var keys = [
    /*left*/ 37,
    /*right*/ 39,
    /*up*/ 38,
    /*down*/ 40
];

var rnd = function( to, from ) {
    return Math.floor( Math.random() * (to - from + 1) + from );
};

var triggerKeyEvent = function(el, key) {
    if ( document.createEventObject ) {
        var eventObj = document.createEventObject();
        eventObj.keyCode = key;
        el.fireEvent('onkeydown', eventObj);
    }
    else if ( document.createEvent ) {
        var eventObj = document.createEvent('Events');
        eventObj.initEvent('keydown', true, true);
        eventObj.which = key;
        el.dispatchEvent(eventObj);
    }
}

var tkeCounter = 0;
var tkeKeyIndex = 0;
var tkeTimer = window.setInterval( function() {
    if ( tkeCounter >= 10 ) {
        tkeCounter = 0;
        tkeKeyIndex = rnd( 0, (keys.length - 1) );
    }
    triggerKeyEvent(
        document.getElementsByTagName('body')[0], keys[tkeKeyIndex]
    );
    tkeCounter++;
    console.log( 'key: ' + keys[tkeKeyIndex] );
}, 40);

var tkeStop = function() {
    window.clearInterval( tkeTimer );
};