疑惑

一、

var arr1=new Array("1","2","3");
var arr2=new Array("4","5","6");
var arr3=Array.prototype.push.apply(arr1,arr2);
alert(arr3); //输出的是6，而不是数组集合 （为什么输出index）
//将push改为concat,则输出两个数组集合
var arr4=Array.prototype.concat.apply(arr1,arr2);
alert(arr4); //输出的是["1", "2", "3", "4", "5", "6"]
var arr5=arr1.push.apply(arr1,arr2);
alert(arr4); //输出的是["1", "2", "3", "4", "5", "6"]

二、

body {
  font-family: sans-serif;
}

.toggle {
  position: relative;
  border: 2px solid #333;
  border-radius: 3px;
  margin: 5px;
  width: 200px;
}

.toggle-header {
  margin: 0;
  padding: 10px;
  background-color: #333;
  color: white;
  text-align: center;
  cursor: pointer;
}

.toggle-height {
  background-color: tomato;
  overflow: hidden;
  transition: max-height .6s ease;
  max-height: 0;
}

.toggle:hover .toggle-height {
  max-height: 1000px;
}

.toggle-transform {
  padding: 5px;
  color: white;
  transition: transform .4s ease;
  transform: translateY(-100%);
}

.toggle:hover .toggle-transform {
  transform: translateY(0);
}

<div class="toggle">
  <div class="toggle-header">
    Toggle!
  </div>
  <div class="toggle-height">
    <div class="toggle-transform">
      <p>Content!</p>
      <p>Content!</p>
      <p>Content!</p>
      <p>Content!</p>
    </div>
  </div>
</div>

<div class="toggle">
  <div class="toggle-header">
    Toggle!
  </div>
  <div class="toggle-height">
    <div class="toggle-transform">
      <p>Content!</p>
      <p>Content!</p>
      <p>Content!</p>
      <p>Content!</p>
    </div>
  </div>
</div>