What I want to do is play a running animation on a sprite, every time a key is pressed

My code (extract) looks like this:

function update() {
  if (keys["w"].isDown) {
    avatar.setVelocityY(-250);
  }
  if (keys["d"].isDown) {
    avatar.setVelocityX(250);
  }
  if (keys["a"].isDown) {
    avatar.setVelocityX(-250);
  }
  if (keys["s"].isDown) {
    avatar.setVelocityY(250);
    avatar.play("run_front");
  }
}

The problem: When I hit s, only the first frame of the animation is played and the animation will only finish when I stop pressing it.

I think this is because the animation is overwritten every time I press s.

So how can I run the animation only if the one before isn't playing yet?

Thanks in advance