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Jest makes unit testing fun again

November 04, 2020 • 7 min read

Written by franleplant: Tech Lead and Software developer with a degree in Engineering. Woodworker, all things Javascript, Rust and Software Architecture. Github


Why I love Jest and why should consider it for your next project.#

Jest is a full-blown, batteries included, ready to rock, testing framework that has similarities with mocha and other testing frameworks but with a much more integrated experience, making it very easy to have a production ready unit test setup out of the box with very little effort.

Let’s cover some of the things I love about Jest, if you are interested in my unit testing mindset and philosophy and an illustrative story check it at the end of this article.

1. Less verbose than similar frameworks#

  • Simple tests can be terse
  • Assertion library is always available
test("my function", () => {
  • You can omit a top level describe when using the BDD interface
beforeEach(() => {...})

it('should do something', () => {...})

Tests should be expressive but abstract, the less code the better, Jest helps with this.

In a lot of cases, if you structure your code a certain way, using one or two test per unit will be more than enough.

2. Jest replaces sinon for the most part#

  • need to mock / spy / stub pieces of code? use the global jest.fn()
test("jest mocks", () => {
  const mockSpyStub = jest.fn();

  mockSpyStub("jest is love");

  expect(mockSpyStub).toHaveBeenCalledWith("jest is love");

Check the docs

3: Jest comes with an assertion library#

  • Comes already installed, integrated and globally available in all tests
  • Covers a wide range of assertions / matchers.
  • Good typescript support
  • Perfect integration with Jest mocks / stubs / spys jest.fn()j
  • Nice diff reporting tool

Check all the included assertions here.

4. Jest comes with code coverage integration#

Note Actually I just checked mocha’s official documentation and it redirects to Wallaby.js which is great but it is a paid tool, instead of linking to nyc (instanbul) and how to integrate with that.

5. Jest has out of the box module mocking#

Jest has an awesome module mocking story:

  • Out of the box support with little config
  • Works with CommonJS and native ES6 modules
  • Works well with local modules and node_modules
  • Simple and terse interface
  • Enables well known practices of unit testing and mocking thoroughly.
  • Enables effective, efficient and terse unit tests of complex pieces of code.

Little implementation detail: Jest achieves this partially by running tests inside an isolated Nodejs vm which gives jest complete control of the test environment.

Let’s cover some really useful patterns I have been using a lot lately:

  • es6 modules: default export mock
import myLib from "my-lib";

// now myLib is a mock
// configure a mock return value, you
(myLib as jest.Mock).mockImplementation(() => "test value");
  • es6 modules: member mock
import { member } from "my-lib";

// now myLib is a mock

// configure a mock return value, you
(member as jest.Mock).mockImplementation(
  () => "test value"

There are libs out there that provide similar strategies but they often fall short, or do not support ES6 modules, or are simply too complicated because they do not have the level of control over how the code is being run like Jest does.

Check the official docs.

6. Jest has first class support for Babel and Typescript#

Easy to integrate with Typescript either by using Babel or by using ts-jest.

If your codebase already uses typescript through Babel then jest will automatically use that config and you won’t need to do anything else.

7. Jest test runner interface is awesome#

Watch mode, interactive mode, filtering, Jest has you covered.

Read more here.

This is a really nice trick I use a lot to focus only on particular tests at a time: reg exp filtering

# runs all the tests

# runs only the tests that fit this patterns
jest pattern
jest myFunction
jest myFunc*

Something similar can be done in the interactive watch mode.

Bonus: My unit test mind set#

  • test in the most simple and minimalistic way
  • do real unit tests, mocking everything else
  • test only the high impact code

In most of the cases I don’t see the value of being obsessed with 100% coverage, because sometimes testing two error handling lines inside a unhappy path branch might require a whole lot of code.

There are cases where a 100% coverage might be adequate or needed but in most high level business apps there’s really no need and the cost of keeping the coverage to 100% is really high. In most cases I try to implement a Pareto approach, do the minimum thing with the highest impact which typically translates into 80% code coverage and ignoring glue code files.

Unit tests files is still code that you need to maintain so it should be treated as close to regular code as possible and it should be as simple and as readable and maintainable as possible.

Story time: how not to unit test#

Some random day I was cursing at my screen and my partner asked me why I was so mad and since she’s not a technical person I came up with this analogy on the fly to explain what was the problem and also how I would solve it. I believe this illustrates really well my unit test mindset:

Let’s say we are building a wall lamp that would typically be used inside your house. The guys who tested this lamp (unit testing) did the unthinkable: they build a testing house, with foundations, wall framing, ceiling, windows, doors, hardwood floors, water pipes, heating system, insulation and a working power grid; they then proceeded to install the lamp that they were testing, and finally flipped the right wall switch to see if it worked.

How I would have done it differently? I would just reveal the contacts and give them power by the simplest possible means (without risking my life), like for example: wiring them to a circuit with power!

My partner hugged me and left feeling sorry for me.

The point of this story is to emphasize that we need to have a clear mind set when unit testing our code, we need to ensure that we are testing an isolated unit, that we are testing it in the simplest way that can provide the verifications we look for, nothing more.

Which brings up my coding mindset: clarity then simplicity then consistency which I will most likely be talking about in a follow up post.

So remember: do not build a house around the lamp you want to test, simply connect it to a cable with power and be done with it.


As you can see, there’s a lot to love about Jest.

Jest can be considered a composition of a bunch of libraries into a nice tight package that works out of the box:

jest = mocha + chai + power-assert + sinon + rewire + instanbul + more

I encourage you to consider it for your next project, it works well for Nodejs and Browser projects, it works well with plain old Javascript or ES6+ or Typescript, and it has really nice integrations with React.js.

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