6 Rules I Follow to Get Simple and Stable Tests

It took me several years of trying and failing to figure out what does work when I write tests. Here are 6 rules that I always follow no matter what kind of tests I write.

Follow AAA pattern

Clean unit tests follow the AAA pattern:

This is the holy grail of simple tests.

Single concept per test

Test single concept in each unit test. If you follow the AAA pattern, this should feel natural.

Oftentimes, I see tests that cover a lot of different concepts in a single unit test. That’s a code smell. It’s better to split up such a test into multiple tests - each testing its own thing.

Your tests will be easier to understand, and easier to fix.

// Bad - mixing two concepts
it('should add new item into cart', () => {
  let cart = new Cart()
  cart.add(new Item('Shoes')) // <-- // Testing adding item into cart
  cart.removeAllItems(); // <-- Testing removing all items from cart

// Better - split it up
it('should add new item into cart', () => {
  let cart = new Cart();
  cart.add(new Item('Shoes');

it('should be possible to remove all items from cart', () => {
  let cart = new Cart([new Item()]);

Avoid logic in your tests

Avoid any if, else, switch, or ternary operator in your test code.

This often happens with a parametrized test where you expect slightly different behavior for different inputs, but also with tests where someone wanted to avoid failure for a flaky test.

In cases like these, it’s always better to split it up and keep it flat and simple.

// Bad
it("should have focus when clicked", () => {
  render(<Input />)
  const input = screen.getByRole("input")

  // this null check is pointless
  if (input !== null) {

// Better
it("should have focus when clicked", () => {
  render(<Input />)

Optimize for simplicity

Good test is short, flat, simple, and delightful to work with. When you look at a test, you should get the intent immediately.

Do not nest tests. It adds unnecessary complexity. Each nesting level adds an extra cognitive complexity to your code.

Refactor common setup code into a fixture to avoid repetition.

Only public API

If your test is doing something that your user can’t, it's most likely testing implementation details.

This is a code smell. Any change in the underlying implementation will break your test.

Test only API that your user can use.

Use TDD to drive your coverage

10/10 times when I use TDD, my code coverage climbs over 80%.

Start with the happy path - make sure that code works for your primary use cases.

Continue with unhappy paths - Cover the error conditions, unexpected input, or any other edge cases.

You can skip testing trivial one-liners, getters, and setters.

With this, you should hit the 80% coverage mark easily.

That’s it

With these few rules, you can write tests that everybody will understand and it will be a pleasure to work with.

Let me know if you have some other rules that you follow, and why.