I did this by inspecting the player progress every x milliseconds, e.g. 50. If the player hasn't advanced as much as it was expected to, then we are buffering. This is quite reliable, since I've found that other events such as waiting or stalled are not fired in all cases of the video buffering.

Note that the interval must be larger than the expected inter-frame difference, but I'm sure that you won't want to be that precise anyway. An estimation of buffering time within ±300ms would still be fine, given that humans most likely cannot perceive differences in that region.

It is important to check whether the user hasn't actively paused the playback though.

var checkInterval  = 50.0 // check every 50 ms (do not use lower values)

var lastPlayPos    = 0

var currentPlayPos = 0

var bufferingDetected = false

var player = document.getElementById('videoPlayer')



setInterval(checkBuffering, checkInterval)

function checkBuffering() {

    currentPlayPos = player.currentTime



    // checking offset should be at most the check interval

    // but allow for some margin

    var offset = (checkInterval - 20) / 1000



    // if no buffering is currently detected,

    // and the position does not seem to increase

    // and the player isn't manually paused...

    if (

            !bufferingDetected 

            && currentPlayPos < (lastPlayPos + offset)

            && !player.paused

        ) {

        console.log(buffering)

        bufferingDetected = true

    }



    // if we were buffering but the player has advanced,

    // then there is no buffering

    if (

        bufferingDetected 

        && currentPlayPos > (lastPlayPos + offset)

        && !player.paused

        ) {

        console.log(not buffering anymore)

        bufferingDetected = false

    }

    lastPlayPos = currentPlayPos

}