I'm trying to temporarily mock node-fetch in an ESM module while still retraining the original implementation so I can access a real endpoint's value. However, this errors with "Must use import to load ES Module." I recognize jest support for ESM is still pending - is there any way to have this behavior in a bination of current Node, ES6, and Jest?

worker.ts (dependency):

export default async () => {
    const response = await fetch(";);
    return await response.json()
  }

main.test.ts:

import { jest } from "@jest/globals";


jest.mock("node-fetch", () => {
    return Promise.resolve({
        json: () => Promise.resolve({ myItem: "abc" }),
    })
})

import doWork from './worker.js';
import mockedFetch from 'node-fetch';

const originalFetch = jest.requireActual('node-fetch') as any;

test("Ensure mock", async () => {
    const result = await doWork();
    expect(result.myItem).toStrictEqual("abc");
    expect(mockedFetch).toBeCalledTimes(1);

    const response = await originalFetch(";);
    expect(response.status).toBe(200);

    const result2 = await doWork();
    expect(result2.myItem).toStrictEqual("abc");
    expect(mockedFetch).toBeCalledTimes(2);
});

I'm trying to temporarily mock node-fetch in an ESM module while still retraining the original implementation so I can access a real endpoint's value. However, this errors with "Must use import to load ES Module." I recognize jest support for ESM is still pending - is there any way to have this behavior in a bination of current Node, ES6, and Jest?

worker.ts (dependency):

export default async () => {
    const response = await fetch("http://example2");
    return await response.json()
  }

main.test.ts:

import { jest } from "@jest/globals";


jest.mock("node-fetch", () => {
    return Promise.resolve({
        json: () => Promise.resolve({ myItem: "abc" }),
    })
})

import doWork from './worker.js';
import mockedFetch from 'node-fetch';

const originalFetch = jest.requireActual('node-fetch') as any;

test("Ensure mock", async () => {
    const result = await doWork();
    expect(result.myItem).toStrictEqual("abc");
    expect(mockedFetch).toBeCalledTimes(1);

    const response = await originalFetch("http://www.example");
    expect(response.status).toBe(200);

    const result2 = await doWork();
    expect(result2.myItem).toStrictEqual("abc");
    expect(mockedFetch).toBeCalledTimes(2);
});

• javascript
• node.js
• jestjs
• fetch

• is there any way to have this behavior in a bination of current Node, ES6, and Jest? I have answered to a similar question here. – Marco Luzzara Commented May 30, 2022 at 5:54

1 Answer 1

First, Jest doesn't support jest.mock in ESM module for tests

Please note that we currently don't support jest.mock in a clean way in ESM, but that is something we intend to add proper support for in the future. Follow this issue for updates.

This is reasonable because import has different semantic than require. All imports are hoisted and will be evaluated at module load before any module code.

So in your case because you are using jest.mock I assume that your test code are transformed. In this case, if you want to use non "CommonJS" package, you should transform it too. You can change transformIgnorePatterns in jest config to []. It means that all packages from node_modules will go through transform. If it's too aggressive, you can pick specific modules which ignore like this "transformIgnorePatterns": [ "node_modules/(?!(node-fetch))" ] but don't forget about transitive dependencies. ;)

• Add/change in jest config

  "transformIgnorePatterns": []

• jest.mock accept module factory which should returns exported code. In your case, you should write it like this.

jest.mock("node-fetch", () => {
  return {
    __esModule: true,
    default: jest.fn(() =>
      Promise.resolve({
        json: () => Promise.resolve({ myItem: "abc" }),
      })
    ),
  };
});

If you have error

The module factory of jest.mock() is not allowed to reference any out-of-scope variables

just remove jest from import and use it as global variable or import it inside the module factory function https://github./facebook/jest/issues/2567

• Change const originalFetch = jest.requireActual('node-fetch') to

const originalFetch = jest.requireActual('node-fetch').default;