Question

我需要测试一个函数（example()），它使用另一个函数（validateDataset）。由于我只想测试example()函数，我嘲笑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，模拟函数总是返回相同的值。

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模拟的好处是，你可以模拟整个模块，并且通过要求它的默认或命名导出，你仍然可以得到一个模拟，你可以实现和重新实现，无论你喜欢什么

我发布了一个预期validateDataset调用失败的测试示例实现。为简洁起见，我也留下了一些评论。

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');
  }
});

希望能解决问题。

Answer 2

您可以使用Rewire更改function.js内的变量。

functions.test.js看起来像：

import * as rewire from 'rewire';
const functionsModule = rewire('functions.js');

describe('example()', () => {
    var functionsModule;

    test('should return object', done => {
        functionsModule.__set__('validateDataset', () => {
            Promise.resolve({
                docMain: {},
                docElement: {
                    data: 'result'
                }
            })
        });
        functionsModule.example('123').then(result => {
            expect(result.docElement.data).toBe('result');
            done(); 
        });

    })

    test('should throw error', done => {
        functionsModule.__set__('validateDataset', () => {
            return Promise.reject();
        });
        functionsModule.example('123').catch(done);
    });

})

JestJS：如何获得模拟函数的不同promise结果并测试抛出错误？

2 个答案: