Jest Preset Configuration for TypeScript Projects

You're tasked with creating a reusable Jest configuration preset for your TypeScript projects. This preset should handle common configurations like transforming TypeScript files, setting up a basic environment, and enabling specific Jest features. This is useful for ensuring consistency across multiple projects and reducing boilerplate in individual Jest configuration files.

Problem Description

Your goal is to create a Jest preset that can be easily extended or used as is in any TypeScript project. This preset should:

1. Handle TypeScript Transformation: Configure Jest to correctly process .ts and .tsx files.
2. Set up a Jest Environment: Define a suitable testing environment, typically node.
3. Enable Common Jest Features: Include configurations for common Jest practices, such as module resolution for imports.
4. Be Extensible: Allow users to override or extend the preset's configurations in their project-specific jest.config.js file.

Key Requirements:

• The preset should be a JavaScript or TypeScript file that exports an object conforming to Jest's preset configuration structure.
• It must include a transform property to handle TypeScript compilation (e.g., using ts-jest).
• It should specify a testEnvironment.
• It should define a moduleFileExtensions array to include .ts and .tsx.
• The preset should be importable and usable by specifying its path in the preset field of a project's jest.config.js.

Expected Behavior:

When a project imports and uses this preset, its Jest tests written in TypeScript should run without compilation errors, and Jest should correctly resolve modules imported within these tests.

Edge Cases to Consider:

• How will the preset handle projects that might already have a transform or moduleFileExtensions configuration? The preset should ideally merge or allow overrides.
• What happens if ts-jest is not installed? While the preset itself won't install dependencies, it should assume ts-jest is available and configured appropriately in its own transform property.

Examples

Example 1: Basic Usage

Input (within a project's jest.config.js):

// jest.config.js
module.exports = {
  preset: './path/to/your/jest-preset.js', // Assuming your preset is here
  // Other project-specific configurations can go here
  testMatch: ["**/__tests__/**/*.ts?(x)", "**/?(*.)+(spec|test).ts?(x)"]
};

Output (Conceptual):

Jest will load configurations from ./path/to/your/jest-preset.js, apply them, and then merge them with the project's jest.config.js. TypeScript files in the project will be transformed by ts-jest as configured in the preset.

Explanation:

The preset field tells Jest to load and use the configuration defined in the specified file. This preset provides the foundational TypeScript-related configurations.

Example 2: Extending the Preset

Input (within a project's jest.config.js):

// jest.config.js
const basePreset = require('./path/to/your/jest-preset.js');

module.exports = {
  ...basePreset, // Spread the base preset configurations
  moduleNameMapper: {
    '^@/(.*)$': '<rootDir>/src/$1', // Add project-specific module mappings
  },
  globals: {
    __DEV__: process.env.NODE_ENV !== 'production',
  },
};

Output (Conceptual):

Jest will start with all configurations from basePreset. Then, it will merge in the moduleNameMapper and globals properties defined in the project's jest.config.js, overriding or adding to the preset's configurations where necessary.

Explanation:

By spreading the base preset and then defining its own configurations, the project-specific file can extend and customize the preset's behavior without rewriting everything.

Constraints

• The preset must be a valid JavaScript object that Jest can parse.
• It must be designed to work with projects that use ts-jest for TypeScript transformation.
• The output of the preset should be a Jest configuration object.
• The preset should not include any project-specific paths or configurations that cannot be generalized (e.g., specific testMatch patterns that are common but not universal).

Notes

• Consider using ts-jest for transforming TypeScript files. You'll need to specify its path in the transform property of your preset.
• Think about common Jest configurations like testEnvironment (node is a good default for most Node.js/TypeScript projects), moduleFileExtensions (ensure .ts and .tsx are included), and potentially setupFiles or setupFilesAfterEnv if you have common setup logic.
• Jest's configuration merging allows presets to be extended. Your preset should be structured in a way that facilitates this. For example, returning an object that can be spread or merged.
• The preset itself does not need to install dependencies; it's assumed that the consuming project will have jest, ts-jest, and @types/jest installed.