Jest - Testing Module Multiple Times in One Test Suite

Question

I have a TypeScript module (should be irrelevant, as I think this affect JS too) and I'm trying to test a module I have. The module imports lots of data from external files and chooses which data should be returned based on the a variable.

I'm attempting to run some tests where I update that variable, re-require the module and run further tests in one file. But my issue is that the require of the file only runs once. I guess it's being cached. Is it possible to tell Jest's require function not to cache or to clear the cache between tests?

Here's some stripped back code of what I'm trying to achieve:

module.ts

import { getLanguage } from "utils/functions";

import * as messagesEn from "resources/translations/en";
import * as messagesFr from "resources/translations/fr";

// Determine the user's default language.
const language: string = getLanguage();

// Set messages based on the language.
let messages: LocaleMessages = messagesEn.default;
if (languageWithoutRegionCode === "fr") {
    messages = messagesFr.default;
}

export { messages, language };

test.ts

import "jest";

// Mock the modules
const messagesEn = { "translation1": "English", "translation2": "Words" };
const messagesFr = { "translation1": "Francais", "translation2": "Mots" };
const getLangTest = jest.fn(() => "te-ST");
const getLangEn = jest.fn(() => "en-GB");
const getLangFr = jest.fn(() => "fr-FR");
jest.mock("resources/translations/en", () => ({"default": messagesEn}));
jest.mock("resources/translations/fr", () => ({"default": messagesFr}));
jest.mock("utils/functions", () => ({
        getLanguage: getLangTest
    })
);

describe("Localisation initialisation", () => {
    it("Sets language", () => {
        const localisation = require("./localisation");
        expect(getLangTest).toHaveBeenCalled();
        expect(localisation.language).toEqual("te-ST");
        expect(localisation.messages).toEqual(messagesEn);
    });

    it("Sets english messages", () => {
        // THIS GETS THE MODULE FROM THE CACHE
        const localisation = require("./localisation");
        expect(getLangEn).toHaveBeenCalled();
        expect(localisation.language).toEqual("en-GB");
        expect(localisation.messages).toEqual(messagesEn);
    });

    it("Sets french messages", () => {
        // THIS GETS THE MODULE FROM THE CACHE
        const localisation = require("./localisation");
        expect(getLangFr).toHaveBeenCalled();
        expect(localisation.language).toEqual("fr-FR");
        expect(localisation.messages).toEqual(messagesFr);
    });
});

I'm aware the second and third tests won't work anyway as I'd need to update the "utils/functions" mock. The issue is that the code in module.ts only runs once.

Answer 1

So, many thanks to the Jest folks on Discord. It's possible to actually clear the modules from the cache with the jest.resetModules() function.

So my test.ts file will look as follows:

describe("Localisation initialisation", () => {
    beforeEach(() => {
        jest.resetModules();
    });

    it("Sets language", () => {
        const localisation = require("./localisation");
        // Perform the tests
    });

    it("Sets english messages", () => {
        const localisation = require("./localisation");
        // Perform the tests
    });

    it("Sets french messages", () => {
        const localisation = require("./localisation");
        // Perform the tests
    });
});

The beforeEach() call to jest.resetModules() ensures we're re-running the code in the module.