You're right hobovsky, didn't pay attention to that. Thank you!

You missed this part:

Note: The parameter is optional. So you have to give it a default value.

There is even a corresponding sample test case: expect(arr()).to.deep.equal([]);

The function is incorrectly typed out of the gate on TypeScript. It spits out this error, independently of whatever I try to do in the function:

TSError: ⨯ Unable to compile TypeScript:
test.ts:6:12 - error TS2554: Expected 1 arguments, but got 0.

6 expect(arr()).to.be.an('array');
~~~~~

solution.ts:1:21
1 export const arr = (n: number[]): number[] => {
~~~~~~~~~~~
An argument for 'n' was not provided.
test.ts:10:12 - error TS2554: Expected 1 arguments, but got 0.

10 expect(arr()).to.deep.equal([]);
~~~~~

solution.ts:1:21
1 export const arr = (n: number[]): number[] => {
~~~~~~~~~~~
An argument for 'n' was not provided.
test.ts:14:16 - error TS2345: Argument of type 'number' is not assignable to parameter of type 'number[]'.

14 expect(arr(4)).to.deep.equal([0,1,2,3]);