如何在笑话测试中模拟节点“createReadStream”和“readline.createInterface”

Question

我在为涉及“createReadStream”和“readline.createInterface”的代码编写单元测试时遇到了这个问题。

下面是我需要测试的代码：

private createReadStreamSafe(filePath: string): Promise<fs.ReadStream> {
        return new Promise((resolve, reject) => {
          const fileStream = fs.createReadStream(filePath)
          console.log('file Stream')
          fileStream
            .on('error', () => {
              reject('create read stream error')
            })
            .on('open', () => {
              resolve(fileStream)
            })
        })
      }
    
      async start() {
        const fileStream = await this.createReadStreamSafe(this.filePath)
    
        const rl = readline.createInterface({
          input: fileStream,
          output: process.stdout,
          terminal: false
        })
    
        for await (const line of rl) {
          ...
        }
      }

下面是我的测试，

it.only('should create an error if creating the read stream from the file path fails', () => {
    const mockedReadStream = new Readable()
    jest.spyOn(fs, 'createReadStream').mockReturnValue(mockedReadStream as any)
    const app = createApp('invalid/file/path')

    expect.assertions(1)
    try {
      app.start()
      mockedReadStream.emit('error', 'Invalid file path')
    } catch (error) {
      expect(getErrorMessage(error)).toBe('Invalid file path')
    }
  })

但是，我明白了：

node:internal/process/promises:246
          triggerUncaughtException(err, true /* fromPromise */);
          ^

[UnhandledPromiseRejection: This error originated either by throwing inside of an async function without a catch block, or by rejecting a promise which was not handled with .catch(). The promise rejected with the reason "undefined".] {
  code: 'ERR_UNHANDLED_REJECTION'
}
node:internal/process/promises:246
          triggerUncaughtException(err, true /* fromPromise */);

Answer 1

模拟会导致未处理的被拒绝承诺。测试应该是异步的并返回一个承诺，即async。 try..catch 无法在同步函数中处理。

由于在调用 mockedReadStream.emit 时 promise 被拒绝，因此需要在 promise 被拒绝后不久与 catch 链接，例如通过 Jest 承诺断言：

let promise = app.start()
mockedReadStream.emit('error', 'Invalid file path')
await expect(promise).rejects.toThrow('Invalid file path')

这揭示了测试单元中的问题，因为reject() 没有传递错误。