在数组的开头添加新元素 - unshift()

<p id="demo">单击按钮在数组中插入元素。</p>

<button onclick="myFunction()">点我</button>

<script>

  function myFunction() {

    var fruits = ["Banana", "Orange", "Apple", "Mango"];

    fruits.unshift("Lemon", "Pineapple");

    var x = document.getElementById("demo");

    x.innerHTML = fruits;

  }

</script>

<p><b>注意:</b> unshift()方法不能用于 Internet Explorer 8 之前的版本，插入的值将被返回成<em> undefined </em>。</p>