I say "deceleration" because I'm not using acceleration at the moment (it is being calculated, it's just set to 0 0); what I mean is moving the velocity back towards zero, eventually stopping.

I'm relatively new to vectors and not so great with physics and such, how is "deceleration" normally handled?

What I have now seems kind of hackish to me but it does work.

update:function(Game, t, dt) {
    var speed = Game.Input.isKeyDown('shift') ? 8 : 4;
    
    if (Game.Input.isKeyDown('a')) {
        this.velocity.i -= speed;
    }
    else if (Game.Input.isKeyDown('d')) {
        this.velocity.i += speed;
    }
    else {
        if (Math.abs(Math.floor(this.velocity.i)) > 3) {
            this.velocity.i += (this.velocity.i > 0) ? -speed : speed;
        }
        else {
            this.velocity.i = 0;
        }
    }
    
    if (Game.Input.isKeyDown('w')) {
        this.velocity.j -= speed;
    }
    else if (Game.Input.isKeyDown('s')) {
        this.velocity.j += speed;
    }
    else {
        if (Math.abs(Math.floor(this.velocity.j)) > 3) {
            this.velocity.j += (this.velocity.j > 0) ? -speed : speed;
        }
        else {
            this.velocity.j = 0;
        }
    }
        
    this.updateVectors(dt);
}

I used 3 because anything lower exhibits weird behaviour, I'm guessing if I raised the speed then it would need to be changed.