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function init() {
  window.addEventListener('scroll', function(e){
    var distanceY = window.pageYOffset || document.documentElement.scrollTop,
      shrinkOn = 300,
      header = document.querySelector("header");
    if (distanceY > shrinkOn) {
      classie.add(header,"smaller");
    } else {
      if (classie.has(header,"smaller")) {
        classie.remove(header,"smaller");
      }
    }
  });
}
window.onload = init();

<header>
    <div class="container clearfix">
        <h1 id="logo">
            LOGO
        </h1>
        <nav>
            <a href="">Lorem</a>
            <a href="">Ipsum</a>
            <a href="">Dolor</a>
        </nav>
    </div>
</header><!-- /header -->

header {
    width: 100%;
    height: 150px;
    overflow: hidden;
    position: fixed;
    top: 0;
    left: 0;
    z-index: 999;
    background-color: #0683c9;
    -webkit-transition: height 0.3s;
    -moz-transition: height 0.3s;
    -ms-transition: height 0.3s;
    -o-transition: height 0.3s;
    transition: height 0.3s;
}
header h1#logo {
    display: inline-block;
    height: 150px;
    line-height: 150px;
    float: left;
    font-family: "Oswald", sans-serif;
    font-size: 60px;
    color: white;
    font-weight: 400;
    -webkit-transition: all 0.3s;
    -moz-transition: all 0.3s;
    -ms-transition: all 0.3s;
    -o-transition: all 0.3s;
    transition: all 0.3s;
}
header nav {
    display: inline-block;
    float: right;
}
header nav a {
    line-height: 150px;
    margin-left: 20px;
    color: #9fdbfc;
    font-weight: 700;
    font-size: 18px;
    -webkit-transition: all 0.3s;
    -moz-transition: all 0.3s;
    -ms-transition: all 0.3s;
    -o-transition: all 0.3s;
    transition: all 0.3s;
}
header nav a:hover {
    color: white;
}
header.smaller {
    height: 75px;
}
header.smaller h1#logo {
    width: 150px;
    height: 75px;
    line-height: 75px;
    font-size: 30px;
}
header.smaller nav a {
    line-height: 75px;
}

@media all and (max-width: 660px) {
    header h1#logo {
        display: block;
        float: none;
        margin: 0 auto;
        height: 100px;
        line-height: 100px;
        text-align: center;
    }
    header nav {
        display: block;
        float: none;
        height: 50px;
        text-align: center;
        margin: 0 auto;
    }
    header nav a {
        line-height: 50px;
        margin: 0 10px;
    }
    header.smaller {
        height: 75px;
    }
    header.smaller h1#logo {
        height: 40px;
        line-height: 40px;
        font-size: 30px;
    }
    header.smaller nav {
        height: 35px;
    }
    header.smaller nav a {
        line-height: 35px;
    }
}