Is mousemove event fire once in per frame?

Question

In this fiddle demo, press and move the mouse will draw the dot according to the mouse's position. The draw method is listen to mousemove callback, but it draw the discrete dot which show that the mousemove event is not fired continuously.

What is more, I collect the event.timeStamp and log in console. I found that the offset between adjacent event is near 16.7ms. So is mousemove event fire once in per frame?

Answer 1

The mousemove event frequency is implementation specific, and not defined by any specification.

A user agent MUST dispatch this event when a pointing device is moved while it is over an element. The frequency rate of events while the pointing device is moved is implementation-, device-, and platform-specific, but multiple consecutive mousemove events SHOULD be fired for sustained pointer-device movement, rather than a single event for each instance of mouse movement. Implementations are encouraged to determine the optimal frequency rate to balance responsiveness with performance.

-- UI Events, W3C Working Draft, 04 August 2016

It's entirely possible some browsers on some platforms will limit it to once per-frame. There is no guarantee any browser that may do this will continue to do so however.

If you want to draw a continuous line, you would need to create interpolation data between each two points.