JestJS:如何获得模拟函数的不同promise结果并测试抛出错误？

Question

我需要测试一个函数(example()),它使用另一个(validateDataset).因为我只想测试example()我嘲笑的功能validateDataset().

当然,每个测试都需要模拟函数的不同结果.但是如何为模拟函数设置不同的promise结果？在我下面显示的尝试中,模拟函数始终返回相同的值.

所以在这个例子中,我无法测试抛出的错误.

functions.js

import { validateDataset } from './helper/validation'

export async function example (id) {
  const { docElement } = await validateDataset(id)
  if (!docElement) throw Error('Missing content')
  return docElement
}

functions.test.js

import { example } from './functions'

jest.mock('./helper/validation', () => ({
  validateDataset: jest.fn(() => Promise.resolve({
    docMain: {},
    docElement: {
      data: 'result'
    }
  }))
}))

describe('example()', () => {
  test('should return object', async () => {
    const result = await example('123')
    expect(result.data).toBe('result')
  })
  test('should throw error', async () => {
    const result = await example('123')
    // How to get different result (`null`) in this test
    // How to test for the thrown error?
  })
})

Answer 1

Jest 模拟的伟大之处在于，您可以模拟整个模块，并且通过要求其默认或命名导出，您仍然可以获得模拟，您可以根据需要在任何地方实现和重新实现该模拟。

validateDataset我已经发布了预计调用失败的测试示例实现。为了简洁起见，我还留下了一些评论。

import { example } from './example';
import { validateDataset } from './helper/validation';

// Declare export of './helper/validation' as a Mock Function.
// It marks all named exports inside as Mock Functions too.
jest.mock('./helper/validation');

test('example() should return object', async () => {
  // Because `validateDataset` is already mocked, we can provide
  // an implementation. For this test we'd like it to be original one.
  validateDataset.mockImplementation(() => {
    // `jest.requireActual` calls unmocked module
    return jest.requireActual('./helper/validation').validateDataset();
  });
  const result = await example('123');
  expect(result.data).toBe('result');
});

test('example() should throw error', async () => {
  // Worth to make sure our async assertions are actually made
  expect.assertions(1);
  // We can adjust the implementation of `validateDataset` however we like,
  // because it's a Mock Function. So we return a null `docElement`
  validateDataset.mockImplementation(() => ({ docElement: null }));
  try {
    await example('123');
  } catch (error) {
    expect(error.message).toEqual('Missing content');
  }
});

希望事情能澄清。