<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8" />
    <meta name="viewport" content="width=device-width, initial-scale=1.0" />
    <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/font-awesome/5.14.0/css/all.min.css" integrity="sha512-1PKOgIY59xJ8Co8+NE6FZ+LOAZKjy+KY8iq0G4B3CyeY6wYHN3yt9PW0XpSriVlkMXe40PTKnXrLnZ9+fkDaog==" crossorigin="anonymous" />
    <link rel="stylesheet" href="styles.css" />
    <title>Hidden Search</title>
</head>

<body>
    <div class="search"> <input type="text" class="input" placeholder="Search...">
        <button class="btn">
            <i class="fas fa-search"></i>
        </button>
    </div>
    <script src="app.js"></script>
</body>

</html>
@import url('https://fonts.googleapis.com/css2?family=Roboto:wght@400;700&display=swap');
* {
    box-sizing: border-box;
}

body {
    background-image: linear-gradient(90deg, #7d5fff, #7158e2);
    font-family: 'Roboto', sans-serif;
    display: flex;
    align-items: center;
    justify-content: center;
    height: 100vh;
    overflow: hidden;
    margin: 0;
}

.search {
    position: relative;
    height: 50px;
}

.search .input {
    background-color: #fff;
    border: 0;
    font-size: 18px;
    padding: 15px;
    height: 50px;
    width: 50px;
    transition: width 0.3s ease;
}

.btn {
    background-color: #fff;
    border: 0;
    cursor: pointer;
    font-size: 24px;
    position: absolute;
    top: 0;
    left: 0;
    height: 50px;
    width: 50px;
    transition: transform 0.3s ease;
}

.btn:focus,
.input:focus {
    outline: none;
}

.search.active .input {
    width: 200px;
}

.search.active .btn {
    transform: translateX(198px);
}
const search = document.querySelector('.search')
const btn = document.querySelector('.btn')
const input = document.querySelector('.input')
btn.addEventListener('click', () => {
    search.classList.toggle('active')
    input.focus()
})

External CSS

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External JavaScript

This Pen doesn't use any external JavaScript resources.