问JQUERY $(document).click vs $(document).ready

EN

$(document).ready(function(e) {
  if ($('#sidebar').hasClass('active')) {
    $('#dismiss, .overlay').on('click', function() {
      $('#sidebar').removeClass('active');
      $('.overlay').removeClass('active');
    });
  } else {
    $('#sidebarCollapse').on('click', function() {
      $('#sidebar').addClass('active');
      $('.overlay').addClass('active');
      $('.collapse.in').toggleClass('in');
      $('a[aria-expanded=true]').attr('aria-expanded', 'false');
    });
  }
});

.fixed-top {
  z-index: 1 !important;
}

#sidebar {
  width: 250px;
  position: fixed;
  top: 0;
  left: -250px;
  height: 100vh;
  z-index: 999;
  background: #7386D5;
  color: #fff;
  transition: all 0.3s;
  overflow-y: hidden;
  box-shadow: 3px 3px 3px rgba(0, 0, 0, 0.2);
}

#sidebar.active {
  left: 0;
}

#dismiss {
  width: 35px;
  height: 35px;
  line-height: 35px;
  text-align: center;
  background: #7386D5;
  position: absolute;
  top: 10px;
  right: 10px;
  cursor: pointer;
  -webkit-transition: all 0.3s;
  -o-transition: all 0.3s;
  transition: all 0.3s;
}

#dismiss:hover {
  background: #fff;
  color: #7386D5;
}

.overlay {
  display: none;
  position: fixed;
  width: 100vw;
  height: 100vh;
  background: rgba(0, 0, 0, 0.7);
  z-index: 998;
  opacity: 0;
  transition: all 0.5s ease-in-out;
}

.overlay.active {
  display: block;
  opacity: 1;
}

#sidebar .sidebar-header {
  padding: 0px;
  background: #6d7fcc;
  text-align: center;
}

#sidebar ul.components {
  padding: 20px 0;
  border-bottom: 1px solid #47748b;
}

#sidebar ul p {
  color: #fff;
  padding: 10px;
}

#sidebar ul li a {
  padding: 10px;
  font-size: 1.1em;
  display: block;
}

#sidebar ul li a:hover {
  color: #7386D5;
  background: #fff;
}

#sidebar ul li.active>a,
a[aria-expanded="true"] {
  color: #fff;
  background: #6d7fcc;
}

a[data-toggle="collapse"] {
  position: relative;
}

.dropdown-toggle::after {
  display: block;
  position: absolute;
  top: 50%;
  right: 20px;
  transform: translateY(-50%);
}

ul ul a {
  font-size: 0.9em !important;
  padding-left: 30px !important;
  background: #6d7fcc;
}

ul ul ul a {
  font-size: 0.7em !important;
  padding-left: 40px !important;
  background: #6d7fcc;
}

ul.CTAs {
  padding: 20px;
}

ul.CTAs a {
  text-align: center;
  font-size: 0.9em !important;
  display: block;
  border-radius: 5px;
  margin-bottom: 5px;
}

a.download {
  background: #fff;
  color: #7386D5;
}

a.article,
a.article:hover {
  background: #6d7fcc !important;
  color: #fff !important;
}

<script src="https://code.jquery.com/jquery-3.3.1.slim.min.js"></script>
<script src="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/js/bootstrap.min.js"></script>
<script src="https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.0/umd/popper.min.js"></script>
<link href="https://stackpath.bootstrapcdn.com/font-awesome/4.7.0/css/font-awesome.min.css" rel="stylesheet" />
<link href="https://stackpath.bootstrapcdn.com/bootstrap/4.1.0/css/bootstrap.min.css" rel="stylesheet" />

<nav id="sidebar">
  <div class="sidebar-header">
    <div id="dismiss">
      <i class="fas fa-arrow-left"></i>
    </div>
  </div>
</nav>

<nav class="navbar navbar-expand-lg navbar-light fixed-top" style="background-color:#6d7fcc;">
  <div class="container-fluid">
    <button type="button" id="sidebarCollapse" class="btn btn-info">
      <i class="fas fa-align-left"></i>
      <span>Table Of Contents</span>
    </button>
  </div>
</nav>

<div class="overlay"></div>

$(document).click(function(e) {
    if ($('#sidebar').('active') && !$(e.target).is('#sidebar')) {
        $('#sidebar').removeClass('active');
        $('.overlay').removeClass('active');
    }
});