Module mocking in ESM

moduleMock.ts

// # https://jestjs.io/docs/ecmascript-modules#module-mocking-in-esm

// #### module.ts ####
import { getData } from "./submodule";

export const myModule = () => {
  return getData();
};

// #### submodule.ts ####
export const getData = () => {
  return "real implementation";
};


// #### module.spec.ts ####
import { jest } from "@jest/globals";

jest.unstable_mockModule("./submodule", () => ({
  getData: jest.fn().mockImplementation(() => "mocked"),
}));

const { myModule } = await import("./module");

describe("esm module", () => {
  it("should return the mocked value for the getData function in submodule.ts", () => {
    expect(myModule()).toBe("mocked");
  });
});


// #### jest.config.ts ####
import type {JestConfigWithTsJest} from 'ts-jest'

const jestConfig: JestConfigWithTsJest = {
  extensionsToTreatAsEsm: [".ts"],
  moduleNameMapper: {
    "^(\\.{1,2}/.*)\\.js$": "$1",
  },
  transform: {
    "^.+\\.(ts|tsx)$": [
      "ts-jest",
      {
        useESM: true,
      },
    ],
  },
  testMatch: ["**/*.spec.ts"],
};

export default jestConfig;