When to enter success and error in AJAX

A quick introduction to Ajax:
Simply put, an Ajax request sends a request through an XMLHttpRequest object, which has four states (readyStates) :
0- uninitialized, 1- initializing, 2- sending data, 3- sending data, 4- done.
When XMLHttpRequest. ReadyState is 4, said ajax request has been completed can get the result.
Ajax’s success and Error methods are triggered based on the response status code. Success () is triggered when XMLHttprequest.status is 200, indicating a successful response. Other status codes trigger error().
In addition to the response status code, Ajax can also use an error method in the following situations:

    return data type is not JSON network interrupt background response interrupt


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