[Solved] How to make methods inside a constructor reference its argument? [closed]

You are not assigning a new value to speed go with -= and += instead of – and +

  this.getSpeed = function () {

   return speed; 
  }

  this.accelerate = function () {

  return speed += acceleration;
  }

  this.decelerate = function () {

  return speed -= acceleration;

  }