JSFiddle - React, Tailwind, and code Playground

HTML

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

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>My Contact List</title>
    <link rel="stylesheet" href="css/style.css">
    <script src="https://kit.fontawesome.com/3ad7573e76.js" crossorigin="anonymous"></script>
</head>

<body>
    <header>
        <div id="logo-div">
            <h1 id="logo">
                <span class="highlight"><i class="fas fa-user-friends"></i></span> My<span
                    class="highlight">Contact</span>List
            </h1>
        </div>
        <div id="search-div">
            <button class="show-btn"><i class="fas fa-search"></i></button>
            <input id="search" type="text" placeholder="Search contacts...">
        </div>
    </header>

    <script src="js/main.js"></script>
</body>

</html>

CSS

/* GENERAL */
:root {
    --light-color: #ccc;
    --lighter-color: #f4f4f4;
    --dark-color: #333;
    --darker-color: #222;
    --brand-color: #ff4;
    --danger: #f44;
    --danger-dark: #c00;
}

* {
    margin: 0;
    padding: 0;
    box-sizing: border-box;
}

body {
    background: var(--dark-color);
    color: var(--light-color);
    font-family: "Trebuchet MS";
}

ul li {
    list-style: none;
}

button,
input {
    outline: none;
}

/* UTILITY */
.highlight {
    color: var(--brand-color);
}

.show {
    display: block !important;
}

/* HEADER */
header {
    background: var(--darker-color);
    display: flex;
    flex-direction: row;
    align-items: center;
    text-align: center;
    justify-content: space-between;
    padding: 1.4rem 6rem;
    width: 100%;
}

#logo {
    font-size: 2.4rem;
    font-weight: 200;
}

#search-div {
    width: auto;
    height: auto;
    display: flex;
    gap: 0.4rem;
}

.show-btn {
    padding: 0.6rem 0.7rem;
    background: var(--light-color);
    border-radius: 5px;
    border: none;
    transition: ease-in 300ms;
    font-size: 1.2rem;
    cursor: pointer;
    height: 100%;
    margin-top: 2px;
}

.show-btn:hover {
    background: var(--brand-color);
    transition: ease-in 300ms;
}

#search {
    display: none;
    background: var(--lighter-color);
    color: var(--darker-color);
    height: 100%;
    width: 100%;
    font-size: 1.2rem;
    padding: 0.6rem 1rem;
    border: black 2px solid;
    border-radius: 2px;
}

JavaScript

const searchDiv = document.querySelector("#search-div");

// ADD EVENT LISTENERS
searchDiv.addEventListener("click", showSearch);

// FUNCTION: SHOW SEARCH BAR ON BUTTON CLICK
function showSearch(e) {
    e.preventDefault;
    if (
        e.target.classList.contains("show-btn") ||
        e.target.classList.contains("fas")
    ) {
        const searchBar = document.querySelector("#search");
        searchBar.classList.add("show");
    }
}