Add Jest support

.eslintrc.cjs

@@ -6,9 +6,11 @@ module.exports = {
   },
   plugins: [
     '@typescript-eslint',
+    'jest',
   ],
   env: {
     node: true,  // for `console`
+    'jest/globals': true,  // describe, test, expect
   },
   extends: [
     'eslint:recommended',

README.md

@@ -75,3 +75,27 @@ Add `"type": "module"` to `package.json`, because [TypeScript can't generate fil
 To be able to run `eslint`, we must create an `.eslintrc.cjs` file, rather than a `.js` one (due to `"type": "module"` in `package.json`). Then, install the required dependencies:
 
     npm i -D eslint @typescript-eslint/eslint-plugin @typescript-eslint/parser
+
+
+# Jest
+
+To fully support Jest with TypeScript and ESLint, we need to:
+
+* `npm install --save-dev jest @types/jest eslint-plugin-jest`
+* add `"jest"` to the `types` array in `tsconfig.json`
+* add the `'jest'` plugin to `.eslintrc.cjs` and also add `'jest/globals': true` to its `env` key
+* create [`jest.config.cjs`](jest.config.cjs)
+
+Now if Jest supported ES Modules, we'd be done, but [it doesn't](https://github.com/facebook/jest/issues/4842), so we'll get this error when running `npm test`:
+
+> SyntaxError: Cannot use import statement outside a module
+
+To work around that, we need to add a `transform` key to `jest.config.cjs` and use Babel to transform the `import` statements from the `.js` files that TypeScript generates:
+
+* `npm install --save-dev @babel/plugin-transform-modules-commonjs`
+* create [`babel.config.cjs`](babel.config.cjs)
+
+Normally, to run Jest from `package.json`, we'd add a `"test": "jest"` line. That won't be sufficient, because we need to pass the `--harmony` flag to node (for optional chaining support). 
+To pass parameters to Node when running Jest, we'll add the following `test` line:
+
+    "test": "node --harmony node_modules/.bin/jest"

babel.config.cjs

@@ -0,0 +1,6 @@
+module.exports = {
+  plugins: [
+    // Because Jest still doesn't support native ES modules - https://github.com/facebook/jest/issues/4842
+    '@babel/plugin-transform-modules-commonjs',
+  ],
+};

jest.config.cjs

@@ -0,0 +1,15 @@
+module.exports = {
+  testEnvironment: 'node',
+  testMatch: [
+    '**/?(*.)test.ts',
+  ],
+  moduleFileExtensions: [
+    'js',  // not used directly, but MANDATORY
+    'ts',  // actually used
+  ],
+  transform: {
+    // Transform .ts files, and also .js ones generated by TypeScript, which scripts load -
+    // https://github.com/microsoft/TypeScript/issues/18442#issuecomment-581738714
+    '^.+\\.(ts|js)$': 'babel-jest',
+  },
+};