First commit

js/ping.js

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+// See LICENSE for usage information
+
+// The following lines allow the ping function to be loaded via commonjs, AMD,
+// and script tags, directly into window globals.
+// Thanks to https://github.com/umdjs/umd/blob/master/templates/returnExports.js
+(function (root, factory) { if (typeof define === 'function' && define.amd) { define([], factory); } else if (typeof module === 'object' && module.exports) { module.exports = factory(); } else { root.ping = factory(); }
+}(this, function () {
+
+    /**
+     * Creates and loads an image element by url.
+     * @param  {String} url
+     * @return {Promise} promise that resolves to an image element or
+     *                   fails to an Error.
+     */
+    function request_image(url) {
+        return new Promise(function(resolve, reject) {
+            var img = new Image();
+            img.onload = function() { resolve(img); };
+            img.onerror = function() { reject(url); };
+            img.src = url + '?random-no-cache=' + Math.floor((1 + Math.random()) * 0x10000).toString(16);
+        });
+    }
+
+    /**
+     * Pings a url.
+     * @param  {String} url
+     * @param  {Number} multiplier - optional, factor to adjust the ping by.  0.3 works well for HTTP servers.
+     * @return {Promise} promise that resolves to a ping (ms, float).
+     */
+    function ping(url, multiplier) {
+        return new Promise(function(resolve, reject) {
+            var start = (new Date()).getTime();
+            var response = function() { 
+                var delta = ((new Date()).getTime() - start);
+                delta *= (multiplier || 1);
+                resolve(delta); 
+            };
+            request_image(url).then(response).catch(response);
+            
+            // Set a timeout for max-pings, 5s.
+            setTimeout(function() { reject(Error('Timeout')); }, 5000);
+        });
+    }
+    
+    return ping;
+}));