@monosmith/jest-preset

The Jest preset every MonoSmith (smith) monorepo uses. One line in a project's Jest config, and no helper files at the workspace root.

// jest.config.js (or .ts, .cjs)
module.exports = {
  preset: '@monosmith/jest-preset',
}

Why this exists

smith convert takes Nx out of a workspace, and Nx provided three things Jest needs: the preset values (@nx/jest/preset), a resolver that falls back to TypeScript (@nx/jest/plugins/resolver) and, for React projects, a transformer for the files a bundler would load as assets (@nx/react/plugins/jest). Convert used to write those out as files at the workspace root (jest.resolver.cjs, jest.asset-transform.cjs, a rewritten jest.preset.js). They looked like clutter, so a tidy user deleted them and every package's tests broke (issue #447). They are this package now: one devDependency, upgraded with npm update, tested here.

What the preset sets

| Key | Value | |---|---| | testMatch | **/?(*.)+(spec\|test).?([mc])[jt]s?(x) | | moduleFileExtensions | ts, js, mts, mjs, cts, cjs, html | | moduleNameMapper | ./x.js resolves to ./x (an ESM-style import of a TypeScript file) | | transform | ts-jest over TypeScript, JavaScript and HTML, reading <rootDir>/tsconfig.spec.json | | resolver | the TypeScript-aware resolver below, wired in by the preset | | testEnvironment | jsdom (set testEnvironment: 'node' in a Node project) | | testEnvironmentOptions | customExportConditions: ['node', 'require', 'default'] | | modulePathIgnorePatterns | <rootDir>/dist/, <rootDir>/out-tsc/ | | coverageReporters | html |

These are the values @nx/jest/preset provided, so a converted workspace keeps testing the way it did. Whatever a project writes beside preset wins, and Jest merges moduleNameMapper and transform instead of replacing them.

The resolver

Jest's own resolution runs first. When it cannot find a module, TypeScript is asked, reading the first of tsconfig.spec.json, tsconfig.test.json, tsconfig.jest.json and tsconfig.json above the project's root. That finds:

• what a tsconfig paths entry names (@lib/greet), and
• a module that only has types: jest.mock('vscode') with just @types/vscode installed, which the mock replaces anyway.

When neither finds it, you get Jest's own Cannot find module error. TypeScript is optional: without it the resolver is Jest's.

To use the resolver without the preset: resolver: require.resolve('@monosmith/jest-preset/resolver').

React projects

module.exports = {
  preset: '@monosmith/jest-preset/react',
}

The base preset plus what @nx/react added to a project's Jest config:

• tsx and jsx join the module file extensions;
• scripts go through babel-jest with @babel/preset-env, @babel/preset-typescript and @babel/preset-react (automatic runtime);
• every other file that is not JavaScript, TypeScript, CSS or JSON goes through the asset transform: an imported SVG is a module whose default export is its file name and whose ReactComponent is a component named as SVGR names it (my-logo.svg is SvgMyLogo); any other file is its file name.

The asset transform alone, for a config with its own transform table, is '@monosmith/jest-preset/asset-transform':

transform: {
  '^(?!.*\\.(js|jsx|ts|tsx|css|json)$)': '@monosmith/jest-preset/asset-transform',
}

Peer dependencies

Nothing is installed for you beyond Jest itself. Add what your project uses:

| Package | When | |---|---| | jest | always | | ts-jest, typescript | the base preset (its transform and the resolver's fallback) | | jest-environment-jsdom | the default testEnvironment (jsdom) | | babel-jest, @babel/preset-env, @babel/preset-typescript, @babel/preset-react | the /react preset (use Babel 7 presets with babel-jest 30) | | @swc/jest | optional: when your transform table uses it, the asset transform hands scripts to it instead of babel-jest |

Layout

The package is plain CommonJS that Jest loads directly: nothing is compiled. The files at the package root are the doors Jest and consumers use (jest-preset.cjs is the name Jest looks for, react/jest-preset.cjs for the sub-preset); the code is three small slices under src/: preset (the values), resolver and asset-transform. The specs sit beside the code and the integration spec runs the real Jest binary over throwaway projects that use the preset.

License

MIT