What are the downsides of defining functions on prototype this way?

Question

Usually people write code like this:

function Shape() {
  this.x = 0;
  this.y = 0;
}

Shape.prototype.move = function(x, y) {
  this.x += x;
  this.y += y;
         


        

Answer 1

If you, or someone in the future trying loop over an object that is extended, the results might be unpredictable and break since the object now contains unknown properties. A little unforeseen gotcha!