How to mock module function using Jest

Jest is a JavaScript testing framework maintained by Facebook, Inc. designed and built by Christoph Nakazawa with a focus on simplicity and support for large web applications. It works with projects using Babel, TypeScript, Node.js, React, Angular, Vue.js and Svelte. Wikipedia

In this post, I will show you an easy way to automatically mock all the functions of the module. Jest also allows you to mock module functions. By the end of this tutorial, you will learn the following things.

• How to mock a module functional automatically
• How to check the function is the mock function
• How to assert on the mock function

To show the demo I am using the following simple javascript module

const utils = {
  getJSON: (data) => JSON.stringify(data),
  isAuthorized: (secret) => secret === "codeguru"
};

module.exports = utils;

As you can see the module has two functions getJSON and isAuthorized now we are going to write the test for this module.

One way is to load the module and then mock the function manually. But there is a better way to mock the module in Jest. Just add the jest.enableAutomock on the top of your test file and then load your module as shown in the following code snippet.

jest.enableAutomock();
const utils = require("./main");

Just adding enableAutomock jest will provide the mock version of the function in your test.

jest.enableAutomock();

const utils = require("./main");

describe("automatic mocks", () => {
  it("should mock all methods of utils", () => {
    expect(utils.getJSON.mock).toBeTruthy();
      expect(jest.isMockFunction(utils.isAuthorized)).toBeTruthy();
  });

  it("mocked implementation", () => {
    utils.getJSON.mockReturnValue(123);
    expect(utils.getJSON({ name: "test" })).toBe(123);
  });
});

As you can see now every function has a mock property and now you can use all the mock functionality of jest on your function