在成功完成了的请求上调用abort不起任何作用，并且仅当浏览器收到一个对旧请求的延时响应的情况下，onreadystatechange才被设置为一个空函数。
Calling abort on a successfully completed request won't do anything and onreadystatechange is set to an empty function just in case the browser gets a delayed response for an old request.
varid =setInterval(fndelay);- 与setTimeout类似，只不过它会持续地调用指定的函数（每次都有一个延时），直到timer被取消为止。
var id = setInterval(fn, delay); - Similar to setTimeout but continually calls the function (with a delay every time) until it is canceled.
事件延迟，毫秒。如果这个值不在定时器支持的最小和最大延时之间的话，函数将返回一个错误。
Event delay, in milliseconds. If this value is not in the range of the minimum and maximum event delays supported by the timer, the function returns an error.
事件延迟，毫秒。如果这个值不在定时器支持的最小和最大延时之间的话，函数将返回一个错误。
Event delay, in milliseconds. If this value is not in the range of the minimum and maximum event delays supported by the timer, the function returns an error.
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