Chaining functions with javascript

I have to write a solution which passes the following test:

describe('Clock', () => {
    describe('Adding and subtracting minutes', () => {
        test('add minutes', () => {
            expect(at(10, 0).plus(3).toString()).toEqual('10:03');
        });
    }
}

Can you please share some online resource for dummies that help me understand this kind of concatenation: "at(10, 0).plus(3)" ?