const Animal = (name, type) => {
  this.name = name
  this.type = type
  this.age = 0
}

Animal.prototype.birthday = function () {
  this.age++
}

const leo = new Animal('Leo', 'Lion')

Arrow functions don’t have their own this. This leads to three errors in our code.

First, we’re adding properties to this in the constructor function. Again, because Arrow Functions don’t have their own this, you can’t do that.

Second, we can’t use the new keyword with an Arrow Function. This will throw a X is not a constructor error.

Third, we can’t add a property on a function’s prototype if that function is an arrow function, again, because there’s no this.

Here’s the solution -

function Animal (name, type) {
  this.name = name
  this.type = type
  this.age = 0
}

Animal.prototype.birthday = function () {
  this.age++
}

const leo = new Animal('Leo', 'Lion')