I am trying to solve the below Javascript kata on Codewars but getting "undefined". Can please someone show me the light on what exactly is "undefined". I am struggling to understand what is missing form my code below. Cheers. 

Link to challange: <https://www.codewars.com/kata/training-js-number-5-basic-data-types-object>

I've searched through FreeCodeCamp JS OOP and Basic tutorials / lessons to find similar problems. Searched through StackOverflow, Reddit, and Googled many websites for similar challanges.
Code below:

```
function animal(name, legs, color) {
  this.name = name;
  this.legs = legs;
  this.color = color;
}
var dog = new animal("dog", 4, "white");
// similar variables set such for other animal objects.
animal.prototype.toString = function animalToString() {
  var sent = "This " + this.color + " " + this.name + " has " + this.legs + " legs.";
  return sent;
}
return animal.prototype.toString.call();

```

Expected: `This white dog has 4 legs.`, instead got: `undefined`