Javascript weird behavior - variable value not same [duplicate]

var _timer;
    function addIdleCheckInMethod() {
        var sessionTimeout = new checkInSessionTimout(); //**code-CreateObject**
        $(this).mousemove(sessionTimeout.resetTimer);
        $(this).keypress(sessionTimeout.resetTimer);
    
       /* _timer = setInterval(function () { sessionTimeout.checkIdleTime(onSessionExpire) }
                                , 1000);
        */

        _timer = setInterval(sessionTimeout.checkIdleTime //**code-setInterval**
                                , 1000, onSessionExpire);
    }
    function onSessionExpire() {
        clearInterval(_timer);
        var user = new user();
        
        console.log('session expired!');
    }
    
    class checkInSessionTimout {
        constructor(args){
            this.IDLE_TIMEOUT = args != null && args.idleTimout ? args.idleTimout : 30;
            this._idleSecondsCounter = 0;
            console.log('constructor');
        }
        checkIdleTime(callback) {
            this._idleSecondsCounter++;
            console.log( "idle timeout: " + this.IDLE_TIMEOUT + ' counter: ' + this._idleSecondsCounter);
            if (this._idleSecondsCounter >= this.IDLE_TIMEOUT) {
                callback();
            }
        }
        resetTimer() {
            this._idleSecondsCounter = 0;
            console.log('reset:' + this._idleSecondsCounter);
        }
    }