Simultaneous keypress causing unwanted behaviour - actionscript-3

Question

I am making a shooter game with action script 3. I have a hero which moves when any of the arrow key is pressed
my code is as following
//some class level variables
private var vx :int = 0;
private var vy :int = 0;
//in the Main constructor
stage.addEventListener(KeyboardEvent.KEY_DOWN , moveHero);
stage.addEventListener(KeyboardEvent.KEY_UP , stopHero);
stage.addEventListener(Event.ENTER_FRAME , onEnter);
//and all the handlers
function moveHero(e:KeyboardEvent)
{
if (e.keyCode == 37)
{
vx = -5;
}
else if (e.keyCode == 38)
{
vy = -10;
}
else if (e.keyCode == 39)
{
vx = 5;
}
else if (e.keyCode == 40)
{
vy = 10;
}
}
function stopHero(e:KeyboardEvent)
{
//when key is up stop miving the hero
vx = 0;
vy = 0;
}
function onEnter(e:Event):void
{
//updtae hero position
hero.x += vx;
hero.y += vy;
}
Now my problem is when user have both up and down keys or left and right keys under his fingers and suddenly press them alternately then the hero shows a notable lag in making response to the key presses

Answer 1

You'll have to do exactly like BotMaster said in the comments, and here's how it looks like in AS3 code:
var keys:Array = new Array(255);
stage.addEventListener(KeyboardEvent.KEY_DOWN , keyDown);
stage.addEventListener(KeyboardEvent.KEY_UP , keyUp);
stage.addEventListener(Event.ENTER_FRAME , loop);
function keyDown(e:KeyboardEvent):void {
keys[e.keyCode] = true;
}
function keyUp(e:KeyboardEvent):void {
keys[e.keyCode] = false;
}
function loop(e:Event):void {
if(keys[Keyboard.LEFT]) {
trace("Moving left...");
} else if(keys[Keyboard.RIGHT]) {
trace("Moving right...");
}
}