Ajax in the open () method has three parameters: 1, method; 2, url; 3, boolean;
Parameter 1: get and post the two values
Parameter 2: Request Path
Parameter 3: boolean values
1. When the boolean value is true, the server request is asynchronous, that is, script send () method does not wait after
The implementation of the results of the server, but continue to execute script code;
2. When the boolean value is false, the server request is synchronous, that is, script send () method and wait
Server to return results of the implementation, if the super-waiting process, then stop waiting and continue behind the script code!