Two Date.now() in one object declaration

Question

Let var o = {a:Date.now(), b:Date.now()}.

Is o.a === o.b always true? (I am mostly interested in Node.JS.)

Answer 1

No.

Before we even get into what the spec might say, Date.now can be replaced with a user-defined function at runtime. This works in both Node and browsers:

let oldNow = Date.now;
Date.now = function () {
  let wait = oldNow() + 1000;
  while (oldNow() < wait) {
    // wait one second
  }
  return oldNow();
}

With that, every invocation will take at least one second, so your two calls will never equal.

When we look at the spec for Date.now (15.9.4.4), it simply says that it returns

the time value designating the UTC date and time of the occurrence of the call to now

which provides no guarantees to two calls ever returning the same value. From what I can tell, the spec specifies that Date objects will have millisecond precision (15.9.1.1) but makes no guarantees as to accuracy.

It is likely that two calls in the same line will probably return the same time, by virtue of the underlying timer being imprecise and the two occurring within the same tick, but the spec does not appear to specify that.