How to Write a Test

When first learning how to write automated tests, it can be easy to get stuck not knowing how to write the test. Tests should be composed using a pattern that makes them easy to write, reason about, and expand. One pattern is the AAA (Arrange-Act-Assert) pattern, which encourages the organization of the test code in a way that allows the most readability and flexibility.

In step one, the Arrange step, you have to perform some setup for your test. For example, in the case of a Cypress end-to-end test, you need to tell Cypress to open the browser and navigate to the correct URL.

cy.visit("http://localhost:8888")

In step two, the Act step, you perform some action. For example, in the case of a todo app, you want to test that you can add a single todo.

cy.get(".new-todo").type("Buy Milk{enter}")

In this example, we are getting the element with the class of .new-todo and then typing "Buy Milk" and simulating pressing the Enter key to add the todo.

Finally, in step three, you Assert. In this step, you assert that the thing you acted upon in step two did what you expected.

cy.get(".todo-list li").should("have.length", 1)

In this example, we are getting the element that contains the todo items that we added to the app and asserting that there is only a single todo.

With these three steps in mind, here is what a simple Cypress end-to-end test looks like for a TodoMVC application.

A simple end-to-end test

The following example is from the React TodoMVC app, which we cover in the "Testing your first application" section.

// app.spec.js

describe("React TodoMVC", () => {
  it("adds a single todo", () => {
    cy.visit("http://localhost:8888") // Arrange
    cy.get(".new-todo").type("Buy Milk{enter}") // Act
    cy.get(".todo-list li").should("have.length", 1) // Assert
  })
})

Before breaking this test file down, it is important to note that Cypress is built upon Mocha. In this example, all of the functions other than those that begin with cy come from Mocha.

Each test file will typically begin with a describe() function or "block." This function provides a way to organize your tests and makes things easier to read. Test files can have multiple describe() within them as it provides the context for the tests written inside them.

Next, we have an it block. This is our actual test and where our test code goes. We first provide a test name "adds a single todo" and then pass in a callback function. Within the body of our test, we are doing three things.

1. Arrange

Telling Cypress to visit the URL http://localhost:8888

2. Act

Getting the element with the class of .new-todo and typing in the string "Buy Milk" and pressing the Enter key.

3. Assert

We are asserting that only a single todo has been added to the app.


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