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  • Repo: eslint/eslint
  • Start Date: 2024-03-09
  • RFC PR: #117
  • Authors: Arya Emami

Add Support for TypeScript Config Files

Summary

Add support for TypeScript config files (eslint.config.ts, eslint.config.mts, eslint.config.cts)

Motivation

The primary motivation for adding support for TypeScript configuration files to ESLint is to enhance the developer experience and accommodate the evolving JavaScript ecosystem. As TypeScript's popularity continues to grow, more projects are adopting TypeScript not only for their source code but also for their configuration files. This shift is driven by TypeScript's ability to provide compile-time type checks and IntelliSense. By supporting eslint.config.ts, eslint.config.mts, and eslint.config.cts, ESLint will offer first-class support to TypeScript users, allowing them to leverage these benefits directly within their ESLint configuration.

Detailed Design

The goal is to seamlessly support TypeScript configuration files in ESLint. To achieve this, ESLint will need to recognize and parse TypeScript configuration files in the same way it does for JavaScript configuration files. This will involve creating the configuration file resolution logic to recognize .ts, .mts, and .cts files as valid ESLint configuration files. We will need to treat eslint.config.mts files as ECMAScript Modules (ESM) and eslint.config.cts files as CommonJS (CJS). The module resolution for eslint.config.ts files will depend on the type field in the nearest package.json. The maintainers of ESLint have raised some valid concerns some of which include:

  • There should not be extra overhead for JavaScript users. This means this change should not have a significant impact (if any at all) affecting users who use plain JavaScript config files.
  • The external tools that are used to parse the config files written in TypeScript should not create side effects. Specifically, it is imperative that these tools do not interfere with Node.js's native module resolution system by hooking into or altering the standard import/require mechanisms. This means tools like ts-node and tsx might not be suitable for this purpose.

So far the tool that seems to be the most suitable for this purpose is jiti. It does not introduce side effects and performs well, demonstrating its reliability. It also seems to be more battle-tested given some established frameworks such as Nuxt, Tailwind CSS and Docusaurus have been using it to load their configuration files.

  • Here is how we would use jiti to load TypeScript configuration files:

inside lib/eslint/eslint.js:

/**
 * Check if the file is a TypeScript file.
 * @param {string} filePath The file path to check.
 * @returns {boolean} `true` if the file is a TypeScript file, `false` if it's not.
 */
function isFileTS(filePath) {
    const fileExtension = path.extname(filePath);

    return fileExtension.endsWith("ts");
}

/**
 * Load the config array from the given filename.
 * @param {string} filePath The filename to load from.
 * @returns {Promise<any>} The config loaded from the config file.
 */
async function loadFlatConfigFile(filePath) {
    debug(`Loading config from ${filePath}`);

    const fileURL = pathToFileURL(filePath);

    debug(`Config file URL is ${fileURL}`);

    const mtime = (await fs.stat(filePath)).mtime.getTime();

    /*
     * Append a query with the config file's modification time (`mtime`) in order
     * to import the current version of the config file. Without the query, `import()` would
     * cache the config file module by the pathname only, and then always return
     * the same version (the one that was actual when the module was imported for the first time).
     *
     * This ensures that the config file module is loaded and executed again
     * if it has been changed since the last time it was imported.
     * If it hasn't been changed, `import()` will just return the cached version.
     *
     * Note that we should not overuse queries (e.g., by appending the current time
     * to always reload the config file module) as that could cause memory leaks
     * because entries are never removed from the import cache.
     */
    fileURL.searchParams.append("mtime", mtime);

    /*
     * With queries, we can bypass the import cache. However, when import-ing a CJS module,
     * Node.js uses the require infrastructure under the hood. That includes the require cache,
     * which caches the config file module by its file path (queries have no effect).
     * Therefore, we also need to clear the require cache before importing the config file module.
     * In order to get the same behavior with ESM and CJS config files, in particular - to reload
     * the config file only if it has been changed, we track file modification times and clear
     * the require cache only if the file has been changed.
     */
    if (importedConfigFileModificationTime.get(filePath) !== mtime) {
        delete require.cache[filePath];
    }

    const isTS = isFileTS(filePath);

    if (isTS) {
        const jiti = (await import("jiti")).default(__filename, { interopDefault: true });

        const config = jiti(fileURL.href);

        return config;
    }

    const config = (await import(fileURL.href)).default;

    importedConfigFileModificationTime.set(filePath, mtime);

    return config;
}

Examples

with eslint.config.mts file:

import eslint from "@eslint/js"
import type { Linter } from "eslint"

const config: Linter.FlatConfig[] = [
  eslint.configs.recommended,
  {
    rules: {
      "no-console": [0],
    },
  },
]

export default config

with eslint.config.cts file:

import type { Linter } from "eslint"
const eslint = require("@eslint/js")

const config: Linter.FlatConfig[] = [
  eslint.configs.recommended,
  {
    rules: {
      "no-console": [0],
    },
  },
]

module.exports = config

with eslint.config.ts file:

import eslint from "@eslint/js"
import type { Linter } from "eslint"

const config: Linter.FlatConfig[] = [
  eslint.configs.recommended,
  {
    rules: {
      "no-console": [0],
    },
  },
]

export default config

It is worth noting that you can already perform some type-checking with the checkJs TypeScript option and a JavaScript config file. Here's an example:

with eslint.config.mjs or (eslint.config.js + "type": "module" in the nearest package.json):

import eslint from "@eslint/js"

/** @type {import('eslint').Linter.FlatConfig[]} */
const config = [
  eslint.configs.recommended,
  {
    rules: {
      "no-console": [0],
    },
  },
]

export default config

with eslint.config.cjs or (eslint.config.js without "type": "module" in the nearest package.json):

const eslint = require("@eslint/js")

/** @type {import('eslint').Linter.FlatConfig[]} */
const config = [
  eslint.configs.recommended,
  {
    rules: {
      "no-console": [0],
    },
  },
]

module.exports = config

Documentation

The documentation for this feature will be added to the ESLint User Guide page. The documentation will explain how to use TypeScript configuration files and the differences between JavaScript and TypeScript configuration files.

Drawbacks

This change will most likely require at least one external tool as either a dependency or a peer dependency.

Backwards Compatibility Analysis

This goal is to minimize the disruption to existing users. The primary focus is to ensure that the existing JavaScript configuration files continue to work as expected. The changes will only affect TypeScript users who are using TypeScript configuration files. The changes will not affect the existing JavaScript configuration files.

Alternatives

While developing this feature, we considered the following alternatives:

  1. Using ts-node to parse TypeScript configuration files. This approach proved to be problematic because ts-node hooks into Node.js's native module resolution system, which could potentially cause side effects.

  2. Using tsx to parse TypeScript configuration files. This approach also proved to be problematic because tsx hooks into Node.js's native module resolution system, which could potentially cause side effects.

  3. Using TypeScript's transpileModule() to parse TypeScript configuration files. This approach proved to be problematic because it requires a significant amount of overhead and is not suitable for this purpose.

Open Questions

  1. How is caching going to work with TypeScript config files? We only cache the computed result of loading a config file, so I don't think this should be a problem.
  2. Should we look at the nearest tsconfig.json file to determine the module resolution for eslint.config.ts files? Most likely not, but it's worth considering.
  3. Should we allow some sort of interoperability between JavaScript and TypeScript configuration files? For example, should we allow a TypeScript configuration file to extend a JavaScript configuration file and vice versa? I don't believe this is an issue as the extends key isn't supported. Users just use import to load anything else they need.
  4. Should we allow eslint.config.ts to be able to use export default as well as module.exports (might be related to TypeScript's automatic Module Detection)?
  5. Tools like Vitest export a defineConfig function to make it easier to write configuration files in TypeScript. Should we consider doing something similar for ESLint?
  6. How does the feature interact with the CLI option --config for specifying a config file? It doesn't behave any differently, same as before. You can do eslint . --config=eslint.config.ts or eslint . -c eslint.config.ts and they just work. Same as with a eslint.config.js file.

Help Needed

I will be implementing this feature. I will need help from the team to review the code and provide feedback.

Frequently Asked Questions

Related Discussions

External References