如何运行时（动态）加载js脚本|JavaScript

如何运行时（动态）加载js脚本呢？看看下边的Demo：

if(typeof(jQuery) == "undefined") {var script = document.createElement("script");script.type = "text/javascript";script.src = "https://ajax.googleapis.com/ajax/libs/jquery/1.7.1/jquery.min.js";document.head.appendChild(script);}$(document).ready(function(){alert("Hello World!");});