Introduction to Jest Runner

Jest Runner is an essential tool for running Jest tests efficiently in your JavaScript applications. This guide introduces Jest Runner and dives into its various APIs with practical examples.

Getting Started with Jest Runner

To begin using Jest Runner, you need to install it in your project:

  npm install jest jest-runner

Overview of Key APIs

Basic Configuration

Jest Runner can be configured within the project’s package.json file or a separate Jest config file:

  {
    "jest": {
      "runner": "jest-runner",
      "testMatch": ["**/__tests__/**/*.js?(x)", "**/?(*.)+(spec|test).js?(x)"]
    }
  }

Running Tests Programmatically

You can run tests programmatically using the Jest Runner API:

  const { runCLI } = require('jest');

  async function runTests() {
    const { results } = await runCLI({}, [process.cwd()]);
    console.log(results.success ? 'Tests passed' : 'Tests failed');
  }

  runTests();

Custom Runners

Create a custom runner by extending the Jest Runner base class:

  const { TestRunner } = require('jest-runner');

  class CustomRunner extends TestRunner {
    async runTests(tests, watcher, onStart, onResult, onFailure, options) {
      // Custom logic for running tests
      return super.runTests(tests, watcher, onStart, onResult, onFailure, options);
    }
  }

  module.exports = CustomRunner;

Using Projects

Configure multiple projects for different setups:

  {
    "jest": {
      "projects": ["/projectA", "/projectB"]
    }
  }

App Example with Jest Runner

The following example demonstrates a simple application tested with Jest Runner:

  // sum.js
  function sum(a, b) {
    return a + b;
  }
  module.exports = sum;

  // sum.test.js
  const sum = require('./sum');

  test('adds 1 + 2 to equal 3', () => {
    expect(sum(1, 2)).toBe(3);
  });

Advanced API Usage

Mock Functions

Utilize mock functions to spy on behavior:

  const myMock = jest.fn();

  myMock.mockReturnValueOnce(10).mockReturnValueOnce('x').mockReturnValue(true);

  console.log(myMock(), myMock(), myMock(), myMock());

Timers

Mock the timers using Jest’s timer mocks:

  jest.useFakeTimers();

  function timerGame(callback) {
    console.log('Ready....go!');
    setTimeout(() => {
      console.log('Time\'s up -- stop!');
      callback && callback();
    }, 1000);
  }

  test('waits 1 second before ending the game', () => {
    const callback = jest.fn();

    timerGame(callback);

    // Fast-forward until all timers have been executed
    jest.runAllTimers();

    expect(callback).toBeCalled();
  });

By mastering these APIs, you can harness the full potential of Jest Runner to automate your tests effectively, ensuring a robust and reliable codebase.

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