export default function createSpy(impl) {
  throw new Error('Not implemented');
}

import createSpy from './createSpy';

describe('createSpy', () => {
  test('records calls and arguments', () => {
    const spy = createSpy();
    spy(1, 2);
    spy('a');

    expect(spy.callCount()).toBe(2);
    expect(spy.calls).toEqual([[1, 2], ['a']]);
    expect(spy.lastCall()).toEqual(['a']);
  });

  test('delegates to the implementation and returns its value', () => {
    const spy = createSpy((a, b) => a + b);
    const out = spy(2, 3);
    expect(out).toBe(5);
    expect(spy.calls[0]).toEqual([2, 3]);
  });

  test('preserves this binding when used as a method', () => {
    const obj = {
      x: 10,
      fn: createSpy(function (n) { return this.x + n; })
    };

    const out = obj.fn(2);

    expect(out).toBe(12);
    expect(obj.fn.thisValues[0]).toBe(obj);
  });

  test('reset clears recorded calls', () => {
    const spy = createSpy();
    spy(1);
    spy.reset();
    expect(spy.callCount()).toBe(0);
    expect(spy.calls).toEqual([]);
    expect(spy.lastCall()).toBe(null);
  });
});