Is there a way track the focus on tab with Javascript?

This should work both on tab switch and on browser window losing focus:

function onBlur() {
    document.body.className = 'blurred';
};
function onFocus(){
    document.body.className = 'focused';
};

if (/*@cc_on!@*/false) { // check for Internet Explorer
    document.onfocusin = onFocus;
    document.onfocusout = onBlur;
} else {
    window.onfocus = onFocus;
    window.onblur = onBlur;
}

I would do something with mousemove and scroll and count a visitor as active as long as either of those trigger within some interval. That will also cover them leaving the browser open and leaving the computer.

Which tab you are talking about? Is it your Nav/menu tab or Browser tab. I feel, you mean browser tab! I think it is not possible accurately. But what if you track few events like mousemove, focus etc and then fire an event that same some data (counter) on server. When user is on your page then he will do something something like move mouse, click somewhere etc. So difference in first page load and last event can tell the usage stat.