Performance - Date.now() vs Date.getTime()

Question

These things are the same (edit semantically; performance is a little better with .now()):

var t1 = Date.now();
var t2 = new Date().getTime();

However, the time value from any already-created Date instance is frozen at the time of its construction (or at whatever time/date it's been set to). That is, if you do this:

var now = new Date();

and then wait a while, a subsequent call to now.getTime() will tell the time at the point the variable was set.

They are effectively equivalent, but you should use Date.now(). It's clearer and about twice as fast.

Edit: Source: http://jsperf.com/date-now-vs-new-date

When you do (new Date()).getTime() you are creating a new Date object. If you do this repeatedly, it will be about 2x slower than Date.now()

The same principle should apply for Array.prototype.slice.call(arguments, 0) vs [].slice.call(arguments, 0)

Yes, that is correct; they are effectively equivalent when using the current time.

Sometimes it's preferable to keep some time tracking variable in a Date object format rather than as just a number of milliseconds, to have access to Date's methods without re-instantiating. In that case, Date.now() still wins over new Date() or the like, though only by about 20% on my Chrome and by a tiny amount on IE.

See my JSPERF on

timeStamp2.setTime(Date.now()); // set to current;

vs.

timeStamp1 = new Date(); // set to current;

http://jsperf.com/new-date-vs-settime