diff --git a/.eslintrc.js b/.eslintrc.js
index 3ff503ba6..3c496dd5d 100644
--- a/.eslintrc.js
+++ b/.eslintrc.js
@@ -128,7 +128,9 @@ module.exports = {
     'no-unused-expressions': 'off', // This rule is replaced with the TSESlint version.
     '@typescript-eslint/no-unused-expressions': 'error', // Support TS stuff.
     '@typescript-eslint/no-throw-literal': 'error', // Recommended in v8 (w/rename to `only-throw-error`)
-    '@typescript-eslint/prefer-namespace-keyword': 'error', // Recommended in v8
+    '@typescript-eslint/prefer-find': 'error', // Recommended in v8
+    '@typescript-eslint/prefer-includes': 'error', // Recommended in v8
+    '@typescript-eslint/prefer-regexp-exec': 'error', // Recommended in v8
 
     // Unsafe
     '@typescript-eslint/no-unsafe-assignment': 'off', // Blocked on typescript-eslint/typescript-eslint#1682.
diff --git a/lib/anonymize.js b/lib/anonymize.js
index 03b599730..65fd2f4ef 100644
--- a/lib/anonymize.js
+++ b/lib/anonymize.js
@@ -1,5 +1,5 @@
 /**
- * This module aims to make the paths and versions used in the CLI generic
+ * @file This module aims to make the paths and versions used in the CLI generic,
  * so that the CLI tests (in the `test/` folder) have the same output on different
  * machines, and also the same output when only the CLI version changes.
  */
diff --git a/lib/app-wrapper.js b/lib/app-wrapper.js
index 2e089e08d..8835f667a 100644
--- a/lib/app-wrapper.js
+++ b/lib/app-wrapper.js
@@ -81,6 +81,7 @@ function initWithWorker(elmModulePath, flags) {
 
 /**
  * @param {string | symbol} port
+ * @returns {Listened<unknown>}
  */
 function send(port) {
   return (/** @type {unknown} */ data) => {
@@ -137,6 +138,7 @@ function initializeListeners() {
 /**
  * @param {string} elmModulePath
  * @param {Flags} flags
+ * @returns {ReviewApp}
  */
 function initWithoutWorker(elmModulePath, flags) {
   const elmModule = loadCompiledElmApp(elmModulePath);
@@ -146,6 +148,9 @@ function initWithoutWorker(elmModulePath, flags) {
   return app;
 }
 
+/**
+ * @returns {void}
+ */
 function stop() {
   if (worker) {
     worker.terminate();
diff --git a/lib/autofix.js b/lib/autofix.js
index 84b838255..f1b540231 100644
--- a/lib/autofix.js
+++ b/lib/autofix.js
@@ -27,6 +27,7 @@ const StyledMessage = require('./styled-message');
  * @param {Options} options
  * @param {ReviewApp} app
  * @param {VersionString} elmVersion
+ * @returns {void}
  */
 function subscribe(options, app, elmVersion) {
   AppState.subscribe(
diff --git a/lib/build.js b/lib/build.js
index cda241d9d..7d3b13cb4 100644
--- a/lib/build.js
+++ b/lib/build.js
@@ -1,10 +1,11 @@
 /**
+ * @import {CompileOptions, Sources} from '../vendor/types/node-elm-compiler';
  * @import {AppHash, BuildResult} from './types/build';
  * @import {ApplicationElmJson, ElmJson} from './types/content';
- * @import {VersionString} from './types/version';
+ * @import {ErrorMessageInfo} from './types/error-message';
  * @import {Options, Template} from './types/options';
  * @import {Path} from './types/path';
- * @import {CompileOptions, Sources} from '../vendor/types/node-elm-compiler';
+ * @import {VersionString} from './types/version';
  */
 const path = require('node:path');
 const crypto = require('node:crypto');
@@ -334,6 +335,7 @@ async function createTemplateProject(
  * @param {Options} options
  * @param {string} userSrc
  * @param {ApplicationElmJson} elmJson
+ * @returns {ApplicationElmJson}
  */
 function updateSourceDirectories(options, userSrc, elmJson) {
   let sourceDirectories = [
@@ -359,10 +361,10 @@ function updateSourceDirectories(options, userSrc, elmJson) {
 
 /**
  * @param {Options} options
- * @param {string} dest
- * @param {string} elmModulePath
+ * @param {Path} dest
+ * @param {Path} elmModulePath
  * @param {Sources} compileTargets
- * @param {string} elmBinary
+ * @param {Path} elmBinary
  * @param {boolean} isReviewApp
  * @returns {Promise<string | null>}
  */
@@ -455,6 +457,7 @@ async function compileElmProject(
 /**
  * @param {Options} options
  * @param {string} stderr
+ * @returns {ErrorMessageInfo}
  */
 function compilationError(options, stderr) {
   if (stderr.includes('DEBUG REMNANTS')) {
@@ -496,6 +499,7 @@ function compilationError(options, stderr) {
  * @param {Options} options
  * @param {Path} reviewElmJsonPath
  * @param {ElmJson} reviewElmJson
+ * @returns {void}
  */
 function validateElmReviewVersion(options, reviewElmJsonPath, reviewElmJson) {
   if (options.localElmReviewSrc) {
diff --git a/lib/debug.js b/lib/debug.js
index 275384965..d9a832d9c 100644
--- a/lib/debug.js
+++ b/lib/debug.js
@@ -4,6 +4,7 @@ const options = AppState.getOptions();
 
 /**
  * @param {string} message
+ * @returns {void}
  */
 function log(message) {
   if (options.debug) {
diff --git a/lib/elm-files.js b/lib/elm-files.js
index c1948cbf3..d39112613 100644
--- a/lib/elm-files.js
+++ b/lib/elm-files.js
@@ -1,5 +1,5 @@
 /**
- * @import {ElmFile, Source, Readme, ElmJson, ElmJsonData, SourceDirectories, Ast} from './types/content';
+ * @import {Ast, ElmFile, ElmJson, ProjectFiles, Readme, Source, SourceDirectories} from './types/content';
  * @import {ReviewOptions} from './types/options';
  * @import {Path} from './types/path';
  */
@@ -19,14 +19,6 @@ const AppState = require('./state');
 
 const defaultGlob = '**/*.elm$';
 
-/**
- * @typedef {object} ProjectFiles
- * @property {ElmJsonData} elmJsonData
- * @property {Readme | null} readme
- * @property {ElmFile[]} elmFiles
- * @property {Path[]} sourceDirectories
- */
-
 /**
  * Get all files from the project.
  *
diff --git a/lib/error-message.js b/lib/error-message.js
index 77c8378b5..27b6efa85 100644
--- a/lib/error-message.js
+++ b/lib/error-message.js
@@ -97,7 +97,7 @@ function stacktrace(debug, error) {
  * @param {boolean} debug
  * @param {CustomError} error
  * @param {Path} [defaultPath]
- * @returns {unknown}
+ * @returns {{type: string; title: string; path: string | undefined; message: string[]; stack: string | undefined}}
  */
 function formatJson(debug, error, defaultPath) {
   return {
diff --git a/lib/fs-wrapper.js b/lib/fs-wrapper.js
index 3a8332a26..e2444b8c1 100644
--- a/lib/fs-wrapper.js
+++ b/lib/fs-wrapper.js
@@ -9,7 +9,7 @@ const fsp = fs.promises;
  * Read a JSON file.
  *
  * @param {Path} file
- * @param {Reviver | undefined} [reviver=undefined]
+ * @param {Reviver} [reviver]
  * @returns {Promise<unknown>}
  */
 async function readJsonFile(file, reviver) {
@@ -19,10 +19,9 @@ async function readJsonFile(file, reviver) {
 }
 
 /**
- *
  * @param {string} data
  * @param {Path} file
- * @param {Reviver | undefined} reviver
+ * @param {Reviver} [reviver]
  * @returns {unknown}
  */
 function parseJsonFromFile(data, file, reviver) {
@@ -46,8 +45,8 @@ function readJsonFileSync(file) {
 /**
  * Read a file using Promises.
  *
- * @param {string} file
- * @param {fs.BaseEncodingOptions & { flag?: fs.OpenMode | undefined }} [options={encoding: 'utf8'}]
+ * @param {Path} file
+ * @param {fs.BaseEncodingOptions & { flag?: fs.OpenMode | undefined }} [options]
  * @returns {Promise<string>}
  */
 async function readFile(file, options) {
@@ -58,10 +57,10 @@ async function readFile(file, options) {
 /**
  * Write a JSON file.
  *
- * @param {string} file
+ * @param {Path} file
  * @param {unknown} content
- * @param {string | number | undefined} [space=undefined]
- * @param {Replacer | undefined} [replacer=undefined]
+ * @param {string | number | undefined} [space]
+ * @param {Replacer} [replacer]
  * @returns {Promise<void>}
  */
 async function writeJson(file, content, space, replacer) {
@@ -85,7 +84,8 @@ function mkdirpSync(dir) {
 }
 
 /**
- * @param {fs.PathLike} file
+ * @param {Path} file
+ * @returns {Promise<void>}
  */
 async function remove(file) {
   await fsp.rm(
diff --git a/lib/help.js b/lib/help.js
index f46d83bf3..7749c0691 100644
--- a/lib/help.js
+++ b/lib/help.js
@@ -56,6 +56,9 @@ ${Flags.buildFlags('fix', null)}
   );
 }
 
+/**
+ * @returns {void}
+ */
 function suppress() {
   const orange = chalk.keyword('orange');
 
@@ -97,6 +100,9 @@ ${Flags.buildFlags('suppress', 'suppress')}
   );
 }
 
+/**
+ * @returns {void}
+ */
 function init() {
   console.log(
     // prettier-ignore
@@ -117,6 +123,9 @@ ${Flags.buildFlags('init', 'init')}
   );
 }
 
+/**
+ * @returns {void}
+ */
 function newPackage() {
   console.log(
     // prettier-ignore
@@ -138,6 +147,9 @@ ${Flags.buildFlags('new-package', 'new-package')}
   );
 }
 
+/**
+ * @returns {void}
+ */
 function newRule() {
   console.log(`The new-rule command adds an empty rule to your review configuration or
 review package.
@@ -161,6 +173,9 @@ ${Flags.buildFlags('new-rule', 'new-rule')}
 `);
 }
 
+/**
+ * @returns {void}
+ */
 function prepareOffline() {
   console.log(`The prepare-offline command allows the tool to run in offline mode using
 the ${chalk.cyan('--offline')} flag.
diff --git a/lib/init.js b/lib/init.js
index 2d49f2ba3..f173d1f6c 100644
--- a/lib/init.js
+++ b/lib/init.js
@@ -52,6 +52,7 @@ async function promptAndCreate(options) {
  * @param {Options} options
  * @param {Template} template
  * @param {string} directory
+ * @returns {Promise<void>}
  */
 async function createFromTemplate(options, template, directory) {
   const configDirectory = path.join(directory, 'src');
@@ -91,6 +92,7 @@ async function createFromTemplate(options, template, directory) {
 /**
  * @param {Options} options
  * @param {Path} directory
+ * @returns {void}
  */
 function logInit(options, directory) {
   const message = options.template
@@ -118,6 +120,7 @@ ${options.template ? templateRecommendation : ''}`
 /**
  * @param {Options} options
  * @param {Path} directory
+ * @returns {string}
  */
 function regularInitMessage(options, directory) {
   return `You can now define your review configuration by editing ${chalk.green(
@@ -130,6 +133,7 @@ const orange = chalk.keyword('orange');
 /**
  * @param {Options} options
  * @param {Path} directory
+ * @returns {string}
  */
 function templateInitMessage(options, directory) {
   return `You chose to use someone's review configuration which can be great to get started
@@ -155,6 +159,7 @@ I recommend you use a mix of the following approaches:
  * @param {Options} options
  * @param {Path} directory
  * @param {string} template
+ * @returns {Promise<void>}
  */
 async function create(options, directory, template) {
   const configDirectory = path.join(directory, 'src');
@@ -170,6 +175,7 @@ async function create(options, directory, template) {
 /**
  * @param {Options} options
  * @param {Path} directory
+ * @returns {Promise<void>}
  */
 async function createElmJson(options, directory) {
   const elmBinary = await getElmBinary(options);
@@ -188,6 +194,7 @@ async function createElmJson(options, directory) {
 /**
  * @param {Path} directory
  * @param {string} template
+ * @returns {void}
  */
 function createReviewConfig(directory, template) {
   fsExtra.copyFileSync(
diff --git a/lib/main.js b/lib/main.js
index 9df6afef5..6a82f7f3d 100644
--- a/lib/main.js
+++ b/lib/main.js
@@ -67,6 +67,9 @@ function errorHandler(err) {
   exit(1);
 }
 
+/**
+ * @returns {Promise<void>}
+ */
 async function runElmReview() {
   const {elmModulePath, reviewElmJson, appHash} = await Builder.build(options);
 
@@ -142,6 +145,9 @@ async function runElmReviewInWatchMode() {
   }
 }
 
+/**
+ * @returns {Promise<void>}
+ */
 async function prepareOffline() {
   const elmBinary = await ElmBinary.getElmBinary(options);
   ElmBinary.downloadDependenciesOfElmJson(
diff --git a/lib/min-version.js b/lib/min-version.js
index c134f3137..974b3907e 100644
--- a/lib/min-version.js
+++ b/lib/min-version.js
@@ -50,7 +50,7 @@ function updateToAtLeastMinimalVersion(version) {
  * @param {Options} options
  * @param {Path} elmJsonPath - path to an elm.json file
  * @param {VersionString} version - version string, e.g. "1.0"
- * @returns void
+ * @returns {void}
  */
 function validate(options, elmJsonPath, version) {
   const [major, minor] = version.split('.');
diff --git a/lib/module-cache.js b/lib/module-cache.js
index 57a71f4d5..21ee2d739 100644
--- a/lib/module-cache.js
+++ b/lib/module-cache.js
@@ -11,6 +11,7 @@ const Cache = require('./cache');
 /**
  * @param {Options} options
  * @param {ReviewApp} app
+ * @returns {void}
  */
 function subscribe(options, app) {
   AppState.subscribe(app.ports.cacheFile, async ({source, ast}) =>
@@ -25,6 +26,7 @@ function subscribe(options, app) {
  * @param {Options} options
  * @param {string} fileHash
  * @param {Ast} ast
+ * @returns {Promise<void>}
  */
 async function cacheFile(options, fileHash, ast) {
   await Cache.cacheJsonFile(
diff --git a/lib/new-package.js b/lib/new-package.js
index 66fdab604..8fef31728 100644
--- a/lib/new-package.js
+++ b/lib/new-package.js
@@ -1,5 +1,6 @@
 /**
- * @import {ApplicationElmJson} from './types/content';
+ * @import {PackageJSON} from '@package-json/types' with {"resolution-mode": "import"};
+ * @import {ApplicationElmJson, PackageElmJson} from './types/content';
  * @import {PackageTitle} from './types/new-package';
  * @import {Options, RuleType} from './types/options';
  * @import {Path} from './types/path';
@@ -19,6 +20,7 @@ const Spinner = require('./spinner');
 
 /**
  * @param {Options} options
+ * @returns {Promise<void>}
  */
 async function create(options) {
   const onCancelOptions = /** @satisfies {prompts.Options} */ ({
@@ -132,6 +134,9 @@ function validatePackageName(packageName) {
   }
 }
 
+/**
+ * @returns {Promise<string | null>}
+ */
 async function askForPackageName() {
   let canceled = false;
 
@@ -162,6 +167,9 @@ async function askForPackageName() {
   return await askForPackageName();
 }
 
+/**
+ * An up-to-date `elm-tooling.json` file.
+ */
 const elmToolingJson = {
   tools: {
     elm: '0.19.1',
@@ -187,6 +195,7 @@ async function writeFile(dir, fileName, content) {
  * @param {string} ruleName
  * @param {RuleType} ruleType
  * @param {string} license
+ * @returns {Promise<void>}
  */
 async function createProject(
   options,
@@ -363,6 +372,7 @@ ElmjutsuDumMyM0DuL3.elm
  * @param {PackageTitle} packageName
  * @param {string} ruleName
  * @param {string} license
+ * @returns {PackageElmJson}
  */
 function elmJson(authorName, packageName, ruleName, license) {
   return {
@@ -388,6 +398,7 @@ function elmJson(authorName, packageName, ruleName, license) {
  * @param {string} authorName
  * @param {PackageTitle} packageName
  * @param {string} ruleName
+ * @returns {string}
  */
 function readme(authorName, packageName, ruleName) {
   return `# ${packageName}
@@ -425,6 +436,7 @@ elm-review --template ${authorName}/${packageName}/example
 /**
  * @param {Options} options
  * @param {PackageTitle} packageName
+ * @returns {PackageJSON}
  */
 function packageJson(options, packageName) {
   return {
diff --git a/lib/new-rule.js b/lib/new-rule.js
index 8afcfdd52..4a0be5d4c 100644
--- a/lib/new-rule.js
+++ b/lib/new-rule.js
@@ -15,6 +15,7 @@ const packageNameRegex = /.+\/(elm-)?review-.+/;
 
 /**
  * @param {ReviewOptions} options
+ * @returns {Promise<void>}
  */
 async function create(options) {
   const elmJson = readElmJson(options);
@@ -81,6 +82,9 @@ function validateRuleName(ruleName) {
   return ruleName;
 }
 
+/**
+ * @returns {Promise<string | null>}
+ */
 async function askForRuleName() {
   let canceled = false;
   /** @type {{ruleName: string}} */
@@ -157,17 +161,18 @@ async function askForRuleType() {
  * @param {string} dir
  * @param {string} fileName
  * @param {string} content
+ * @returns {void}
  */
 function writeFile(dir, fileName, content) {
   fs.writeFileSync(path.join(dir, fileName), content);
 }
 
 /**
- *
  * @param {ReviewOptions} options
  * @param {PackageElmJson} elmJson
  * @param {string} ruleName
  * @param {RuleType} ruleType
+ * @returns {Promise<void>}
  */
 async function addRule(options, elmJson, ruleName, ruleType) {
   const ruleNameFolder = ruleName.split('.').slice(0, -1).join('/');
@@ -248,6 +253,7 @@ async function addRule(options, elmJson, ruleName, ruleType) {
  * @param {string} dir
  * @param {PackageElmJson} elmJson
  * @param {string} ruleName
+ * @returns {void}
  */
 function injectRuleInPreview(dir, elmJson, ruleName) {
   try {
@@ -272,6 +278,7 @@ function injectRuleInPreview(dir, elmJson, ruleName) {
  * @param {PackageElmJson} elmJson
  * @param {string} ruleName
  * @param {string} content
+ * @returns {void}
  */
 function injectRuleInReadme(dir, elmJson, ruleName, content) {
   const lines = content.split('\n');
@@ -283,10 +290,13 @@ function injectRuleInReadme(dir, elmJson, ruleName, content) {
 const ruleSectionRegex = /^#+.*rules/i;
 
 /**
+ * @remarks
+ * Mutates `lines`!
+ *
  * @param {PackageElmJson} elmJson
  * @param {string} ruleName
  * @param {string[]} lines
- * @description - Mutates `lines`!
+ * @returns {void}
  */
 function insertRuleDescription(elmJson, ruleName, lines) {
   const rulesSectionIndex = lines.findIndex((line) =>
@@ -315,6 +325,7 @@ function insertRuleDescription(elmJson, ruleName, lines) {
  * @param {string} ruleName
  * @param {number} rulesSectionIndex
  * @param {string[]} lines
+ * @returns {boolean}
  */
 function alreadyHasRuleDescription(ruleName, rulesSectionIndex, lines) {
   const nextSectionIndex = findNextSectionIndex(rulesSectionIndex, lines);
@@ -329,6 +340,7 @@ function alreadyHasRuleDescription(ruleName, rulesSectionIndex, lines) {
 /**
  * @param {number} previousSectionIndex
  * @param {string[]} lines
+ * @returns {number}
  */
 function findNextSectionIndex(previousSectionIndex, lines) {
   const nextSectionIndex = lines
@@ -343,12 +355,14 @@ function findNextSectionIndex(previousSectionIndex, lines) {
 }
 
 /**
- * @remarks Mutates `lines`!
+ * @remarks
+ * Mutates `lines`!
  *
  * @param {string} configurationPath
  * @param {PackageElmJson} elmJson
  * @param {string} ruleName
  * @param {string[]} lines
+ * @returns {void}
  */
 function insertRuleInConfiguration(
   configurationPath,
@@ -370,6 +384,7 @@ function insertRuleInConfiguration(
  * @param {string} configurationPath
  * @param {string} ruleName
  * @param {string[]} lines
+ * @returns {void}
  */
 function insertImport(configurationPath, ruleName, lines) {
   const firstImportIndex = lines.findIndex((line) => line.startsWith('import'));
@@ -402,6 +417,7 @@ function insertImport(configurationPath, ruleName, lines) {
  * @param {string} ruleName
  * @param {string} someOtherRule
  * @param {string[]} lines
+ * @returns {void}
  */
 function insertRuleInConfigList(ruleName, someOtherRule, lines) {
   const indexOfOtherRuleExample = lines.findIndex((line) =>
@@ -429,6 +445,7 @@ function insertRuleInConfigList(ruleName, someOtherRule, lines) {
  * @param {string} fullPackageName
  * @param {string} ruleName
  * @param {RuleType} ruleType
+ * @returns {string}
  */
 function newSourceFile(fullPackageName, ruleName, ruleType) {
   return `module ${ruleName} exposing (rule)
@@ -520,8 +537,8 @@ expressionVisitor node context =
 /**
  * @param {string} ruleName
  * @returns {string}
- * @todo Create a branded `ElmSource` type.
  */
+// TODO(@lishaduck): Create a branded `ElmSource` type.
 function projectRuleTemplate(ruleName) {
   return `rule : Rule
 rule =
diff --git a/lib/npx.js b/lib/npx.js
index 9956e933e..ea6b476bf 100644
--- a/lib/npx.js
+++ b/lib/npx.js
@@ -1,5 +1,9 @@
 /**
- * A fast `npx` alternative.
+ * @file A fast `npx` alternative.
+ */
+
+/**
+ * @import {Path} from './types/path';
  */
 
 const path = require('node:path');
@@ -56,6 +60,8 @@ const pathKey = getPathKey();
  *   That’s an odd use case, but was supported due to the `npx` approach.
  *
  * This can be removed in a major version.
+ *
+ * @returns {Path}
  */
 function backwardsCompatiblePath() {
   return [
diff --git a/lib/options.js b/lib/options.js
index e94aa38bf..542514766 100644
--- a/lib/options.js
+++ b/lib/options.js
@@ -101,6 +101,9 @@ function compute(processArgv) {
     }
   }
 
+  /**
+   * @returns {Path}
+   */
   function projectToReview() {
     if (!elmJsonPath) {
       throw new ErrorMessage.CustomError(
@@ -142,6 +145,9 @@ try re-running it with ${chalk.cyan('--elmjson <path-to-elm.json>')}.`,
 
   const namespace = args.namespace || 'cli';
 
+  /**
+   * @returns {Path}
+   */
   function elmStuffFolder() {
     return path.join(
       projectToReview(),
@@ -154,12 +160,16 @@ try re-running it with ${chalk.cyan('--elmjson <path-to-elm.json>')}.`,
     );
   }
 
+  /**
+   * @returns {Path}
+   */
   function userSrc() {
     return args.config
       ? path.resolve(process.cwd(), args.config)
       : path.join(projectToReview(), 'review');
   }
 
+  /** @returns {Path} */
   function suppressedErrorsFolder() {
     return path.join(userSrc(), 'suppressed');
   }
@@ -518,7 +528,11 @@ function suggestions(flagName) {
       distance: levenshtein.distance(flagName, f.name)
     }))
     .sort(
-      /** @returns {-1 | 0 | 1} */
+      /**
+       * @param {Flag & {distance: number}} a
+       * @param {Flag & {distance: number}} b
+       * @returns {-1 | 0 | 1}
+       */
       (a, b) => {
         if (a.distance < b.distance) {
           return -1;
@@ -542,7 +556,10 @@ function suggestions(flagName) {
  */
 function checkForMissingArgs(subcommand, args) {
   for (const flag of Flags.flags.filter(
-    /** @returns {flag is Flag & Multi} */
+    /**
+     * @param {Flag} flag
+     * @returns {flag is Flag & Multi}
+     */
     (flag) => flag.boolean === false
   )) {
     if (typeof args[flag.name] === 'boolean') {
diff --git a/lib/parse-elm.js b/lib/parse-elm.js
index 1cc71ccb9..f21514855 100644
--- a/lib/parse-elm.js
+++ b/lib/parse-elm.js
@@ -1,24 +1,18 @@
 /**
+ * @file Parse Elm files in parallel using worker threads.
+ *
  * This code was very much inspired by
  * <https://blog.logrocket.com/complete-guide-threads-node-js/>.
  */
 
 /**
- * @import {Source, ElmFile, Ast} from './types/content';
- * @import {ParseJob} from './types/parse-elm';
+ * @import {Ast, ElmFile, Source} from './types/content';
+ * @import {CustomWorker, ParseJob} from './types/parse-elm';
  * @import {Path} from './types/path';
  */
 const os = require('node:os');
 const path = require('node:path');
 
-/**
- * A worker and the information whether it is currently busy.
- *
- * @typedef {object} CustomWorker
- * @property {Worker} worker
- * @property {boolean} busy
- */
-
 // We want to have as many worker threads as CPUs on the user's machine.
 // Since the main thread is already one, we spawn (numberOfCpus - 1) workers.
 const numberOfThreads = Math.max(1, os.cpus().length - 1);
@@ -49,6 +43,9 @@ function prepareWorkers(Worker) {
   };
 }
 
+/**
+ * @returns {void}
+ */
 function terminateWorkers() {
   for (const {worker} of workers) worker.terminate();
 }
@@ -109,25 +106,34 @@ function findInactiveWorker() {
 function runWorker(availableWorker, queueItem) {
   availableWorker.busy = true;
 
-  /** @param {ElmFile} result */
+  /**
+   * @param {ElmFile} result
+   * @returns {void}
+   */
   const messageCallback = (result) => {
     queueItem.callback(undefined, result);
     cleanUp();
   };
 
-  /** @param {Error} error */
+  /**
+   * @param {Error} error
+   * @returns {void}
+   */
   function errorCallback(error) {
     queueItem.callback(error);
     cleanUp();
   }
 
+  /**
+   * @returns {void}
+   */
   function cleanUp() {
     availableWorker.worker.removeAllListeners('message');
     availableWorker.worker.removeAllListeners('error');
     availableWorker.busy = false;
     const nextJob = queue.pop();
     if (nextJob === undefined) {
-      return null;
+      return;
     }
 
     runWorker(availableWorker, nextJob);
diff --git a/lib/project-dependencies.js b/lib/project-dependencies.js
index f1b0917fe..c6d69201a 100644
--- a/lib/project-dependencies.js
+++ b/lib/project-dependencies.js
@@ -1,5 +1,5 @@
 /**
- * @import {ElmJson} from './types/content';
+ * @import {Dependencies, ElmJson, PackageElmJson, PackageName} from './types/content';
  * @import {Options} from './types/options';
  * @import {VersionString} from './types/version';
  */
@@ -10,6 +10,7 @@ const ProjectJsonFiles = require('./project-json-files');
  * @param {Options} options
  * @param {ElmJson} elmJson
  * @param {VersionString} elmVersion
+ * @returns {Promise<Dependencies>}
  */
 async function collect(options, elmJson, elmVersion) {
   const dependenciesEntries =
@@ -29,7 +30,8 @@ async function collect(options, elmJson, elmVersion) {
   let hasDependenciesThatCouldNotBeDownloaded = false;
 
   const projectDepsPromises = Object.entries(dependenciesEntries).map(
-    async ([name, constraint]) => {
+    async ([pkgname, constraint]) => {
+      const name = /** @type {PackageName} */ (pkgname);
       const packageVersion = /** @type {VersionString} */ (
         constraint.split(' ')[0]
       );
@@ -80,8 +82,9 @@ I will try to review the project anyway, but you might get unexpected results…
 }
 
 /**
- * @param {string} name
+ * @param {PackageName} name
  * @param {VersionString} packageVersion
+ * @returns {PackageElmJson}
  */
 function defaultElmJson(name, packageVersion) {
   return {
diff --git a/lib/project-json-files.js b/lib/project-json-files.js
index a53036a3e..d8f1dd5e2 100644
--- a/lib/project-json-files.js
+++ b/lib/project-json-files.js
@@ -2,7 +2,7 @@
  * @import {Path} from './types/path';
  * @import {VersionString} from './types/version';
  * @import {Options} from './types/options';
- * @import {PackageElmJson} from './types/content';
+ * @import {PackageElmJson, PackageName} from './types/content';
  */
 const os = require('node:os');
 const path = require('node:path');
@@ -17,12 +17,12 @@ const elmRoot =
   );
 
 /**
- * Get the elm.json file for a dependency.
+ * Get the `elm.json` file for a dependency.
  *
  * @param {Options} options
  * @param {VersionString} elmVersion
- * @param {string} name
- * @param {string} packageVersion
+ * @param {PackageName} name
+ * @param {VersionString} packageVersion
  * @returns {Promise<PackageElmJson>}
  */
 async function getElmJson(options, elmVersion, name, packageVersion) {
@@ -117,7 +117,7 @@ function getPackagePathInElmHome(elmVersion, name) {
  *
  * @param {Options} options
  * @param {VersionString} elmVersion
- * @param {string} name
+ * @param {PackageName} name
  * @param {VersionString} packageVersion
  * @returns {Promise<unknown>}
  */
diff --git a/lib/promisify-port.js b/lib/promisify-port.js
index b7d6bb97e..56acb7681 100644
--- a/lib/promisify-port.js
+++ b/lib/promisify-port.js
@@ -1,5 +1,5 @@
 /**
- * @import { PortsToPromise } from './types/promisify-port';
+ * @import {PortsToPromise} from './types/promisify-port';
  */
 
 module.exports = promisifyPort;
@@ -16,7 +16,7 @@ function promisifyPort({subscribeTo, sendThrough, data}) {
   return new Promise((resolve) => {
     /**
      * @param {DataOut} result
-     * @returns void
+     * @returns {void}
      */
     const handler = (result) => {
       subscribeTo.unsubscribe(handler);
diff --git a/lib/remote-template.js b/lib/remote-template.js
index 040d7c28f..b5e760c51 100644
--- a/lib/remote-template.js
+++ b/lib/remote-template.js
@@ -1,5 +1,6 @@
 /**
  * @import {OptionsOfJSONResponseBody} from 'got';
+ * @import {ErrorMessageInfo} from './types/error-message';
  * @import {Options, Template} from './types/options';
  * @import {ApplicationElmJson} from './types/content';
  * @import {Path} from './types/path';
@@ -168,6 +169,7 @@ ${error.message}`
   }
 }
 
+/** @type {ErrorMessageInfo} */
 const rateLimitErrorMessage = {
   title: 'GITHUB RATE LIMIT EXCEEDED',
   message: `It looks like you exceeded the GitHub rate limit by using "--template" too many
@@ -185,6 +187,7 @@ elm-review in your project:
 
 /**
  * @param {string} repoName
+ * @returns {ErrorMessageInfo}
  */
 function repoNotFoundErrorMessage(repoName) {
   return {
@@ -198,8 +201,9 @@ with private ones at the moment.`
 }
 
 /**
- * @param {unknown} repoName
- * @param {unknown} reference
+ * @param {string} repoName
+ * @param {string} reference
+ * @returns {ErrorMessageInfo}
  */
 function commitNotFoundErrorMessage(repoName, reference) {
   return {
@@ -220,6 +224,7 @@ Check the spelling and make sure it has been pushed.`
  * @param {string} commit
  * @param {string} basePath
  * @param {ApplicationElmJson} reviewElmJson
+ * @returns {Promise<void>}
  */
 async function downloadSourceDirectories(
   options,
@@ -228,7 +233,7 @@ async function downloadSourceDirectories(
   basePath,
   reviewElmJson
 ) {
-  return await Promise.all(
+  await Promise.all(
     reviewElmJson['source-directories'].map(async (sourceDirectory) => {
       await downloadDirectory(
         options,
@@ -247,6 +252,7 @@ async function downloadSourceDirectories(
  * @param {string} commit
  * @param {string} basePath
  * @param {string} directory
+ * @returns {Promise<void>}
  */
 async function downloadDirectory(
   options,
@@ -300,7 +306,7 @@ async function downloadDirectory(
  * Returns a promise that resolves to null if the download was successful, or an error if it failed.
  *
  * @param {string} url
- * @param {string} dest
+ * @param {Path} dest
  * @returns {Promise<NodeJS.ErrnoException | null | undefined>}
  */
 async function downloadFile(url, dest) {
@@ -315,16 +321,12 @@ async function downloadFile(url, dest) {
   });
 }
 
-/**
- * @typedef {{message: string}} JsonResponse
- */
-
 /**
  * Make a request to the GitHub API.
  *
  * @param {Options} options
  * @param {string} url
- * @param {((arg: JsonResponse) => {title: string, message: string})} [handleNotFound]
+ * @param {((errorBody: {message: string}) => ErrorMessageInfo)} [handleNotFound]
  * @returns {Promise<unknown>}
  */
 async function makeGitHubApiRequest(options, url, handleNotFound) {
diff --git a/lib/report.js b/lib/report.js
index cea73f9de..3cc0aad2d 100644
--- a/lib/report.js
+++ b/lib/report.js
@@ -1,14 +1,14 @@
 /**
- * @import { Options } from './types/options';
- * @import { StyledMessage as StyledMsg } from './types/styled-message';
- * @import { SuppressedErrorsFile } from './types/suppressed';
+ * @import {Options} from './types/options';
+ * @import {Report} from './types/report';
  */
 const Benchmark = require('./benchmark');
 const StyledMessage = require('./styled-message');
 
 /**
  * @param {Options} options
- * @param {{ errors?: { errors: unknown[]; path: unknown; }[] & StyledMsg; extracts?: unknown; suppressedErrors?: SuppressedErrorsFile[] }} result
+ * @param {Report} result
+ * @returns {Promise<void>}
  */
 async function report(options, result) {
   Benchmark.start(options, 'Writing error report');
@@ -25,27 +25,24 @@ async function report(options, result) {
 
 /**
  * @param {Options} options
- * @param {{ errors?: { errors: unknown[]; path: unknown; }[]; extracts?: unknown; }} json
+ * @param {Report} json
+ * @returns {Promise<void>}
  */
 async function print(options, json) {
   if (options.reportOnOneLine) {
     if ((json.errors?.length ?? 0) > 0) {
       await safeConsoleLog(
         json.errors
-          ?.map(
-            (
-              /** @type {{ errors: unknown[]; path: unknown; }} */ errorForFile
-            ) => {
-              return errorForFile.errors
-                .map((error) => {
-                  return JSON.stringify({
-                    path: errorForFile.path,
-                    .../** @type {object} */ (error)
-                  });
-                })
-                .join('\n');
-            }
-          )
+          ?.map((errorForFile) => {
+            return errorForFile.errors
+              .map((error) => {
+                return JSON.stringify({
+                  path: errorForFile.path,
+                  ...error
+                });
+              })
+              .join('\n');
+          })
           .join('\n') ?? ''
       );
     }
@@ -63,15 +60,16 @@ async function print(options, json) {
 /**
  * Prints a message in a way that will not be cut off.
  *
+ * @remarks
  * Printing large outputs to stdout is not recommended because at times
  * console.log is asynchronous and returns before ensuring that the whole
  * output has been printed. Check out these links for more details:
  *
- * - https://nodejs.org/api/process.html#process_process_exit_code
- * - https://github.com/nodejs/node/issues/6456
- * - https://github.com/nodejs/node/issues/19218
+ * - <https://nodejs.org/api/process.html#process_process_exit_code>
+ * - nodejs/node#6456
+ * - nodejs/node#19218
  *
- * Using process.stdout.write and passing a function ensures that
+ * Using {@linkcode process.stdout.write} and passing a function ensures that
  * the whole output has been written.
  *
  * @param {string} message - Message to print.
diff --git a/lib/result-cache-json.js b/lib/result-cache-json.js
index 01ff6cafd..c15d8d15d 100644
--- a/lib/result-cache-json.js
+++ b/lib/result-cache-json.js
@@ -1,7 +1,7 @@
 // @ts-expect-error(TS6133): There's a bug in noUnusedLocals (microsoft/TypeScript#58368).
 /** @import {Replacer, Reviver} from './types/json'; */
 // @ts-expect-error(TS6133): There's a bug in noUnusedLocals (microsoft/TypeScript#58368).
-/** @import {Value, ValueStruct} from './types/elm-internals'; */
+/** @import {ElmArray, Value, ValueStruct} from './types/elm-internals'; */
 
 /** @type {Replacer} */
 function replacer(_key, value_) {
@@ -57,6 +57,7 @@ function replacer(_key, value_) {
 
 /**
  * @param {ValueStruct} xs
+ * @returns {ElmArray[] | null}
  */
 function prudentListToArray(xs) {
   const out = [];
@@ -113,7 +114,8 @@ function reviver(_, value_) {
 const _ListNilPROD = {$: 0};
 
 /**
- * @param {string | string[]} array
+ * @param {ElmArray} array
+ * @returns {ValueStruct}
  */
 function _ListFromArrayPROD(array) {
   let out = _ListNilPROD;
diff --git a/lib/result-cache-worker.js b/lib/result-cache-worker.js
index c560d7c11..5c87a11ef 100644
--- a/lib/result-cache-worker.js
+++ b/lib/result-cache-worker.js
@@ -12,6 +12,7 @@ if (parentPort) {
 
 /**
  * @param {MessagePort} parentPort
+ * @returns {void}
  */
 function subscribe(parentPort) {
   parentPort.on('message', ({filePath, cacheEntry, cacheKey}) => {
diff --git a/lib/result-cache.js b/lib/result-cache.js
index c602f5c4a..f53e132fe 100644
--- a/lib/result-cache.js
+++ b/lib/result-cache.js
@@ -124,7 +124,6 @@ async function load(options, ignoredDirs, ignoredFiles, cacheFolder) {
 }
 
 /**
- *
  * @param {RuleName} ruleName
  * @param {RuleId} ruleId
  * @returns {CacheKey}
diff --git a/lib/review-dependencies.js b/lib/review-dependencies.js
index 3f05a1f2e..68a4dca53 100644
--- a/lib/review-dependencies.js
+++ b/lib/review-dependencies.js
@@ -8,6 +8,7 @@ const ProjectJsonFiles = require('./project-json-files');
 /**
  * @param {ApplicationDependencyList} reviewDirectDependencies
  * @param {VersionString} elmVersion
+ * @returns {Promise<Record<string, string>>}
  */
 async function collectRuleLinks(reviewDirectDependencies, elmVersion) {
   const elmJsonForReviewDependenciesPromises = Object.entries(
@@ -36,6 +37,7 @@ async function collectRuleLinks(reviewDirectDependencies, elmVersion) {
 /**
  * @param {ApplicationDependencyList} reviewDirectDependencies
  * @param {PackageElmJson[]} elmJsonForReviewDependencies
+ * @returns {Record<string, string>}
  */
 function computeLinksToRuleDocs(
   reviewDirectDependencies,
@@ -76,6 +78,7 @@ function computeLinksToRuleDocs(
  * @param {PackageName} dependencyName
  * @param {VersionString} packageVersion
  * @param {string} moduleName
+ * @returns {string}
  */
 function linkToModule(dependencyName, packageVersion, moduleName) {
   const urlModuleName = moduleName.split('.').join('-');
diff --git a/lib/run-review.js b/lib/run-review.js
index 129096537..0111c2499 100644
--- a/lib/run-review.js
+++ b/lib/run-review.js
@@ -66,6 +66,7 @@ let isVeryFirstRun = true;
 /**
  * @param {Options} options
  * @param {ReviewApp} app
+ * @returns {void}
  */
 function startReview(options, app) {
   if (options.report !== 'json' && !isVeryFirstRun) {
@@ -79,6 +80,7 @@ function startReview(options, app) {
 /**
  * @param {Options} options
  * @param {ReviewApp} app
+ * @returns {void}
  */
 function requestReview(options, app) {
   if (AppState.requestReview()) {
diff --git a/lib/runner.js b/lib/runner.js
index 22351a3e0..0b5478014 100755
--- a/lib/runner.js
+++ b/lib/runner.js
@@ -1,12 +1,14 @@
 #!/usr/bin/env node
 
 /**
- * @import {Options} from './types/options';
  * @import {ReviewApp, FileReceipt} from './types/app';
- * @import {ElmFile, Readme, ElmJsonData, Dependencies, LinksToRuleDocs, NonElmFiles, ExtraFileRequest, ApplicationElmJson} from './types/content'
- * @import {SuppressedErrorsFile} from './types/suppressed';
  * @import {AppHash} from './types/build';
+ * @import {ApplicationElmJson, Dependencies, ElmFile, ElmJsonData, ExtraFileRequest, LinksToRuleDocs, NonElmFiles, Readme} from './types/content';
  * @import {FixMode} from './types/fix';
+ * @import {Options} from './types/options';
+ * @import {ReportedError} from './types/report';
+ * @import {RunOptions} from'./types/runner';
+ * @import {SuppressedErrorsFile} from './types/suppressed';
  */
 const exit = require('../vendor/exit');
 const AppWrapper = require('./app-wrapper');
@@ -104,6 +106,7 @@ async function sendProjectContent(
  * @param {string} elmModulePath
  * @param {ApplicationElmJson} reviewElmJson
  * @param {AppHash} appHash
+ * @returns  {Promise<RunOptions>}
  */
 async function initializeApp(options, elmModulePath, reviewElmJson, appHash) {
   const app = AppWrapper.init(options, elmModulePath, {
@@ -131,7 +134,10 @@ async function initializeApp(options, elmModulePath, reviewElmJson, appHash) {
   });
   AppState.subscribe(
     app.ports.abortForConfigurationErrors,
-    /** @returns {never} */
+    /**
+     * @param {ReportedError[]} errors
+     * @returns {never}
+     */
     (errors) => {
       Report.report(options, {errors});
       exit(1);
@@ -245,6 +251,7 @@ function fixMode(options) {
 
 /**
  * @param {string} message
+ * @returns {void}
  */
 function throwOnError(message) {
   throw new ErrorMessage.CustomError(
diff --git a/lib/spinner.js b/lib/spinner.js
index 00c8a5de3..bcef9dba8 100644
--- a/lib/spinner.js
+++ b/lib/spinner.js
@@ -74,6 +74,7 @@ function fail(text) {
 /**
  * @template {(v: string) => void} T
  * @param {T} func
+ * @returns {(T | (() => void))}
  */
 function exportFunc(func) {
   return options.report === 'json' ? () => {} : func;
diff --git a/lib/state.js b/lib/state.js
index 416ab20db..71c4fe823 100644
--- a/lib/state.js
+++ b/lib/state.js
@@ -217,6 +217,9 @@ function getFileFromMemoryCache(filePath) {
   return model.elmFilesCacheForWatch.get(filePath);
 }
 
+/**
+ * @returns {FilesProposedByCurrentFix}
+ */
 function filesProposedByCurrentFix() {
   return model.filesProposedByCurrentFix;
 }
diff --git a/lib/styled-message.js b/lib/styled-message.js
index aa95dd485..3cbb1c427 100644
--- a/lib/styled-message.js
+++ b/lib/styled-message.js
@@ -14,6 +14,7 @@ const wrap = require('wrap-ansi');
  * @param {Options} options
  * @param {StyledMessage} message
  * @param {boolean} clearFixLine
+ * @returns {void}
  */
 function clearAndLog(options, message, clearFixLine) {
   if (options.watch && !options.debug && options.report !== 'json') {
@@ -29,6 +30,7 @@ function clearAndLog(options, message, clearFixLine) {
 /**
  * @param {Options} options
  * @param {StyledMessage} message
+ * @returns {void}
  */
 function log(options, message) {
   const colorEnabled = options.color !== false && options.report === null;
@@ -40,7 +42,7 @@ function log(options, message) {
 /**
  * @param {boolean} colorEnabled
  * @param {StyledMessage} message
- * @returns
+ * @returns {string}
  */
 function styleMessage(colorEnabled, message) {
   return message
@@ -102,6 +104,9 @@ function wrapLinesForTerminal(message) {
 
 // CLEARING CONSOLE
 
+/**
+ * @returns {void}
+ */
 function clearConsole() {
   process.stdout.write(
     process.platform === 'win32'
diff --git a/lib/template-dependencies.js b/lib/template-dependencies.js
index 2a4b8c1cc..4222ab156 100644
--- a/lib/template-dependencies.js
+++ b/lib/template-dependencies.js
@@ -335,6 +335,9 @@ function update(options, elmJson) {
   return elmJson;
 }
 
+/**
+ * @returns {void}
+ */
 function teardownDependenciesProvider() {
   if (dependencyProvider) {
     dependencyProvider.tearDown();
diff --git a/lib/types/app.ts b/lib/types/app.ts
index 54a9a1078..066c4414b 100644
--- a/lib/types/app.ts
+++ b/lib/types/app.ts
@@ -8,10 +8,12 @@ import type {
   Readme,
   Source
 } from './content.ts';
-import type {ElmNamespace, ElmApp} from './elm-js.js';
+import type {ElmNamespace, ElmApp} from './elm-js.ts';
+import type {ErrorMessageInfo} from './error-message.ts';
 import type {Flags} from './flags.ts';
 import type {Path} from './path.ts';
 import type {SendPort, SubscribePort} from './promisify-port.ts';
+import type {ReportedError} from './report.ts';
 import type {FilesProposedByCurrentFix} from './state.ts';
 import type {StyledMessage} from './styled-message.ts';
 import type {SuppressedErrorsFile} from './suppressed.ts';
@@ -52,10 +54,8 @@ export type Ports = {
   fixConfirmationStatus: SubscribePort<boolean>;
   askForFixConfirmationStatus: SendPort<null>;
   abort: SubscribePort<string>;
-  abortWithDetails: SubscribePort<{title: string; message: string}>;
-  abortForConfigurationErrors: SubscribePort<
-    {errors: unknown[]; path: unknown}[] & StyledMessage
-  >;
+  abortWithDetails: SubscribePort<ErrorMessageInfo>;
+  abortForConfigurationErrors: SubscribePort<ReportedError[]>;
 };
 
 export type FileReceipt = {
diff --git a/lib/types/content.ts b/lib/types/content.ts
index 11b53d666..a1fcaedb5 100644
--- a/lib/types/content.ts
+++ b/lib/types/content.ts
@@ -67,12 +67,23 @@ export type ExtraFileRequest = {
   excludedDirectories: string[];
 };
 
+export type ProjectFiles = {
+  elmJsonData: ElmJsonData;
+  readme: Readme | null;
+  elmFiles: ElmFile[];
+  sourceDirectories: Path[];
+};
+
 export type Dependencies = Dependency[];
 
-export type Dependency = unknown;
+export type Dependency = {
+  name: PackageName;
+  docsJson: unknown;
+  elmJson: PackageElmJson;
+};
 
 export type LinksToRuleDocs = Record<string, string>;
 
-export type Source = string;
+export type Source = Path;
 
 export type Ast = Record<string, unknown>;
diff --git a/lib/types/elm-internals.ts b/lib/types/elm-internals.ts
index b503d55fe..5f4cae514 100644
--- a/lib/types/elm-internals.ts
+++ b/lib/types/elm-internals.ts
@@ -1,7 +1,8 @@
 export type Value = null | ValueStruct | number | string;
 export type ValueStruct = {
-  readonly $: 1 | -1 | 0 | `$L`;
-  readonly a?: string[] | string;
+  readonly $: 1 | -1 | 0 | '$L';
+  readonly a?: ElmArray;
   readonly b?: ValueStruct;
   readonly c?: unknown;
 };
+export type ElmArray = string[] | string;
diff --git a/lib/types/error-message.ts b/lib/types/error-message.ts
new file mode 100644
index 000000000..8c1a0563c
--- /dev/null
+++ b/lib/types/error-message.ts
@@ -0,0 +1,4 @@
+export type ErrorMessageInfo = {
+  title: string;
+  message: string;
+};
diff --git a/lib/types/parse-elm.ts b/lib/types/parse-elm.ts
index 99d1617f9..1d461250d 100644
--- a/lib/types/parse-elm.ts
+++ b/lib/types/parse-elm.ts
@@ -1,7 +1,8 @@
-import type {ElmApp, ElmNamespace} from './elm-js.js';
+import type {ElmApp, ElmNamespace} from './elm-js.ts';
 import type {Ast, ElmFile, Source} from './content.ts';
 import type {Path} from './path.ts';
 import type {SendPort, SubscribePort} from './promisify-port.ts';
+import type {Worker} from 'node:worker_threads';
 
 export type ParseJob = {
   elmParserPath: Path;
@@ -21,3 +22,11 @@ type ParserPorts = {
   requestParsing: SendPort<string>;
   parseResult: SubscribePort<ElmFile>;
 };
+
+/**
+ * A worker thread and whether it is currently busy.
+ */
+export type CustomWorker = {
+  worker: Worker;
+  busy: boolean;
+};
diff --git a/lib/types/report.ts b/lib/types/report.ts
new file mode 100644
index 000000000..a699d12ec
--- /dev/null
+++ b/lib/types/report.ts
@@ -0,0 +1,14 @@
+import type {StyledMessagePart} from './styled-message.ts';
+import type {SuppressedErrorsFile} from './suppressed.ts';
+
+export type ReportedError = StyledMessagePart & {
+  errors: object[];
+  path: unknown;
+  extracts?: unknown;
+};
+
+export type Report = {
+  errors?: ReportedError[];
+  extracts?: unknown;
+  suppressedErrors?: SuppressedErrorsFile[];
+};
diff --git a/lib/types/runner.ts b/lib/types/runner.ts
new file mode 100644
index 000000000..caef57ec2
--- /dev/null
+++ b/lib/types/runner.ts
@@ -0,0 +1,12 @@
+import type {ReviewApp} from './app.ts';
+import type {ElmFile, ElmJsonData, ExtraFileRequest} from './content.ts';
+import type {VersionString} from './version.js';
+
+export type RunOptions = {
+  app: ReviewApp;
+  elmVersion: VersionString;
+  elmJsonData: ElmJsonData;
+  elmFiles: ElmFile[];
+  sourceDirectories: string[];
+  requestedExtraFiles: ExtraFileRequest[];
+};
diff --git a/lib/types/version.ts b/lib/types/version.ts
index 9ccf209fe..733046d10 100644
--- a/lib/types/version.ts
+++ b/lib/types/version.ts
@@ -1,9 +1,9 @@
 /**
- * A major.minor version string, e.g. "1.0.0"
+ * A "major.minor.patch" version string, e.g. `1.0.0`.
  */
 export type VersionString = `${number}.${number}.${number}`;
 
 /**
- * A dependency support version, e.g. "1.0.0 <= v < 2.0.0"
+ * A dependency support version, e.g. `1.0.0 <= v < 2.0.0`.
  */
 export type VersionRange = `${VersionString} <= v < ${VersionString}`;
diff --git a/lib/types/watch.ts b/lib/types/watch.ts
index ba3051606..26cf733c0 100644
--- a/lib/types/watch.ts
+++ b/lib/types/watch.ts
@@ -1,18 +1,8 @@
-import type {ReviewApp} from './app.ts';
-import type {
-  ApplicationElmJson,
-  ElmFile,
-  ElmJsonData,
-  ExtraFileRequest
-} from './content.ts';
+import type {ApplicationElmJson} from './content.ts';
 import type {Path} from './path.ts';
+import type {RunOptions} from './runner.ts';
 
-export type WatchOptions = {
-  app: ReviewApp;
-  elmJsonData: ElmJsonData;
-  elmFiles: ElmFile[];
-  sourceDirectories: Path[];
+export type WatchOptions = RunOptions & {
   reviewElmJson: ApplicationElmJson;
   reviewElmJsonPath: Path | null;
-  requestedExtraFiles: ExtraFileRequest[];
 };
diff --git a/lib/utils.js b/lib/utils.js
index f0ed6f087..fd49ab6d5 100644
--- a/lib/utils.js
+++ b/lib/utils.js
@@ -14,6 +14,7 @@ function unique(array) {
  * Otherwise, wrap it in a generic error.
  *
  * @param {unknown} error
+ * @returns {Error}
  */
 function intoError(error) {
   return error instanceof Error ? error : new Error(String(error));
diff --git a/lib/watch.js b/lib/watch.js
index d254f0e86..eb665f746 100644
--- a/lib/watch.js
+++ b/lib/watch.js
@@ -438,6 +438,9 @@ function createExtraFilesWatcher(options, app, runReview, onError, request) {
  * @returns {FSWatcher}
  */
 function createSuppressedFilesWatcher(options, app, onError) {
+  /**
+   * @returns {void}
+   */
   function updateSuppressedErrors() {
     // TODO(@jfmengels): Write last save time for each of these in appstate,
     //   and compare with the last update time that is given as argument to
@@ -468,6 +471,9 @@ function createSuppressedFilesWatcher(options, app, onError) {
     .on('error', onError);
 }
 
+/**
+ * @returns {void}
+ */
 function clearConsole() {
   process.stdout.write(
     process.platform === 'win32'
diff --git a/new-package/elm-review-package-tests/check-previews-compile.js b/new-package/elm-review-package-tests/check-previews-compile.js
index 7df56f84e..13699cdc5 100644
--- a/new-package/elm-review-package-tests/check-previews-compile.js
+++ b/new-package/elm-review-package-tests/check-previews-compile.js
@@ -16,6 +16,7 @@ for (const example of findPreviewConfigurations()) {
 
 /**
  * @param {string} exampleConfiguration
+ * @returns {void}
  */
 function checkThatExampleCompiles(exampleConfiguration) {
   const exampleConfigurationElmJson = require(`${exampleConfiguration}/elm.json`);
@@ -67,6 +68,7 @@ and make the necessary changes to make it compile.`
 
 /**
  * @param {string} config
+ * @returns {void}
  */
 function success(config) {
   console.log(`${Ansi.green('✔')} ${path.relative(root, config)}/ compiles`);
@@ -75,6 +77,7 @@ function success(config) {
 /**
  * @param {string} exampleConfiguration
  * @param {Record<string, string>} previewDependencies
+ * @returns {void}
  */
 function checkDepsAreCompatible(exampleConfiguration, previewDependencies) {
   for (const [depName, constraint] of Object.entries(packageDependencies)) {
@@ -109,6 +112,7 @@ function checkDepsAreCompatible(exampleConfiguration, previewDependencies) {
  * @param {string} depName
  * @param {string} constraint
  * @param {string} version
+ * @returns {void}
  */
 function checkConstraint(exampleConfiguration, depName, constraint, version) {
   const [minVersion] = constraint.split(' <= v < ').map(splitVersion);
@@ -128,6 +132,7 @@ function checkConstraint(exampleConfiguration, depName, constraint, version) {
 
 /**
  * @param {string} version
+ * @returns {number[]}
  */
 function splitVersion(version) {
   return version.split('.').map((n) => Number.parseInt(n, 10));
diff --git a/new-package/elm-review-package-tests/helpers/ansi.js b/new-package/elm-review-package-tests/helpers/ansi.js
index 5eaa78f5a..24eb0148b 100644
--- a/new-package/elm-review-package-tests/helpers/ansi.js
+++ b/new-package/elm-review-package-tests/helpers/ansi.js
@@ -1,5 +1,6 @@
 /**
  * @param {string} text
+ * @returns {string}
  */
 function red(text) {
   return `\u001B[31m${text}\u001B[39m`;
@@ -7,6 +8,7 @@ function red(text) {
 
 /**
  * @param {string} text
+ * @returns {string}
  */
 function green(text) {
   return `\u001B[32m${text}\u001B[39m`;
@@ -14,6 +16,7 @@ function green(text) {
 
 /**
  * @param {string} text
+ * @returns {string}
  */
 function yellow(text) {
   return `\u001B[33m${text}\u001B[39m`;
diff --git a/new-package/maintenance/update-examples-from-preview.js b/new-package/maintenance/update-examples-from-preview.js
index bd82369e8..7c229ef0c 100755
--- a/new-package/maintenance/update-examples-from-preview.js
+++ b/new-package/maintenance/update-examples-from-preview.js
@@ -28,6 +28,9 @@ if (require.main === module) {
  * @property {Record<string, string>} indirect
  */
 
+/**
+ * @returns {void}
+ */
 function copyPreviewsToExamples() {
   const previewFolders = findPreviewConfigurations();
   for (const folder of previewFolders) {
@@ -37,6 +40,7 @@ function copyPreviewsToExamples() {
 
 /**
  * @param {string} pathToPreviewFolder
+ * @returns {void}
  */
 function copyPreviewToExample(pathToPreviewFolder) {
   const pathToExampleFolder = `${pathToPreviewFolder}/`.replace(
diff --git a/package-lock.json b/package-lock.json
index 18379a50a..5bdb2c2ac 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -35,6 +35,7 @@
       },
       "devDependencies": {
         "@eslint-community/eslint-plugin-eslint-comments": "^4.4.1",
+        "@package-json/types": "^0.0.11",
         "@types/cross-spawn": "~6.0.6",
         "@types/folder-hash": "~3.3.0",
         "@types/fs-extra": "~9.0.13",
@@ -1717,6 +1718,12 @@
         "node": "^14.17.0 || ^16.13.0 || >=18.0.0"
       }
     },
+    "node_modules/@package-json/types": {
+      "version": "0.0.11",
+      "resolved": "https://registry.npmjs.org/@package-json/types/-/types-0.0.11.tgz",
+      "integrity": "sha512-allOTUn4Xi2bQMs+mthzHWekgjRBVno+DLOcXk9+6haG5oFu5rlz0pszT3sh1OAkQVFLYrAS4V5CSxWyVwUf7g==",
+      "dev": true
+    },
     "node_modules/@pkgjs/parseargs": {
       "version": "0.11.0",
       "resolved": "https://registry.npmjs.org/@pkgjs/parseargs/-/parseargs-0.11.0.tgz",
@@ -12074,6 +12081,12 @@
         }
       }
     },
+    "@package-json/types": {
+      "version": "0.0.11",
+      "resolved": "https://registry.npmjs.org/@package-json/types/-/types-0.0.11.tgz",
+      "integrity": "sha512-allOTUn4Xi2bQMs+mthzHWekgjRBVno+DLOcXk9+6haG5oFu5rlz0pszT3sh1OAkQVFLYrAS4V5CSxWyVwUf7g==",
+      "dev": true
+    },
     "@pkgjs/parseargs": {
       "version": "0.11.0",
       "resolved": "https://registry.npmjs.org/@pkgjs/parseargs/-/parseargs-0.11.0.tgz",
diff --git a/package.json b/package.json
index 9f30f108f..3fbcc3719 100644
--- a/package.json
+++ b/package.json
@@ -83,6 +83,7 @@
   },
   "devDependencies": {
     "@eslint-community/eslint-plugin-eslint-comments": "^4.4.1",
+    "@package-json/types": "^0.0.11",
     "@types/cross-spawn": "~6.0.6",
     "@types/folder-hash": "~3.3.0",
     "@types/fs-extra": "~9.0.13",
diff --git a/test/jest-helpers/cli.js b/test/jest-helpers/cli.js
index 53f8cf1d5..d1b0fd241 100644
--- a/test/jest-helpers/cli.js
+++ b/test/jest-helpers/cli.js
@@ -12,7 +12,8 @@ expect.extend({toMatchFile});
 
 /**
  * @param {string} args
- * @param {Options} [options=undefined]
+ * @param {Options} [options]
+ * @returns
  */
 async function run(args, options) {
   return await internalExec(`--FOR-TESTS ${args}`, options);
@@ -20,7 +21,7 @@ async function run(args, options) {
 
 /**
  * @param {string} args
- * @param {Options | undefined} [options=undefined]
+ * @param {Options | undefined} [options]
  * @returns {Promise<unknown>}
  */
 async function runAndExpectError(args, options) {
@@ -36,7 +37,8 @@ async function runAndExpectError(args, options) {
 
 /**
  * @param {string} args
- * @param {Options | undefined} [options=undefined]
+ * @param {Options | undefined} [options]
+ * @returns
  */
 async function runWithoutTestMode(args, options) {
   return await internalExec(args, options);
@@ -44,7 +46,8 @@ async function runWithoutTestMode(args, options) {
 
 /**
  * @param {string} args
- * @param {Options} [options=undefined]
+ * @param {Options} [options]
+ * @returns
  */
 async function internalExec(args, options = {}) {
   // Overriding FORCE_COLOR because Jest forcefully adds it as well,
@@ -67,6 +70,7 @@ async function internalExec(args, options = {}) {
 
 /**
  * @param {Options} options
+ * @returns
  */
 function cwdFromOptions(options) {
   if (options.project) {
@@ -78,6 +82,7 @@ function cwdFromOptions(options) {
 
 /**
  * @param {Options} options
+ * @returns
  */
 function reportMode(options) {
   if (!options.report) {
@@ -89,6 +94,7 @@ function reportMode(options) {
 
 /**
  * @param {Options} options
+ * @returns
  */
 function colors(options) {
   if (options.colors) {
diff --git a/test/run-snapshots/elm-review-something-for-new-rule/elm-review-package-tests/check-previews-compile.js b/test/run-snapshots/elm-review-something-for-new-rule/elm-review-package-tests/check-previews-compile.js
index a577b2c98..dcafe0169 100644
--- a/test/run-snapshots/elm-review-something-for-new-rule/elm-review-package-tests/check-previews-compile.js
+++ b/test/run-snapshots/elm-review-something-for-new-rule/elm-review-package-tests/check-previews-compile.js
@@ -15,6 +15,7 @@ for (const example of findPreviewConfigurations()) {
 
 /**
  * @param {string} exampleConfiguration
+ * @returns {void}
  */
 function checkThatExampleCompiles(exampleConfiguration) {
   const exampleConfigurationElmJson = require(`${exampleConfiguration}/elm.json`);
@@ -66,6 +67,7 @@ and make the necessary changes to make it compile.`
 
 /**
  * @param {string} config
+ * @returns {void}
  */
 function success(config) {
   console.log(`${Ansi.green('✔')} ${path.relative(root, config)}/ compiles`);
@@ -74,6 +76,7 @@ function success(config) {
 /**
  * @param {string} exampleConfiguration
  * @param {Record<string, string>} previewDependencies
+ * @returns {void}
  */
 function checkDepsAreCompatible(exampleConfiguration, previewDependencies) {
   for (const [depName, constraint] of Object.entries(packageDependencies)) {
@@ -108,6 +111,7 @@ function checkDepsAreCompatible(exampleConfiguration, previewDependencies) {
  * @param {string} depName
  * @param {string} constraint
  * @param {string} version
+ * @returns {void}
  */
 function checkConstraint(exampleConfiguration, depName, constraint, version) {
   const [minVersion] = constraint.split(' <= v < ').map(splitVersion);
@@ -127,6 +131,7 @@ function checkConstraint(exampleConfiguration, depName, constraint, version) {
 
 /**
  * @param {string} version
+ * @returns {number[]}
  */
 function splitVersion(version) {
   return version.split('.').map((n) => Number.parseInt(n, 10));
diff --git a/test/run-snapshots/elm-review-something-for-new-rule/elm-review-package-tests/helpers/ansi.js b/test/run-snapshots/elm-review-something-for-new-rule/elm-review-package-tests/helpers/ansi.js
index 5eaa78f5a..24eb0148b 100644
--- a/test/run-snapshots/elm-review-something-for-new-rule/elm-review-package-tests/helpers/ansi.js
+++ b/test/run-snapshots/elm-review-something-for-new-rule/elm-review-package-tests/helpers/ansi.js
@@ -1,5 +1,6 @@
 /**
  * @param {string} text
+ * @returns {string}
  */
 function red(text) {
   return `\u001B[31m${text}\u001B[39m`;
@@ -7,6 +8,7 @@ function red(text) {
 
 /**
  * @param {string} text
+ * @returns {string}
  */
 function green(text) {
   return `\u001B[32m${text}\u001B[39m`;
@@ -14,6 +16,7 @@ function green(text) {
 
 /**
  * @param {string} text
+ * @returns {string}
  */
 function yellow(text) {
   return `\u001B[33m${text}\u001B[39m`;
diff --git a/test/run-snapshots/elm-review-something-for-new-rule/maintenance/update-examples-from-preview.js b/test/run-snapshots/elm-review-something-for-new-rule/maintenance/update-examples-from-preview.js
index bd82369e8..7c229ef0c 100755
--- a/test/run-snapshots/elm-review-something-for-new-rule/maintenance/update-examples-from-preview.js
+++ b/test/run-snapshots/elm-review-something-for-new-rule/maintenance/update-examples-from-preview.js
@@ -28,6 +28,9 @@ if (require.main === module) {
  * @property {Record<string, string>} indirect
  */
 
+/**
+ * @returns {void}
+ */
 function copyPreviewsToExamples() {
   const previewFolders = findPreviewConfigurations();
   for (const folder of previewFolders) {
@@ -37,6 +40,7 @@ function copyPreviewsToExamples() {
 
 /**
  * @param {string} pathToPreviewFolder
+ * @returns {void}
  */
 function copyPreviewToExample(pathToPreviewFolder) {
   const pathToExampleFolder = `${pathToPreviewFolder}/`.replace(
diff --git a/test/run-snapshots/elm-review-something/elm-review-package-tests/check-previews-compile.js b/test/run-snapshots/elm-review-something/elm-review-package-tests/check-previews-compile.js
index a577b2c98..dcafe0169 100644
--- a/test/run-snapshots/elm-review-something/elm-review-package-tests/check-previews-compile.js
+++ b/test/run-snapshots/elm-review-something/elm-review-package-tests/check-previews-compile.js
@@ -15,6 +15,7 @@ for (const example of findPreviewConfigurations()) {
 
 /**
  * @param {string} exampleConfiguration
+ * @returns {void}
  */
 function checkThatExampleCompiles(exampleConfiguration) {
   const exampleConfigurationElmJson = require(`${exampleConfiguration}/elm.json`);
@@ -66,6 +67,7 @@ and make the necessary changes to make it compile.`
 
 /**
  * @param {string} config
+ * @returns {void}
  */
 function success(config) {
   console.log(`${Ansi.green('✔')} ${path.relative(root, config)}/ compiles`);
@@ -74,6 +76,7 @@ function success(config) {
 /**
  * @param {string} exampleConfiguration
  * @param {Record<string, string>} previewDependencies
+ * @returns {void}
  */
 function checkDepsAreCompatible(exampleConfiguration, previewDependencies) {
   for (const [depName, constraint] of Object.entries(packageDependencies)) {
@@ -108,6 +111,7 @@ function checkDepsAreCompatible(exampleConfiguration, previewDependencies) {
  * @param {string} depName
  * @param {string} constraint
  * @param {string} version
+ * @returns {void}
  */
 function checkConstraint(exampleConfiguration, depName, constraint, version) {
   const [minVersion] = constraint.split(' <= v < ').map(splitVersion);
@@ -127,6 +131,7 @@ function checkConstraint(exampleConfiguration, depName, constraint, version) {
 
 /**
  * @param {string} version
+ * @returns {number[]}
  */
 function splitVersion(version) {
   return version.split('.').map((n) => Number.parseInt(n, 10));
diff --git a/test/run-snapshots/elm-review-something/elm-review-package-tests/helpers/ansi.js b/test/run-snapshots/elm-review-something/elm-review-package-tests/helpers/ansi.js
index 5eaa78f5a..24eb0148b 100644
--- a/test/run-snapshots/elm-review-something/elm-review-package-tests/helpers/ansi.js
+++ b/test/run-snapshots/elm-review-something/elm-review-package-tests/helpers/ansi.js
@@ -1,5 +1,6 @@
 /**
  * @param {string} text
+ * @returns {string}
  */
 function red(text) {
   return `\u001B[31m${text}\u001B[39m`;
@@ -7,6 +8,7 @@ function red(text) {
 
 /**
  * @param {string} text
+ * @returns {string}
  */
 function green(text) {
   return `\u001B[32m${text}\u001B[39m`;
@@ -14,6 +16,7 @@ function green(text) {
 
 /**
  * @param {string} text
+ * @returns {string}
  */
 function yellow(text) {
   return `\u001B[33m${text}\u001B[39m`;
diff --git a/test/run-snapshots/elm-review-something/maintenance/update-examples-from-preview.js b/test/run-snapshots/elm-review-something/maintenance/update-examples-from-preview.js
index bd82369e8..7c229ef0c 100755
--- a/test/run-snapshots/elm-review-something/maintenance/update-examples-from-preview.js
+++ b/test/run-snapshots/elm-review-something/maintenance/update-examples-from-preview.js
@@ -28,6 +28,9 @@ if (require.main === module) {
  * @property {Record<string, string>} indirect
  */
 
+/**
+ * @returns {void}
+ */
 function copyPreviewsToExamples() {
   const previewFolders = findPreviewConfigurations();
   for (const folder of previewFolders) {
@@ -37,6 +40,7 @@ function copyPreviewsToExamples() {
 
 /**
  * @param {string} pathToPreviewFolder
+ * @returns {void}
  */
 function copyPreviewToExample(pathToPreviewFolder) {
   const pathToExampleFolder = `${pathToPreviewFolder}/`.replace(
diff --git a/vendor/types/node-elm-compiler.ts b/vendor/types/node-elm-compiler.ts
index e5bd98e33..8ce80923d 100644
--- a/vendor/types/node-elm-compiler.ts
+++ b/vendor/types/node-elm-compiler.ts
@@ -1,17 +1,19 @@
 import type {ChildProcess, SpawnOptions} from 'node:child_process';
 import type {ReportMode} from '../../lib/types/options';
+import type {Path} from '../../lib/types/path';
+import type {Source} from '../../lib/types/content';
 
 export type CompileOptions = {
   spawn?: Spawner;
 
-  cwd: string;
-  output: string;
+  cwd: Path;
+  output: Path;
   debug: boolean;
   optimize: boolean;
   verbose: boolean;
   warn: boolean;
   report: ReportMode;
-  pathToElm: string;
+  pathToElm: Path;
   help?: undefined;
   docs?: undefined;
   processOpts: ProcessOptions;
@@ -23,9 +25,9 @@ export interface ProcessOptions {
 }
 
 export type Spawner = (
-  pathToElm: string,
+  pathToElm: Path,
   processArgs: SpawnOptions,
   processOpts: ProcessOptions
 ) => ChildProcess;
 
-export type Sources = string | string[];
+export type Sources = Source | Source[];