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HTML 

              
                <div class="widget-wrap">
  <div id="sort"></div>
  <h1>SEAT RESERVATION</h1>
  
  <!-- (A) SEAT LAYOUT -->
  <div id="layout"></div>

  <!-- (B) LEGEND -->
  <div id="legend">
    <div class="seat"></div> <div class="txt">Available</div>
    <div class="seat taken"></div> <div class="txt">Taken</div>
    <div class="seat selected"></div> <div class="txt">Your Chosen Seats</div>
  </div>

  <!-- (C) SAVE SELECTION -->
  <button id="save" onclick="reserve.save()">Reserve Seats</button>
  
  <!-- (X) VISIT CODE-BOXX -->
  <div id="code-boxx">
    Visit
    <a href="https://code-boxx.com/seat-reservation-javascript/"
       target="_blank">
      Code Boxx
    </a> for more details.
  </div>
</div>
!

CSS 

              
                /* (A) SEAT & "COLOR CODE" */
.seat {
  text-align: center;
  padding: 20px 10px;
  border-radius: 10px;
  background: #f1f1f1;
}
.taken { background: #df0000; color: #fff; }
.selected { background: #87ff96; }

/* (B) SEATS LAYOUT */
#layout {
  max-width: 400px;
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  grid-gap: 10px;
  margin-bottom: 20px;
}

/* (C) LEGEND */
#legend {
  display: grid;
  grid-gap: 10px;
  grid-template-columns: 50px auto ;
}
#legend .txt {
  display: flex;
  align-items: center;
}

/* (D) SAVE */
#save {
  font-size: 20px;
  margin-top: 20px;
  padding: 10px 20px;
  border: 0;
  color: #fff;
  background: #00479f;
}

/* (X) DOES NOT MATTER */
/* PAGE & BODY */
* {
  font-family: arial, sans-serif;
  box-sizing: border-box;
}
body {
  display: flex;
  align-items: center; justify-content: center;
  min-height: 100vh;
  background-image: url(https://images.unsplash.com/photo-1574267432644-f410f8ec2474?crop=entropy&cs=srgb&fm=jpg&ixid=MnwzMjM4NDZ8MHwxfHJhbmRvbXx8fHx8fHx8fDE2NTIyNDkwNzE&ixlib=rb-1.2.1&q=85);
  background-repeat: no-repeat;
  background-position: center;
  background-size: cover;
}

/* WIDGET */
.widget-wrap {
  min-width: 600px;
  padding: 30px;
  border-radius: 20px;
  background: rgba(255, 255, 255, 0.8);
}

/* SVG */
#sort {
  width: 100px; height:100px;
  background-image: url('data:image/svg+xml;utf8,<svg viewBox="0 0 512 512" width="100" xmlns="http://www.w3.org/2000/svg"><path d="M349.565 98.783C295.978 98.783 251.721 64 184.348 64c-24.955 0-47.309 4.384-68.045 12.013a55.947 55.947 0 0 0 3.586-23.562C118.117 24.015 94.806 1.206 66.338.048 34.345-1.254 8 24.296 8 56c0 19.026 9.497 35.825 24 45.945V488c0 13.255 10.745 24 24 24h16c13.255 0 24-10.745 24-24v-94.4c28.311-12.064 63.582-22.122 114.435-22.122 53.588 0 97.844 34.783 165.217 34.783 48.169 0 86.667-16.294 122.505-40.858C506.84 359.452 512 349.571 512 339.045v-243.1c0-23.393-24.269-38.87-45.485-29.016-34.338 15.948-76.454 31.854-116.95 31.854z" /></svg>');
  background-repeat: no-repeat;
  background-position: center;
}

/* FOOTER */
#code-boxx {
  font-weight: 600;
  margin-top: 50px;
}
#code-boxx a {
  display: inline-block;
  padding: 5px;
  text-decoration: none;
  background: #b90a0a;
  color: #fff;
}
!

JS 

              
                var reserve = {
  // (A) INIT
  init : () => {
    // (A1) GET LAYOUT WRAPPER
    let layout = document.getElementById("layout");

    // (A2) GENERATE SEATS
    for (let i=65; i<=68; i++) { for (let j=1; j<=4; j++) {
      let seat = document.createElement("div");
      seat.innerHTML = String.fromCharCode(i) + j;
      seat.className = "seat";
      seat.onclick = () => reserve.toggle(seat);
      layout.appendChild(seat);
    }}

    // (A3) FOR DEMO ONLY - RANDOM TAKEN SEATS
    let all = document.querySelectorAll("#layout .seat"),
        len = all.length - 1, rnd = [];
    while (rnd.length != 3) {
      let r = Math.floor(Math.random() * len);
      if (!rnd.includes(r)) { rnd.push(r); }
    }
    for (let i of rnd) {
      all[i].classList.add("taken");
      all[i].onclick = "";
    }
  },

  // (B) CHOOSE THIS SEAT
  toggle : seat => seat.classList.toggle("selected"),

  // (C) SAVE RESERVATION
  save : () => {
    // (C1) GET SELECTED SEATS
    let selected = document.querySelectorAll("#layout .selected");

    // (C2) ERROR!
    if (selected.length == 0) { alert("No seats selected."); }

    // (C3) SELECTED SEATS
    else {
      // (C3-1) GET SELECTED SEAT NUMBERS
      let seats = [];
      for (let s of selected) { seats.push(s.innerHTML); }

      // (C3-2) DO SOMETHING WITH IT...
      alert(JSON.stringify(seats));
    }
  }
};
window.addEventListener("DOMContentLoaded", reserve.init);
!
999px

Console