Smooth character movement in canvas game using keyboard controls

I'm creating a side-scrolling endless space themed game using canvas and JavaScript. I'm controlling a spaceship just by using the up and down arrows and I want to implement some kind of movement easing so that the ship doesn't just stop dead when I let go of the keys. I've looked around and haven't found anything plus my own attempts just aren't working. This is what I've tried.

Jet.prototype.checkDirection = function () {
if (this.isUpKey) {
    this.drawY -= this.speed;
    if (this.speed < 5) {
        this.speed += 0.1;
    }
}
if (this.isDownKey) {
    this.drawY += this.speed;
    if (this.speed < 5) {
        this.speed += 0.1;
    }
}
if (!this.isUpKey) {
    if (!this.isDownKey) {
        if (this.speed >= 0) {
            this.drawY -= this.speed;
            this.speed -= 1;
        }
    }
}
if (!this.isDownKey) {
    if (!this.isUpKey) {
        if (this.speed >= 0) {
            this.drawY += this.speed;
            this.speed -= 1;
        }
    }
}