josh-frank / react-hooks-jest-testing

Running Tests in React

Overview

In this lab, we'll discuss how the tests are set up for other labs throughout this phase, and some tips for running tests.

Objectives

1. Explain how tests are set up in React
2. Use Jest to run tests on specific files
3. Practice running tests

Running Jest Tests

There are a number of JavaScript test frameworks out there. For testing vanilla JavaScript applications, one popular choice is Mocha, which we use for the tests in Phase 3.

Jest is another popular choice for JavaScript developers, and in particular the React community. Jest, like React, was developed by Facebook and is an open source project. You can read the docs here if you're curious to learn more!

Jest comes preinstalled when you generate a React project using Create React App, so all you have to do to run tests in React labs is run npm test, which will execute the test script found in the package.json file. You can also run learn test, which will run the test files as well as sync your progress with Canvas.

Running learn test or npm test should produce an output like this in your terminal:

This command will run all tests in the src directory, looking for files that have .test.js in the file name (you'll typically find them in the __tests__ directory for our labs).

After running learn test, any changes you make to your components will cause the tests to run again, so you can keep the tests running as you work!

Code Along

There are a few tests defined for this lab so you can get some practice.

To get started, run npm install (if you haven't already), then run npm test or learn test.

You should see something like this in the output:

 FAIL  src/__tests__/Header.test.js
  ● displays the text 'hello from the Header!'

    expect(received).toBeInTheDocument()

    received value must be an HTMLElement or an SVGElement.
    Received has value: null

       8 |   render(<Header />);
       9 |
    > 10 |   expect(screen.queryByText("hello from the Header!")).toBeInTheDocument();
         |                                                        ^
      11 | });
      12 |

      at __EXTERNAL_MATCHER_TRAP__ (node_modules/expect/build/index.js:342:30)
      at Object.<anonymous> (src/__tests__/Header.test.js:10:56)

 FAIL  src/__tests__/Article.test.js
  ● displays the text 'please pass this test'

    expect(received).toBeInTheDocument()

    received value must be an HTMLElement or an SVGElement.
    Received has value: null

       8 |   render(<Article />);
       9 |
    > 10 |   expect(screen.queryByText("please pass this test")).toBeInTheDocument();
         |                                                        ^
      11 | });
      12 |

      at __EXTERNAL_MATCHER_TRAP__ (node_modules/expect/build/index.js:342:30)
      at Object.<anonymous> (src/__tests__/Article.test.js:10:56)

Test Suites: 2 failed, 2 total
Tests:       2 failed, 2 total
Snapshots:   0 total
Time:        3.486 s
Ran all test suites.

Watch Usage
 › Press f to run only failed tests.
 › Press o to only run tests related to changed files.
 › Press q to quit watch mode.
 › Press p to filter by a filename regex pattern.
 › Press t to filter by a test name regex pattern.
 › Press Enter to trigger a test run.

Let's focus on the Header.test.js file first. To tell Jest to only run tests on this one file, press the p key in your terminal (this will let you filter out tests by their filename). In the next screen, type in Header:

Pattern Mode Usage
 › Press Esc to exit pattern mode.
 › Press Enter to filter by a filenames regex pattern.

 pattern › Header

 Pattern matches 1 file
 › src/__tests__/Header.test.js

Then, press the Enter key to run tests in the Header.test.js file only.

See if you can get this test passing by updating the code in /src/components/Header.js!

Next, press the a key in your terminal to tell Jest to run all tests. Try getting the tests for the Article component to pass too!

When you've finished, you can hit the q key to exit Jest.

Resources

• Jest