toThrow doesn't seem to capture the error...

[joetidee]

I am doing this:

expect(async () => {await doSomething()}).toThrow('myThrownErrorMessage');

but this throws an error in the terminal:

(node:12725) UnhandledPromiseRejectionWarning: Unhandled promise rejection (rejection id: 575): Error: myThrownErrorMessage

Is this simply a case of node capturing the error before Jest does?

[SimenB]

return expect(async () => {await doSomething()}).rejects.toThrow('myThrownErrorMessage'); (you might need jest@test)

https://facebook.github.io/jest/docs/en/tutorial-async.html#rejects

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[joetidee]

I'm raising it as a bug because I would expect the toThrow function to handle the thrown error.

[SimenB]

What happens if you make the surrounding it async?

[joetidee]

Nothing different happens. The only way to get this to work is to do:

const mockError = new Error('someErrorMessage');
try {
    const e = await doSomething();
    expect(e).not.toEqual(mockError);
} catch (e) {
    expect(e).toEqual(mockError);
}

[magwas]

This issue is easier to find than the relevant Stack overflow question: https://stackoverflow.com/questions/50807170/jest-test-fails-when-trying-to-test-an-asynchronous-function-throws
The answer of @SimenB here actually does not work. The way to make it work is to make the lambda in expect async, and put .rejects. before .toThrow

Example production code:

import fetch from "node-fetch";

export async function ConfigLoaderService():Promise<void> {
  try {
    let response = await fetch("/config.json");
  } catch(error) {
    throw new Error("Config can't be loaded: "+ error);
  }
}

example test code:

import { ConfigLoaderService } from "../../src/com.kodekonveyor.frontend/config/ConfigLoaderService";
import { enableFetchMocks } from 'jest-fetch-mock'

enableFetchMocks()

describe("ConfigLoaderService", () => {
  it("throws error if config.json not found", async () => {
    fetchMock.mockAbort()
    expect(async () => {await ConfigLoaderService()}).rejects.toThrow(Error);
  })
});

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