Car With Prototype

car.js

function Car() {
    this._speed = 0;
}

Car.prototype = {

    currentSpeed: function() {
        return this._speed;
    },
    stop: function() {
        this._speed = 0;
        return this.currentSpeed();
    },

    accelerate: function(delta) {
        this._speed += delta;
        return this.currentSpeed();
    },

    slow: function(delta) {
        this._speed -= delta;
        if (this._speed < 0) {
            this._speed = 0;
        }
        return this.currentSpeed();
    }
}