Javascr ipt 在浏览器中的性能，可认为是开发者所要面对的最重要的可用性问题。此问题因 Javascr ipt

的阻塞特征而复杂，也就是说，当 Javascr ipt 运行时其他的事情不能被浏览器处理。事实上，大多数浏览

器使用单进程处理 UI 更新和 Javascr ipt 运行等多个任务，而同一时间只能有一个任务被执行。Javascr ipt

运行了多长时间，那么在浏览器空闲下来响应用户输入之前的等待时间就有多长-Javascr ipt performance in the browser is arguably the most important usability issue facing developers. The

problem is complex because of the blocking nature of Javascr ipt, which is to say that nothing else can happen

while Javascr ipt code is being　d. In fact, most browsers use a single process for both user interface (UI)

updates and Javascr ipt execution, so only one can happen at any given moment in time. The longer Javascr ipt

takes to　, the longer it takes before the browser is free to respond to user input.