Jest and Typescript installation / configuration

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1 - About

How to configure and install Jest with Typescript

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3 - Steps

3.1 - Jest

  • Cli
yarn global add jest
# or npm
npm install -g jest
  • Locally for the project
yarn # if jest is already in package.json
yarn add --dev jest
# or with npm
npm install # if jest is already in package.json
npm install --save-dev jest
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3.2 - Package.json

Add the npm test script

package.json
{
  "scripts": {
    "test": "jest"
  }
}

3.3 - Configuration file

Generating a conf file

jest --init
jest.config.js
module.exports = {
  // All imported modules in your tests should be mocked automatically
  // automock: false,
 
  // Stop running tests after `n` failures
  // bail: 0,
 
  // Respect "browser" field in package.json when resolving modules
  // browser: false,
 
  // The directory where Jest should store its cached dependency information
  // cacheDirectory: "C:\\Users\\gerardnico\\AppData\\Local\\Temp\\jest",
 
  // Automatically clear mock calls and instances between every test
  // clearMocks: false,
 
  // Indicates whether the coverage information should be collected while executing the test
  // collectCoverage: false,
 
  // An array of glob patterns indicating a set of files for which coverage information should be collected
  // collectCoverageFrom: null,
 
  // The directory where Jest should output its coverage files
  // coverageDirectory: null,
 
  // An array of regexp pattern strings used to skip coverage collection
  // coveragePathIgnorePatterns: [
  //   "\\\\node_modules\\\\"
  // ],
 
  // A list of reporter names that Jest uses when writing coverage reports
  // coverageReporters: [
  //   "json",
  //   "text",
  //   "lcov",
  //   "clover"
  // ],
 
  // An object that configures minimum threshold enforcement for coverage results
  // coverageThreshold: null,
 
  // A path to a custom dependency extractor
  // dependencyExtractor: null,
 
  // Make calling deprecated APIs throw helpful error messages
  // errorOnDeprecated: false,
 
  // Force coverage collection from ignored files using an array of glob patterns
  // forceCoverageMatch: [],
 
  // A path to a module which exports an async function that is triggered once before all test suites
  // globalSetup: null,
 
  // A path to a module which exports an async function that is triggered once after all test suites
  // globalTeardown: null,
 
  // A set of global variables that need to be available in all test environments
  // globals: {},
 
  // The maximum amount of workers used to run your tests. Can be specified as % or a number. E.g. maxWorkers: 10% will use 10% of your CPU amount + 1 as the maximum worker number. maxWorkers: 2 will use a maximum of 2 workers.
  // maxWorkers: "50%",
 
  // An array of directory names to be searched recursively up from the requiring module's location
  // moduleDirectories: [
  //   "node_modules"
  // ],
 
  // An array of file extensions your modules use
  // moduleFileExtensions: [
  //   "js",
  //   "json",
  //   "jsx",
  //   "ts",
  //   "tsx",
  //   "node"
  // ],
 
  // A map from regular expressions to module names that allow to stub out resources with a single module
  // moduleNameMapper: {},
 
  // An array of regexp pattern strings, matched against all module paths before considered 'visible' to the module loader
  // modulePathIgnorePatterns: [],
 
  // Activates notifications for test results
  // notify: false,
 
  // An enum that specifies notification mode. Requires { notify: true }
  // notifyMode: "failure-change",
 
  // A preset that is used as a base for Jest's configuration
  // preset: null,
 
  // Run tests from one or more projects
  // projects: null,
 
  // Use this configuration option to add custom reporters to Jest
  // reporters: undefined,
 
  // Automatically reset mock state between every test
  // resetMocks: false,
 
  // Reset the module registry before running each individual test
  // resetModules: false,
 
  // A path to a custom resolver
  // resolver: null,
 
  // Automatically restore mock state between every test
  // restoreMocks: false,
 
  // The root directory that Jest should scan for tests and modules within
  // rootDir: null,
 
  // A list of paths to directories that Jest should use to search for files in
  // roots: [
  //   "<rootDir>"
  // ],
 
  // Allows you to use a custom runner instead of Jest's default test runner
  // runner: "jest-runner",
 
  // The paths to modules that run some code to configure or set up the testing environment before each test
  // setupFiles: [],
 
  // A list of paths to modules that run some code to configure or set up the testing framework before each test
  // setupFilesAfterEnv: [],
 
  // A list of paths to snapshot serializer modules Jest should use for snapshot testing
  // snapshotSerializers: [],
 
  // The test environment that will be used for testing
  // testEnvironment: "jest-environment-jsdom",
 
  // Options that will be passed to the testEnvironment
  // testEnvironmentOptions: {},
 
  // Adds a location field to test results
  // testLocationInResults: false,
 
  // The glob patterns Jest uses to detect test files
  // testMatch: [
  //   "**/__tests__/**/*.[jt]s?(x)",
  //   "**/?(*.)+(spec|test).[tj]s?(x)"
  // ],
 
  // An array of regexp pattern strings that are matched against all test paths, matched tests are skipped
  // testPathIgnorePatterns: [
  //   "\\\\node_modules\\\\"
  // ],
 
  // The regexp pattern or array of patterns that Jest uses to detect test files
  // testRegex: [],
 
  // This option allows the use of a custom results processor
  // testResultsProcessor: null,
 
  // This option allows use of a custom test runner
  // testRunner: "jasmine2",
 
  // This option sets the URL for the jsdom environment. It is reflected in properties such as location.href
  // testURL: "http://localhost",
 
  // Setting this value to "fake" allows the use of fake timers for functions such as "setTimeout"
  // timers: "real",
 
  // A map from regular expressions to paths to transformers
  transform: {}, // babel-jest is automatically installed when installing Jest and will automatically transform files
 
  // An array of regexp pattern strings that are matched against all source file paths, matched files will skip transformation
  // transformIgnorePatterns: [
  //   "\\\\node_modules\\\\"
  // ],
 
  // An array of regexp pattern strings that are matched against all modules before the module loader will automatically return a mock for them
  // unmockedModulePathPatterns: undefined,
 
  // Indicates whether each individual test should be reported during the run
  // verbose: null,
 
  // An array of regexp patterns that are matched against all source file paths before re-running tests in watch mode
  // watchPathIgnorePatterns: [],
 
  // Whether to use watchman for file crawling
  // watchman: true,
};
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3.4 - Babel

Babel is needed with Jest:

  • to understand ES6 modules (no import statement in Node). Babel will be used to transpiler the import statement of Es6.
  • to supports TypeScript

webpack 2 offers native support for ES modules. However, Jest runs in Node, and thus requires ES modules to be transpiled to CommonJS modules. As such, if you are using webpack 2, you most likely will want to configure Babel to transpile ES modules to CommonJS modules only in the test environment.

Installation:

  • Babel
yarn add --dev babel-jest @babel/core babel-cli
# or
npm install --save-dev babel-jest babel-core babel-cli
yarn add --dev @babel/preset-env #es module
yarn add --dev @babel/preset-typescript # typescript
# or
npm install --save-dev babel/preset-env
npm install --save-dev @babel/preset-typescript #typescript
  • To enable the preset
babel.config.js
module.exports = api => {
    const isTest = api.env('test');
    // You can use isTest to determine what presets and plugins to use.
 
    return {
        presets: [
            [
                '@babel/preset-env',
                '@babel/preset-typescript',
                {
                    targets: {
                        node: 'current',
                    },
                },
            ],
        ],
    };
 
};

3.5 - Typescript

The babel preset should be installed. See section above babel.

Then …

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3.5.1 - Types

yarn add --dev @types/jest 
# or
npm install --save-dev @types/jest
  • Add jest in the types compiler option.
tsconfig.json
{
  "compilerOptions": {
    ........
    "types": [
      "jest"
    ]
    ........
}

3.5.2 - ts-jest

ts-jest is a TypeScript preprocessor with source map support for Jest that lets you use Jest to test projects written in TypeScript.

It's needed because typeScript support in Babel is transpilation, Jest will not type-check the tests as they are run.

yarn add --dev ts-jest
yarn ts-jest config:init
  • It should create a jest.config.js config file with the following options
jest.config.js
module.exports = {
  preset: 'ts-jest',
  testEnvironment: 'node'
}

4 - Support

4.1 - SyntaxError: Cannot use import statement outside a module

> jest

 FAIL  __tests__/country.test.ts
  ● Test suite failed to run

    D:\dokuwiki\lib\plugins\webcomponent\__tests__\country.test.ts:1
    ({"Object.<anonymous>":function(module,exports,require,__dirname,__filename,global,jest){import * as Country from "../js/Country";
                                                                                             ^^^^^^

    SyntaxError: Cannot use import statement outside a module

      at ScriptTransformer._transformAndBuildScript (node_modules/@jest/transform/build/ScriptTransformer.js:537:17)
      at ScriptTransformer.transform (node_modules/@jest/transform/build/ScriptTransformer.js:579:25)

If you are using typescript, add ts-jest

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