用 Jest 模拟返回函数的 node_modules？

Question

I am writing a typeScript program which hits an external API. In the process of writing tests for this program, I have been unable to correctly mock-out the dependency on the external API in a way that allows me to inspect the values passed to the API itself.

A simplified version of my code that hits the API is as follows:

const api = require("api-name")();

export class DataManager {
  setup_api = async () => {
    const email = "email@website.ext";
    const password = "password";
    try {
      return api.login(email, password);
    } catch (err) {
      throw new Error("Failure to log in: " + err);
    }
  };

My test logic is as follows:

jest.mock("api-name", () => () => {
  return {
    login: jest.fn().mockImplementation(() => {
      return "200 - OK. Log in successful.";
    }),
  };
});

import { DataManager } from "../../core/dataManager";
const api = require("api-name")();

describe("DataManager.setup_api", () => {
  it("should login to API with correct parameters", async () => {
    //Arrange
    let manager: DataManager = new DataManager();

    //Act
    const result = await manager.setup_api();

    //Assert
    expect(result).toEqual("200 - OK. Log in successful.");
    expect(api.login).toHaveBeenCalledTimes(1);
  });
});

What I find perplexing is that the test assertion which fails is only expect(api.login).toHaveBeenCalledTimes(1) . Which means the API is being mocked, but I don't have access to the original mock. I think this is because the opening line of my test logic is replacing login with a NEW jest.fn() when called. Whether or not that's true, I don't know how to prevent it or to get access to the mock function-which I want to do because I am more concerned with the function being called with the correct values than it returning something specific.

I think my difficulty in mocking this library has to do with the way it's imported: const api = require("api-name")(); where I have to include an opening and closing parenthesis after the require statement. But I don't entirely know what that means, or what the implications of it are re:testing.

Answer 1

I came across an answer in this issue thread for ts-jest. Apparently, ts-jest does NOT "hoist" variables which follow the naming pattern mock* , as regular jest does. As a result, when you try to instantiate a named mock variable before using the factory parameter for jest.mock() , you get an error that you cannot access the mock variable before initialization.

Per the previously mentioned thread, the jest.doMock() method works in the same way as jest.mock() , save for the fact that it is not "hoisted" to the top of the file. Thus, you can create variables prior to mocking out the library.

Thus, a working solution is as follows:

const mockLogin = jest.fn().mockImplementation(() => {
  return "Mock Login Method Called";
});

jest.doMock("api-name", () => () => {
  return {
    login: mockLogin,
  };
});

import { DataManager } from "../../core/dataManager";

describe("DataManager.setup_api", () => {
  it("should login to API with correct parameters", async () => {
    //Arrange
    let manager: DataManager = new DataManager();

    //Act
    const result = await manager.setup_api();

    //Assert
    expect(result).toEqual("Mock Login Method Called");
    expect(mockLogin).toHaveBeenCalledWith("email@website.ext", "password");
  });
});

Again, this is really only relevant when using ts-jest , as using babel to transform your jest typescript tests WILL support the correct hoisting behavior. This is subject to change in the future, with updates to ts-jest , but the jest.doMock() workaround seems good enough for the time being.