@swissquote/crafty-preset-jest

Compatible Runners	Not tied to any runner
Linters	N/A
Commands	test: Jest integrates itself with the crafty test command
Related presets	Babel TypeScript

Crafty provides a preset that will run Jest once crafty test is executed.

It adds safe defaults to be able to run your tests with your configuration and provides an extension hook that allows you and other presets to extend its configuration.

Jest is a Node-based runner. This means that the tests always run in a Node environment and not in a real browser. This lets us enable fast iteration speed and prevent flakiness.

While Jest provides browser global variables such as window thanks to jsdom, they are approximations of the real browser behavior. Jest is intended to be used for unit tests of your logic and your components rather than the DOM quirks.

We recommend that you use a separate tool for browser end-to-end tests if you need them.

[TOC]

File name Conventions

Jest will look for test files with any of the following popular naming conventions:

• Files with .js suffix in __tests__ folders.
• Files with .test.js suffix.
• Files with .spec.js suffix.

The .test.js / .spec.js files (or the __tests__ folders) can be located at any depth under the src top level folder.

We recommend to put the test files (or __tests__ folders) next to the code they are testing so that relative imports appear shorter. For example, if App.test.js and App.js are in the same folder, the test needs to import App from './App' instead of a long relative path.

Using crafty-preset-babel will add jsx as a supported test file extension and crafty-preset-typescript will add the support TypeScript extensions ts, tsx, mts and cts

To be able to use TypeScript test files, you'll have to add @types/jest as a dependency :

npm install --save @types/jest

crafty test

Running crafty test will run all test and exit. But you can use any option provided by Jest itself.

For example crafty test --watch will run your tests in watch mode. This mode will run all your tests once and once it's done will wait for code or test changes to re-run the concerned tests.

Writing Tests

To create tests, add it() (or test()) blocks with the name of the test and its code. You may optionally wrap them in describe() blocks for logical grouping but this is neither required nor recommended.

Jest provides a built-in expect() global function for making assertions. A basic test could look like this:

import sum from "./sum";

it("sums numbers", () => {
  expect(sum(1, 2)).toEqual(3);
  expect(sum(2, 2)).toEqual(4);
});

All expect() matchers supported by Jest are extensively documented here.
You can also use jest.fn() and expect(fn).toBeCalled() to create “spies” or mock functions.

Focusing and Excluding Tests

You can replace it() with xit() to temporarily exclude a test from being executed.
Similarly, fit() lets you focus on a specific test without running any other tests.

Coverage Reporting

Jest has an integrated coverage reporter that works well with EcmaScript 2015+ and requires no configuration.
Run crafty test --coverage to include a coverage report like this:

Note that tests run much slower with coverage. We recommend to run it separately from your normal workflow.

Snapshot Testing

Snapshot testing is a feature of Jest that automatically generates text snapshots of your components and saves them on the disk so if the UI output changes, you get notified without manually writing any assertions on the component output. Read more about snapshot testing.

Extending the configuration

Each preset and crafty.config.js can define the jest(crafty, options) function to override Jest's configuration.

const path = require("path");
const MODULES = path.join(__dirname, "..", "node_modules");

module.exports = {
  /**
   * Represents the extension point for Jest configuration
   * @param {Crafty} crafty - The instance of Crafty.
   * @param {Object} options - The Jest configuration object
   */
  jest(crafty, options) {
    // Adds this directory to resolve modules
    options.moduleDirectories.push(MODULES);

    // Add a transformer for TypeScript
    options.transform["^.+\\.(ts|tsx|mts|cts)$"] = require.resolve("ts-jest");

    // Add file extensions to resolve imports
    options.moduleFileExtensions.push("ts");
    options.moduleFileExtensions.push("tsx");
    options.moduleFileExtensions.push("mts");
    options.moduleFileExtensions.push("cts");
  }
};

The full list of available configuration option is available on the official website.