
.box {
  width: 10rem;
  height: 10rem;
  margin: 5rem auto;
  display: flex;
  justify-content: center;
  align-items: center;
  font-size: 2rem;
  background-color: #febf00;
  transition: .4s;  
}

.box.active {
  width: 20rem;
  background: #f03d3d;
  color: #fff;
}

/* ------------- */

* {
  margin: 0;
  padding: 0;
  outline: none;
  box-sizing: border-box;
}

body {
  background-color: #f8f8f8;
}

@import url("https://cdn.jsdelivr.net/gh/orioncactus/pretendard/dist/web/static/pretendard.css");

* {
  font-family: Pretendard;
}
const options = {
  root: null, // viewport
  rootMargin: "0px",
  threshold: 1.0,  // 50%가 viewport에 들어와 있어야 callback 실행
}

const observer = new IntersectionObserver(entries => {
  entries.forEach(entry => {
    if (entry.isIntersecting) {
      entry.target.classList.add('active');
    } else {
      entry.target.classList.remove('active');
    }
  });
}, options);

const boxList = document.querySelectorAll('.box');

// 반복문을 돌려 모든 DOM에 적용
boxList.forEach(el => observer.observe(el));

External CSS

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External JavaScript

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