If a callback function is provided, this function sets the onreadystatechange property of the XHR object to that callback function.

如果提供了回调，那么这个函数将XHR对象的onreadystatechange属性设置为这个回调函数。

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The sendRequest() function (see Listing 9) encodes the parameters, opens the request, adds the headers, sets the Ajax callback if async is true, and sends the HTTP request.

sendRequest()函数（见清单9）将参数编码，打开请求，添加头部，当async 为true 时设置Ajax回调，并发送 HTTP 请求。

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Gets or sets the default failed callback function for the service.

获取或设置服务的默认失败回调函数。

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Gets or sets the default failed callback function for the service.

获取或设置服务的默认失败回调函数。

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