滚动时改变导航条固定顶部的颜色

代码中的错别字(length) -应该是:

if ($('.navbar').length> 0) {
    $(window).on("scroll load resize", function(){
        checkScroll();
    });
}

addClass:

 $('.navbar').addClass("scrolled");

这里是错误

$('.navbar')addClass("scrolled");

.addClass

的例子:

function checkScroll() {
  var startY = $('.navbar').height() * 2;
  if ($(window).scrollTop() > startY) {
    $('.navbar').addClass("scrolled");
  } else {
    $('.navbar').removeClass("scrolled");
  }
}
if ($('.navbar').length > 0) {
  $(window).on("scroll load resize", function() {
    checkScroll();
  });
}

.navbar {
  background: transparent;
  border: none;
  -webkit-transition: all 0.6s ease-out;
  -moz-transition: all 0.6s ease-out;
  -o-transition: all 0.6s ease-out;
  -ms-transition: all 0.6s ease-out;
  transition: all 0.6s ease-out;
}
.navbar.scrolled {
  background: #B0171F;
}

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.0/jquery.min.js"></script>
<link href="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/css/bootstrap.min.css" rel="stylesheet" />
<script src="https://maxcdn.bootstrapcdn.com/bootstrap/3.3.6/js/bootstrap.min.js"></script>
<nav class="navbar navbar-default navbar-fixed-top">
  <div class="container-fluid">
    <div class="navbar-header">
      <button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false">
        <span class="sr-only">Toggle navigation</span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
        <span class="icon-bar"></span>
      </button>
      <a class="navbar-brand" href="#">Brand</a>
    </div>
    <div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">
      <ul class="nav navbar-nav">
        <li class="active"><a href="#">Link <span class="sr-only">(current)</span></a>
        </li>
        <li><a href="#">Link</a>
        </li>
        <li class="dropdown">
          <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
          <ul class="dropdown-menu">
            <li><a href="#">Action</a>
            </li>
            <li><a href="#">Another action</a>
            </li>
            <li><a href="#">Something else here</a>
            </li>
            <li role="separator" class="divider"></li>
            <li><a href="#">Separated link</a>
            </li>
            <li role="separator" class="divider"></li>
            <li><a href="#">One more separated link</a>
            </li>
          </ul>
        </li>
      </ul>
      <form class="navbar-form navbar-left" role="search">
        <div class="form-group">
          <input type="text" class="form-control" placeholder="Search">
        </div>
        <button type="submit" class="btn btn-default">Submit</button>
      </form>
      <ul class="nav navbar-nav navbar-right">
        <li><a href="#">Link</a>
        </li>
        <li class="dropdown">
          <a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">Dropdown <span class="caret"></span></a>
          <ul class="dropdown-menu">
            <li><a href="#">Action</a>
            </li>
            <li><a href="#">Another action</a>
            </li>
            <li><a href="#">Something else here</a>
            </li>
            <li role="separator" class="divider"></li>
            <li><a href="#">Separated link</a>
            </li>
          </ul>
        </li>
      </ul>
    </div>
  </div>
</nav>
<div class="jumbotron">
  <h1>Theme example</h1>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
</div>
<div class="container">
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
  <p>This is a template showcasing the optional theme stylesheet included in Bootstrap. Use it as a starting point to create something more unique by building on or modifying it.</p>
</div>

function checkScroll(){
    var startY = $('.navbar').height() * 2;
    if ($(window).scrollTop() > startY) { 
        $('.navbar').addClass("scrolled");
    } else {
        $('.navbar').removeClass("scrolled");
    }
}
if ($('.navbar').length > 0) {
    $(window).on("scroll load resize", function(){
        checkScroll();
    });
}