Remove the new Date() stuff

event-timestamp/demo.js

@@ -1,47 +1,30 @@
-var offsetFromEpoch = 0;
-var previousX = 0;
-var previousY = 0;
-var previousT = 0;
-
-window.addEventListener('load', function(event) {
-  // Use a simple heuristic to determine the offset from the epoch:
-  // If the timestamp value is less than the current time,
-  // then it's almost certainly a DOMHighResTimeStamp. Adding timestamp to
-  // performance.timing.navigationStart should give a value relative to
-  // the epoch, which could be passed in to the Date constructor.
-  // Otherwise, it's a DOMTimeStamp, and is already relative to the epoch.
-  if (event.timeStamp < Date.now()) {
-    offsetFromEpoch = performance.timing.navigationStart;
+var previousX;
+var previousY;
+var previousT;
+
+window.addEventListener('mousemove', function(event) {
+  // Don't update velocity until we have an initial value for X, Y, and T.
+  if (!(previousX === undefined ||
+        previousY === undefined ||
+        previousT === undefined)) {
+    var Δx = event.screenX - previousX;
+    var Δy = event.screenY - previousY;
+    var Δd = Math.sqrt(Math.pow(Δx, 2) + Math.pow(Δy, 2));
+
+    // event.timeStamp will always represent a value in milliseconds.
+    // In supported browsers, the value will have a microsecond resolution, and
+    // therefore might include a fractional number of milliseconds.
+    // Older browsers only support a millisecond resolution, and the value will
+    // always be a whole number.
+    var Δt = event.timeStamp - previousT;
+
+    // Multiply by 1000 to go from milliseconds to seconds.
+    var velocityInPixelsPerSecond = Δd / Δt * 1000;
+
+    ChromeSamples.setStatus(velocityInPixelsPerSecond);
   }
 
-  // While a 'mousemove' listener is being used for the purposes of this
-  // example, the new timeStamp resolution applies to all Event types.
-  window.addEventListener('mousemove', calculateVelocity);
-});
-
-function calculateVelocity(event) {
-  var Δx = event.screenX - previousX;
-  var Δy = event.screenY - previousY;
-  var Δd = Math.sqrt(Math.pow(Δx, 2) + Math.pow(Δy, 2));
-
-  // event.timeStamp will always represent a value in milliseconds.
-  // In supported browsers, the value will have a microsecond resolution, and
-  // therefore might include a fractional number of milliseconds.
-  // Older browsers only support a millisecond resolution, and the value will
-  // always be a whole number.
-  var Δt = event.timeStamp - previousT;
-
-  // Multiply by 1000 to go from milliseconds to seconds.
-  var velocityInPixelsPerSecond = Δd / Δt * 1000;
-
-  // Contrived example to illustrate how to use offsetFromEpoch to get a Date
-  // that corresponds to event.timeStamp.
-  var date = new Date(event.timeStamp + offsetFromEpoch);
-
-  ChromeSamples.setStatus(velocityInPixelsPerSecond + ' as of ' +
-    date.toLocaleTimeString());
-
   previousX = event.screenX;
   previousY = event.screenY;
   previousT = event.timeStamp;
-}
+});