export default function once(fn) {
  throw new Error('Not implemented');
}

import once from './file';

describe('once', () => {
  test('calls the function only once', () => {
    let calls = 0;
    const fn = once(() => { calls += 1; return calls; });
    expect(fn()).toBe(1);
    expect(fn()).toBe(1);
    expect(calls).toBe(1);
  });

  test('returns the first result on later calls', () => {
    const add = once((a, b) => a + b);
    expect(add(1, 2)).toBe(3);
    expect(add(5, 6)).toBe(3);
  });

  test('preserves this context', () => {
    const obj = { x: 2, get: once(function (n) { return this.x + n; }) };
    expect(obj.get(3)).toBe(5);
  });
});