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Asynchronous AJAX:
The main thread: . "Hello, AJAX thread you please help me send a HTTP request, and I put the request addresses and parameters gave you."
AJAX thread: "Okay, I'll go to the main thread of hair, but it may take a little time, you can go to busy with something else.."
The main thread: : "Thank you, you get a response back to tell me ah."
(Then, the main thread to do other things to go after a meal time, it received a notification response arrives.)
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Synchronous AJAX:
The main thread: . "Hello, AJAX thread you please help me send a HTTP request, and I put the request addresses and parameters gave you."
AJAX thread: "......"
The main thread: : "Hey, AJAX thread, how do you not speak?"
AJAX thread: "......"
The main thread: : "Hey Hey Hey!!"
AJAX thread: "......"
(After a stick of incense)
The main thread: : "! Hey Please say something!"
AJAX thread: "the main thread, sorry, I can not talk at work when your request has been made over, to get the response data, and to you.."
It is because JavaScript is single-threaded, non-blocking and asynchronous easy to implement, so in JavaScript or time-consuming operation for the operation time of uncertainty, the use of asynchronous became the inevitable choice.