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POJavascript XMLHttpRequest issue
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copied!<p>I am using the following script to monitor whether I can connect to a web site in a regular interval (10 seconds in my sample code). I met with two issues, any ideas how to solve them?</p> <ol> <li><p>If a web site is very slow and no response within 10 seconds (making PingWebSite not return), I find 2 second call to PingWebSite will be executed because of 10 second interval arrives. My purpose is I want only one call to PingWebSite is under execution, and if 10 seconds interval arrives and previous PingWebSite is executing, I want to prevent current PingWebSite from execution. Any ideas how to solve this?</p></li> <li><p>I find a strange issue, when I connect to a very slow web site, and code path executes to "alert("connecting");", then I expect exception to be thrown for timeout, but in my debug, no exception is thrown. Any ideas how to catch timeout exception?</p></li> </ol> <p>Here is my code,</p> <pre><code>var index = 0; function setup() { window.setInterval(PingWebSite, (10 * 1000)); } function PingWebSite() { var http_request = new XMLHttpRequest(); try { http_request.open("GET", "http://www.google.com", true); http_request.onreadystatechange = function() { if (http_request.readyState == 4) { if (http_request.status == 200) { MonitorInformation.innerText = "http://www.google.com" + " Connection ok"; alert("ok"); } else { alert("fail"); } http_request = null; } // if if (http_request.readyState == 4) else { // if execute here, no exception will be thrown alert("connecting"); } } // end of function http_request.send(null); } // try catch (e) { alert("service is not available"); } } </code></pre> <p>EDIT 1: I have followed advice here to modify my code. Here is the below version. The new issue is index value (e.g. 0) will be prompted in alert message box before ok/fail alert message box. I think index value (e.g. 0) should be prompted in alert message box after ok/fail alert message box. Any ideas why?</p> <pre><code>var index = 0; var http_request; var xhrTimeout; var chkConn; function setup() { chkConn = window.setInterval(PingWebSite, (10 * 1000)); } function WebMonitorTimeout() { http_request.abort(); alert("timeout"); index = index + 1; } function PingWebSite() { http_request = new XMLHttpRequest(); http_request.open("GET", "http://www.google.com", true); http_request.onreadystatechange = function() { if (http_request.readyState == 4) { if (chkConn) { clearInterval(chkConn); } if (http_request.status == 200) { alert("ok"); index = index + 1; if (xhrTimeout) { clearTimeout(xhrTimeout); } } else { alert("fail"); index = index + 1; if (xhrTimeout) { clearTimeout(xhrTimeout); } } http_request = null; } //if (http_request.readyState == 4) } // end of event function http_request.send(null); xhrTimeout = setTimeout("WebMonitorTimeout();", 30000); alert(index); chkConn = window.setInterval(PingWebSite, (30 * 1000)); } </code></pre> <p>thanks in advance, George</p>

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