Add ESLint (#48)

Author: artnc

.dockerignore

@@ -2,6 +2,7 @@
 !.clang-format
 !.editorconfig
 !entry.ts
+!eslint.config.js
 !ruff.toml
 !stylesheet.xml
 !svgo.config.js

Dockerfile

@@ -16,7 +16,7 @@ RUN tsc \
 # eclipse-temurin:21-alpine java --list-modules`, then removing modules by trial
 # and error until `make test` throws ClassNotFoundException. When first
 # implemented, this custom JRE reduced our image size from 574 MB to 469 MB
-FROM amazoncorretto:21.0.6-alpine3.21 as jre
+FROM amazoncorretto:21.0.6-alpine3.21 AS jre
 RUN apk add binutils && jlink \
   --add-modules java.se,jdk.compiler,jdk.unsupported \
   --compress zip-6 \
@@ -57,10 +57,27 @@ echo 'source /black21-venv/bin/activate && black "$@"' > /usr/bin/black21
 chmod +x /usr/bin/black21
 
 # Install Node dependencies
+#
+# We stay on eslint-plugin-unicorn 56.0.1 because 57+ removed support for
+# importing this plugin into the ESLint config as CommonJS. (We use CommonJS
+# instead of ESM in the ESLint config mainly because the latter requires a new
+# local package.json file that declares `"type": "module"`, which interferes
+# with other tools like SVGO). I couldn't get the `deasync` hack to work - it
+# just hung forever. TODO: Try updating this plugin after updating Node.js to
+# v22+, which has experimental support for synchronously require()-ing ESM?
+# https://github.com/sindresorhus/eslint-plugin-unicorn/releases/tag/v57.0.0
+# https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c
+# https://nodejs.org/en/blog/announcements/v22-release-announce#support-requireing-synchronous-esm-graphs
+# https://github.com/eslint/eslint/issues/13684#issuecomment-722949152
 npm install -g \
   @prettier/plugin-xml@3.4.1 \
-  prettier@3.3.3 \
-  svgo@3.3.2
+  eslint@9.23.0 \
+  eslint-plugin-jsdoc@50.6.9 \
+  eslint-plugin-sort-keys@2.3.5 \
+  eslint-plugin-unicorn@56.0.1 \
+  prettier@3.5.3 \
+  svgo@3.3.2 \
+  typescript-eslint@8.29.0
 
 # Install Scala dependencies
 wget https://github.com/coursier/coursier/releases/download/v2.1.17/coursier -O /bin/coursier
@@ -108,3 +125,4 @@ COPY --from=entry /entry.js /entry
 # https://github.com/coursier/coursier/issues/1955#issuecomment-956697764
 ENV COURSIER_CACHE=/tmp/coursier-cache
 ENV COURSIER_JVM_CACHE=/tmp/coursier-jvm-cache
+ENV NODE_PATH=/usr/local/lib/node_modules

README.md

@@ -1,8 +1,9 @@
 # pre-commit hooks
 
-This repo currently contains a single [pre-commit](https://pre-commit.com/) hook that internally runs several code formatters in parallel.
+This repo currently contains a single [pre-commit](https://pre-commit.com/) hook that internally runs several code formatters in parallel:
 
-- [Prettier](https://github.com/prettier/prettier) v3.3.3 for CSS, HTML, JS, JSX, Markdown, Sass, TypeScript, XML, YAML
+- [Prettier](https://github.com/prettier/prettier) v3.5.3 for CSS, HTML, JS, JSX, Markdown, Sass, TypeScript, XML, YAML
+- [ESLint](https://eslint.org/) v9.23.0 for JS, TypeScript
 - [Ruff](https://docs.astral.sh/ruff/) v0.7.3 for Python 3
 - [Black](https://github.com/psf/black) v21.12b0 for Python 2
 - [autoflake](https://github.com/myint/autoflake) v1.7.8 for Python <!-- TODO: Upgrade to v2+, restrict to Python 2, and reenable Ruff rule F401 once our Python 3 repos that were converted from Python 2 no longer use type hint comments: https://github.com/PyCQA/autoflake/issues/222#issuecomment-1419089254 -->
@@ -23,6 +24,8 @@ This repo currently contains a single [pre-commit](https://pre-commit.com/) hook
   - Replacing empty Python collections like `list()` with literal equivalents
   - Replacing empty Kotlin collections like `arrayOf()` with `empty` equivalents
 
+To minimize developer friction, we enable only rules whose violations can be fixed automatically and disable all rules whose violations require manual correction.
+
 We run this hook on developer workstations and enforce it in CI for all production repos at Duolingo.
 
 ## Usage

entry.ts

@@ -10,6 +10,9 @@ import { createInterface } from "readline";
  */
 const EMPTY_FILE = "/emptyfile";
 
+/** Minified JS files to exclude from formatting */
+const MINIFIED_JS_REGEX = /\bmin\b|\.(custom|pack)\./;
+
 /** CLI options to use in all Prettier invocations */
 const PRETTIER_OPTIONS = [
   "--arrow-parens",
@@ -27,6 +30,8 @@ const PRETTIER_OPTIONS = [
   "warn",
   "--no-config",
   "--no-editorconfig",
+  "--object-wrap",
+  "collapse",
   "--write",
 ];
 
@@ -75,6 +80,7 @@ const enum HookName {
   Autoflake = "autoflake",
   Black = "Black",
   ClangFormat = "ClangFormat",
+  EsLint = "ESLint",
   Gofmt = "gofmt",
   GoogleJavaFormat = "google-java-format",
   Isort = "isort",
@@ -184,6 +190,25 @@ const HOOKS: Record<HookName, LockableHook> = {
     include: /\.(cpp|proto$)/,
     runAfter: [HookName.Sed],
   }),
+  [HookName.EsLint]: createLockableHook({
+    action: async sources => {
+      try {
+        await run(
+          "eslint",
+          "--fix",
+          "--config",
+          "/eslint.config.js",
+          ...sources,
+        );
+      } catch {
+        // We swallow nonzero exit codes because we care only about autofixable
+        // errors (which should now be fixed), not about non-autofixable errors
+      }
+    },
+    exclude: MINIFIED_JS_REGEX,
+    include: /\.[jt]sx?$/,
+    runAfter: [HookName.Sed],
+  }),
   [HookName.Gofmt]: createLockableHook({
     action: sources => run("/gofmt", "-s", "-w", ...sources),
     include: /\.go$/,
@@ -238,9 +263,9 @@ const HOOKS: Record<HookName, LockableHook> = {
         "es5",
         ...sources,
       ),
-    exclude: /\b(compressed|custom|min|minified|pack|prod|production)\b/,
+    exclude: MINIFIED_JS_REGEX,
     include: /\.jsx?$/,
-    runAfter: [HookName.Sed],
+    runAfter: [HookName.Sed, HookName.EsLint],
   }),
   [HookName.PrettierNonJs]: createLockableHook({
     action: sources =>
@@ -253,7 +278,7 @@ const HOOKS: Record<HookName, LockableHook> = {
         ...sources,
       ),
     include: /\.(css|html?|markdown|md|scss|tsx?|xml|ya?ml)$/,
-    runAfter: [HookName.Sed, HookName.Xsltproc],
+    runAfter: [HookName.Sed, HookName.Xsltproc, HookName.EsLint],
   }),
   [HookName.Ruff]: createLockableHook({
     action: async (sources, args) => {