Блог

 function online() {
  var online = document.getElementById("online");
  var requests = document.getElementById("requests");
  var blocks = document.getElementById("blocks");

  if (online.style.display === "none") {
    online.style.display = "block";
    requests.style.display = "none";
    blocks.style.display = "none";
  } else {
    online.style.display = "block";
    requests.style.display = "none";
    blocks.style.display = "none";
  }
}


function requests() {
  var online = document.getElementById("online");
  var requests = document.getElementById("requests");
  var blocks = document.getElementById("blocks");

  if (requests.style.display === "none") {
    online.style.display = "none";
    requests.style.display = "block";
    blocks.style.display = "none";
  } else {
    online.style.display = "none";
    requests.style.display = "block";
    blocks.style.display = "none";
  }
}

function blocks() {
  var online = document.getElementById("online");
  var requests = document.getElementById("requests");
  var blocks = document.getElementById("blocks");

  if (blocks.style.display === "none") {
    online.style.display = "none";
    requests.style.display = "none";
    blocks.style.display = "block";
  } else {
    online.style.display = "none";
    requests.style.display = "none";
    blocks.style.display = "block";
  }
} 

 .friend-list-window {
  border: 3px solid rgb(0, 0, 0);
  background-color: rgb(160, 156, 156);
  color: #fff;
  border-collapse: collapse;
  border-radius: 5px;
  text-align: center;
  list-style-type: none;
  position: absolute;
  top: 50%;
  left: 50%;
  transform: translate(-50%, -50%);
}

.friend-list-header {
  background: #000;
  padding: 4px 0;
  display: block;
}

.friend-list-header h4 {
  display: inline-block;
  width: 90%;
}

.friend-list-details button {
  width: 30%;
  margin: 5px 0;
  padding-top: 2px;
  border-bottom: none;
  border-top-left-radius: 10px;
  border-top-right-radius: 10px;
  display: inline-block;
  border: none;
  text-decoration: none;
  text-align: center;
  cursor: pointer;
  transition: background 250ms ease-in-out, transform 150ms ease;
  -webkit-appearance: none;
  -moz-appearance: none;
  background-color: rgb(182, 175, 175);
}

.friend-list-window input {
  width: 90%;
  border-radius: 5px;
  margin: 5px;
  padding: 2px;
  height: 15px;
}

.online-users,
.new-requests,
.block-list {
  display: inline-block;
  width: 90%;
}

.user-available,
.user-request,
.user-block {
  display: inline-block;
  width: 100%;
}

.user-available ul,
.user-request ul,
.user-block ul {
  float: left;
  font-size: small;
  display: inline-block;
  font-weight: bold;
  color: black;
}

.user-available button,
.user-request button,
.user-block button {
  width: 50px;
  float: right;
  margin-left: 5px;
  border-radius: 5px;
  text-decoration: none;
  text-align: center;
  cursor: pointer;
  transition: background 250ms ease-in-out, transform 150ms ease;
  -webkit-appearance: none;
  -moz-appearance: none;
}

.friend-list-details button:hover {
  background-color: greenyellow;
}

.friend-list-details button {
  background: ivory;
}

.remove-friend,
.reject-request {
  border: 1px solid white;
  background-color: red;
  color: white;
  font-weight: bold;
  font-size: x-small;
}

.message-friend,
.accept-request,
.unblock-user {
  border: 1px solid white;
  background-color: green;
  color: white;
  font-weight: bold;
  font-size: x-small;
} 

 <div class="friend-list-window" id="frndlist">
  <div class="friend-list-header">
    <h4>Friend List</h4>
  </div>
  <div class="friend-list-details">
    <button type="button" onclick="online()"><i class="fas fa-users"></i> Online</button>
    <button type="button" onclick="requests()"><i class="fas fa-user-plus"></i> Requests</button>
    <button type="button" onclick="blocks()"><i class="fas fa-users-slash"></i> Block List</button>
  </div>
  <input type="text" placeholder="Search">
  <div class="online-users" id="online">
    <div class="user-available">
      <ul>Groot</ul>
      <button class="remove-friend">Unfriend</button>
      <button class="message-friend">Chat</button>
    </div>
    <div class="user-available">
      <ul>Groot</ul>
      <button class="remove-friend">Unfriend</button>
      <button class="message-friend">Chat</button>
    </div>
    <div class="user-available">
      <ul>Groot</ul>
      <button class="remove-friend">Unfriend</button>
      <button class="message-friend">Chat</button>
    </div>
  </div>
  <div class="new-requests" id="requests">
    <div class="user-request">
      <ul>Groot</ul>
      <button class="reject-request">Reject</button>
      <button class="accept-request">Accept</button>
    </div>
    <div class="user-request">
      <ul>Groot</ul>
      <button class="reject-request">Reject</button>
      <button class="accept-request">Accept</button>
    </div>
    <div class="user-request">
      <ul>Groot</ul>
      <button class="reject-request">Reject</button>
      <button class="accept-request">Accept</button>
    </div>
  </div>

  <div class="block-list">
    <div class="blocked-user" id="blocks">
      <div class="user-block">
        <ul>Groot</ul>
        <button class="unblock-user">Unblock</button>
      </div>
      <div class="user-block">
        <ul>Groot</ul>
        <button class="unblock-user">Unblock</button>
      </div>
      <div class="user-block">
        <ul>Groot</ul>
        <button class="unblock-user">Unblock</button>
      </div>
    </div>
  </div>
</div>