如何在Jest中跟踪失败的异步测试? [英] How to trace a failed asynchronous test in Jest?

问题描述

-更新-

已确定问题.

在实际的代码库中，将断言传递给导入的回调，一旦回调以失败的测试执行，就会引发承诺拒绝.

In the actual codebase the assertion is passed to a imported callback, and once the callback executes with a failed test, it raises a promise rejection.

因此，这与测试的实际编写方式很接近:

So, this is close to how the test was actually written:

describe( "file system", () => {

  it( "should check if the file exists", async () => {

    call( async () => {

      const received = await fileExists();
      const expected = true;

      expect( received ).toBe( expected );

    });
  });
});

并以更简单的方式显示复杂的回调以产生相同的问题:

and the complex callback is presented in a simpler way to produce the same issue:

export function call( callback) {

  callback();

}

---

-更新-

以下代码有效.

我从大型代码库中提取了一小部分代码，以提高可视性.如果我只运行下面的代码，它将按预期工作.我认为实际的代码库中存在问题.

I picked up a small portion of the code from a large codebase for better visibility. If I run just the following piece of code, it works as expected. I think there's an issue in the actual codebase.

@Flask关于处理未处理的诺言拒绝的建议为该问题增加了很大的价值.

@Flask's recommendation of handling the unhandled promise rejections centrally added a great value to the question.

请考虑以下测试:

import fileExists, { call } from "./exists";

describe( "file system", () => {

  it( "should check if the file exists", async () => {

    const received = await fileExists();
    const expected = true;

    expect( received ).toBe( expected );
  });
});

适用于以下来源:

import fs, { constants } from "fs";
import { promisify } from "util";

export default async function fileExists() {

  const path    = ".nonexistent";
  const access  = promisify( fs.access );

  try {

    await access( path, constants.F_OK );

  } catch {

    return false;

  }

  return true;

}

当fileExists 拒绝返回false时，将收到预期的UnhandledPromiseRejectionWarning .但这无助于追踪失败测试的根源.

When fileExists rejects returns false, an UnhandledPromiseRejectionWarning is received as expected. But this does not help trace the source of the failed test.

对于同步测试，Jest显示测试路径(即file system › should check if the file exists)，这有助于跟踪失败测试的来源.

For synchronous tests, Jest shows the path to the test (i.e. file system › should check if the file exists) which helps trace the source of the failed test.

实现异步测试的最佳方法是什么?

What is the best way to achieve this for asynchronous tests?

推荐答案

UnhandledPromiseRejectionWarning在这里不是必需的.它不等于失败的测试，因为它不会在断言通过时阻止测试通过.这意味着代码编写方式错误，并且包含未链接的承诺.

UnhandledPromiseRejectionWarning is not expected here. It's not equivalent to failed test because it doesn't prevent a test to pass if assertions pass. It means that the code was written the wrong way and contains unchained promises.

只有在测试中省略了await时，它才会发生:

It can happen only if await was omitted in the test:

fileExists(); // no await

或fileExists函数包含未处理的宽松承诺:

Or fileExists function contains loose unhandled promises:

fileExists() {
  whatever() // no return
  .then(() => {
    whatever() // no return
  })  // no catch to suppress errors
}

在setupFiles中拥有一个好习惯:

process.on('unhandledRejection', console.error);

与UnhandledPromiseRejectionWarning相比，它提供了更多有用的输出，并允许基于错误堆栈来调试问题.

It provides more useful output than UnhandledPromiseRejectionWarning and allows to debug the problem based on error stack.

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