jquery ajax timeout含义是设置请求超时时间，不涉及服务器响应时间

ajax访问超时怎么办？服务器处理时间过长，或网速很慢，服务器返回数据很慢等，于是想到使用timeout设置，网上有类似代码：

经测试：async必须设置为同步ture，timeout才生效；如果设置为异步，则锁住浏览器，禁止一切操作，直到请求有返回结果。

官网说明

• timeout

  Type: Number

  Set a timeout (in milliseconds) for the request. This will override any global timeout set with $.ajaxSetup(). The timeout period starts at the point the$.ajaxcall is made; if several other requests are in progress and the browser has no connections available, it is possible for a request to time out before it can be sent. In jQuery 1.4.x and below, the XMLHttpRequest object will be in an invalid state if the request times out; accessing any object members may throw an exception. In Firefox 3.0+ only, script and JSONP requests cannot be cancelled by a timeout; the script will run even if it arrives after the timeout period.