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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?
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readyState is both 2 (when the headers are received) and 4 (when the response has been downloaded) when any status code is returned.
State 4 means that the request had been sent, the server had finished returning the response and the browser had finished downloading the response content.
What is XHR readyState=3 ? Having the readyState with a value of 3 it means that the current state is LOADING . So when is a readyStateChange event for that state fired? Whenever an asynchrounous request does switch to the LOADING state.
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.
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