Array.prototype (you will learn this later in the exercises) is the series of methods that every array inherits from the Array class constructor.
I want to say that Array is the parent class of arr but I don’t know enough about js to know if that is technically true.
You’ll note that arr is also an Object which means it could also take advantage of any methods in the Object prototype, like Object.prototype.getOwnPropertyNames().
You can verify this your self by running the method on your array:
This is a very long-winded way of telling you that you are doing something the assignment specifically asks you not to do.
You are using a built-in method to solve the problem but the exercise asks you to build your own custom function to flatten the array in an effort to understand what is happening behind the scenes on this method that can be executed on any instance of an Array.