问题
XMLHttpRequest has 5 readyStates, and I only use 1 of them (the last one, 4).
What are the others for, and what practical applications can I use them in?
回答1:
The full list of readyState values is:
State Description
0 The request is not initialized
1 The request has been set up
2 The request has been sent
3 The request is in process
4 The request is complete
(from https://www.w3schools.com/js/js_ajax_http_response.asp)
In practice you almost never use any of them except for 4.
Some XMLHttpRequest implementations may let you see partially received responses in responseText when readyState==3, but this isn't universally supported and shouldn't be relied upon.
回答2:
kieron's answer contains w3schools ref. to which nobody rely , bobince's answer gives link , which actually tells native implementation of IE ,
so here is the original documentation quoted to rightly understand what readystate represents :
The XMLHttpRequest object can be in several states. The readyState attribute must return the current state, which must be one of the following values:
UNSENT (numeric value 0)
The object has been constructed.OPENED (numeric value 1)
The open() method has been successfully invoked. During this state request headers can be set using setRequestHeader() and the request can be made using the send() method.HEADERS_RECEIVED (numeric value 2)
All redirects (if any) have been followed and all HTTP headers of the final response have been received. Several response members of the object are now available.LOADING (numeric value 3)
The response entity body is being received.DONE (numeric value 4)
The data transfer has been completed or something went wrong during the transfer (e.g. infinite redirects).
Please Read here : W3C Explaination Of ReadyState
回答3:
Original definitive documentation
0, 1 and 2 only track how many of the necessary methods to make a request you've called so far.
3 tells you that the server's response has started to come in. But when you're using the XMLHttpRequest object from a web page there's almost nothing(*) you can do with that information, since you don't have access to the extended properties that allow you to read the partial data.
readyState 4 is the only one that holds any meaning.
(*: about the only conceivable use I can think of for checking for readyState 3 is that it signals some form of life at the server end, so you could possibly increase the amount of time you wait for a full response when you receive it.)
回答4:
onreadystatechange Stores a function (or the name of a function) to be called automatically each time the readyState property changes readyState Holds the status of the XMLHttpRequest. Changes from 0 to 4:
0: request not initialized
1: server connection established
2: request received
3: processing request
4: request finished and response is ready
status 200: "OK"
404: Page not found
回答5:
- 0 : UNSENT Client has been created. open() not called yet.
- 1 : OPENED open() has been called.
- 2 : HEADERS_RECEIVED send() has been called, and headers and status are available.
- 3 : LOADING Downloading; responseText holds partial data.
- 4 : DONE The operation is complete.
(From https://developer.mozilla.org/en-US/docs/Web/API/XMLHttpRequest/readyState)
来源：https://stackoverflow.com/questions/632774/what-do-the-different-readystates-in-xmlhttprequest-mean-and-how-can-i-use-them