javascript - Will an ajax request ever reach readyState 4 if a local firewall is blocking the URL requested? -

it's understanding local (or local network) firewall blocking url prevent ajax request url reaching readystate 4. correct?

yes will. example, @ code.

function readystatecallback () {   jsprint(this.readystate); };  var oreq = new xmlhttprequest(); oreq.onreadystatechange = readystatecallback; oreq.open("get", "http://foobarbaz.nnn", true); oreq.send(); 

you can try on this fiddle. every request triggers ready state of 1 , 4, not ready states 2 or 3.

see xmlhttprequest mdn docs.

0   unsent               open() has not been called yet. 1   opened               send() has not been called yet. 2   headers_received     send() has been called, , headers , status available. 3   loading              downloading; responsetext holds partial data. 4   done                 operation complete. 

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to determine if request failed (such being blocked firewall), check this.status. request 200. request didn't receive response, 0.

the status attribute must return result of running these steps:

1. if state unsent or opened, return 0 , terminate these steps.
2. if error flag set, return 0 , terminate these steps.
3. return http status code.

_{source: w3 xmlhttprequest::status}

network errors described as,

dns errors, tls negotiation failure, or other type of network errors discusses onreadystatechange event fired in case of error. readystate 4.

a firewall either prohibit dns resolution or tls packets being sent.