如何使用 jest 模拟在回调函数中发回的结果

Question

Is there a way to mock the storage object so I can control the err and v passed back in the callback function when I do storage.get ?

const mod = (() => {
  let value;

  return {
    init: (storage) => {
      storage.get('key', (err, v) => {
        value = v;
      });
    },
    get: () => value
  };
})();

Since I care about the side effects of storage.get I cannot simply do mockImplementation to override the function. is that correct?

Answer 1

Create a mocked storage object and pass it into mod.init method.

Eg

index.ts :

const mod = (() => {
  let value;

  return {
    init: (storage) => {
      storage.get('key', (err, v) => {
        value = v;
      });
    },
    get: () => value,
  };
})();

export { mod };

index.test.ts :

import { mod } from './';

describe('63640360', () => {
  it('should pass', () => {
    const mStorage = {
      get: jest.fn().mockImplementationOnce((key, callback) => {
        callback(null, 'teresa teng');
      }),
    };
    mod.init(mStorage);
    const actual = mod.get();
    expect(actual).toBe('teresa teng');
    expect(mStorage.get).toBeCalledWith('key', expect.any(Function));
  });
});

unit test result with coverage report:

 PASS  src/stackoverflow/63640360/index.test.ts
  63640360
    ✓ should pass (5ms)

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |      100 |      100 |      100 |      100 |                   |
 index.ts |      100 |      100 |      100 |      100 |                   |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       1 passed, 1 total
Snapshots:   0 total
Time:        4.079s, estimated 9s