How to know if a timer is cleared or timed out in javascript?

If you're looking for something more formal, you could build javascript class that encapsulates the setTimeout/clearTimeout functionality.

Such a class might look something like this:

/** class Timer **/
var Timer = function(delayMs, callbackFunc) {
    this.delayMs = delayMs;
    this.callbackFunc = callbackFunc;
    this.timerState = 'new';
}
Timer.prototype.start = function() {
    if( this.tmr ) return;

    var self = this;
    this.timerState = 'running';
    this.tmr = setTimeout(function() { self._handleTmr(); }, this.delayMs);
}
Timer.prototype.cancel = function() {
    if( ! this.tmr ) return;

    clearTimeout(this.tmr);
    this.tmr = null;
    this.timerState = 'canceled';
}
Timer.prototype._handleTmr = function() {
    this.tmr = null;
    this.timerState = 'completed';
    this.callbackFunc();
}

I've also included a timerState attribute that would let you easily determine whether the timer was "completed" or "canceled".

You would use it like this:

var t = new Timer(500, function() {
    alert('timer completed');
});
t.start();

// do whatever...

// now cancel the timer if it hasn't completed yet.
t.cancel();

// maybe you do some other stuff...
// then check the timerState, and act accordingly.
//
if( t.timerState == 'canceled' ) {
   alert("the timer was canceled!");
} else {
   alert("the timer completed uneventfully.");
}

You can extend the same basic idea to include additional functionality if you need it (eg. repeating timer, start/stop/resume, etc.)

assign null to the timer after the clearTimeout(timer)