用 Jest 测试 Express 时，有没有办法解决这个“TypeError：express.json 不是函数”？答案

【问题标题】：Is there a way to fix this "TypeError: express.json is not a function" when testing Express with Jest?用 Jest 测试 Express 时，有没有办法解决这个“TypeError：express.json 不是函数”？
【发布时间】：2021-01-23 07:44:38
【问题描述】：

我正在尝试学习如何在 express.js 上运行 Jest 测试，但出现此错误

TypeError: express.json is not a function

但是，如果我从 index.js 中注释掉这两行：

app.use(express.json({limit: '50mb'}));
app.use(express.urlencoded({limit: '50mb', extended: true}));

然后它会工作并通过前 2 个测试。我该如何解决这个错误？

Here the index.js

and here the index.test.js

【问题讨论】：

标签： node.js unit-testing express jestjs backend

【解决方案1】：

你没有模拟 express.json 方法。

例如

index.js:

const express = require('express');
const cors = require('cors');
const app = express();
const corsOptions = {
  origin: true,
};
const PORT = process.env.PORT || 4002;
app.use(cors(corsOptions));
app.use(express.json({ limit: '50mb' }));
app.use(express.urlencoded({ limit: '50mb', extended: true }));

app.listen(PORT, (err) => {
  if (err) {
    console.log('Rumble in the Bronx! ' + err);
  } else {
    console.log(`? <(Communications active at port http://localhost:${PORT}/)`);
  }
});

index.test.js:

const express = require('express');

const useSpy = jest.fn();
const listenSpy = jest.fn();
const urlencodedMock = jest.fn();
const jsonMock = jest.fn();

jest.mock('express', () => {
  return () => ({
    listen: listenSpy,
    use: useSpy,
  });
});

express.json = jsonMock;
express.urlencoded = urlencodedMock;

describe('64259504', () => {
  test('should initialize an express server', () => {
    require('./index');
    expect(jsonMock).toBeCalled();
    expect(urlencodedMock).toBeCalled();
    expect(listenSpy).toHaveBeenCalled();
  });

  test('should call listen fn', () => {
    require('./index');
    expect(jsonMock).toBeCalled();
    expect(urlencodedMock).toBeCalled();
    expect(listenSpy).toHaveBeenCalled();
  });
});

带有覆盖率报告的单元测试结果：

 PASS  src/stackoverflow/64259504/index.test.js (13.203s)
  64259504
    ✓ should initialize an express server (626ms)
    ✓ should call listen fn (1ms)

----------|----------|----------|----------|----------|-------------------|
File      |  % Stmts | % Branch |  % Funcs |  % Lines | Uncovered Line #s |
----------|----------|----------|----------|----------|-------------------|
All files |       75 |       50 |        0 |       75 |                   |
 index.js |       75 |       50 |        0 |       75 |          13,14,16 |
----------|----------|----------|----------|----------|-------------------|
Test Suites: 1 passed, 1 total
Tests:       2 passed, 2 total
Snapshots:   0 total
Time:        15.01s

【讨论】：

`无效的变量访问：listenSpy` - 我收到了这个错误
感谢幻灯片。明确地将 json 设置为相同的模拟对我有帮助。