diff --git a/.eslintrc.json b/.eslintrc.json
index 0e3b939a..d4ae215b 100644
--- a/.eslintrc.json
+++ b/.eslintrc.json
@@ -1,7 +1,14 @@
 {
+    "root": true,
     "extends": "eslint:recommended",
     "parserOptions": {
-        "ecmaVersion": 8
+        "ecmaVersion": 8,
+        "sourceType": "script"
+    },
+    "env": {
+        "browser": true,
+        "es2017": true,
+        "webextensions": true
     },
     "ignorePatterns": [
         "/ext/mixed/lib/",
@@ -16,13 +23,57 @@
         "no-case-declarations": "error",
         "no-const-assign": "error",
         "no-constant-condition": "off",
-        "no-undef": "off",
+        "no-global-assign": "error",
+        "no-undef": "error",
         "no-unused-vars": ["error", {"vars": "local", "args": "after-used", "argsIgnorePattern": "^_", "caughtErrors": "none"}],
+        "no-unused-expressions": "error",
         "no-var": "error",
         "prefer-const": ["error", {"destructuring": "all"}],
         "quote-props": ["error", "consistent"],
         "quotes": ["error", "single", "avoid-escape"],
         "require-atomic-updates": "off",
         "semi": "error"
-    }
+    },
+    "overrides": [
+        {
+            "files": ["*.js"],
+            "excludedFiles": ["ext/mixed/js/core.js"],
+            "globals": {
+                "yomichan": "readonly",
+                "errorToJson": "readonly",
+                "jsonToError": "readonly",
+                "logError": "readonly",
+                "isObject": "readonly",
+                "hasOwn": "readonly",
+                "toIterable": "readonly",
+                "stringReverse": "readonly",
+                "promiseTimeout": "readonly",
+                "stringReplaceAsync": "readonly",
+                "parseUrl": "readonly",
+                "EventDispatcher": "readonly",
+                "EXTENSION_IS_BROWSER_EDGE": "readonly"
+            }
+        },
+        {
+            "files": ["ext/mixed/js/core.js"],
+            "globals": {
+                "chrome": "writable"
+            }
+        },
+        {
+            "files": ["ext/bg/js/settings/*.js"],
+            "env": {
+                "jquery": true
+            }
+        },
+        {
+            "files": ["test/**/*.js"],
+            "env": {
+                "browser": false,
+                "es2017": false,
+                "webextensions": false,
+                "node": true
+            }
+        }
+    ]
 }
diff --git a/ext/bg/js/anki.js b/ext/bg/js/anki.js
index 10a07061..39c6ad51 100644
--- a/ext/bg/js/anki.js
+++ b/ext/bg/js/anki.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global requestJson*/
 
 /*
  * AnkiConnect
diff --git a/ext/bg/js/audio.js b/ext/bg/js/audio.js
index d1e4af9e..6389528b 100644
--- a/ext/bg/js/audio.js
+++ b/ext/bg/js/audio.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global jpIsStringEntirelyKana, audioGetFromSources*/
 
 const audioUrlBuilders = new Map([
     ['jpod101', async (definition) => {
diff --git a/ext/bg/js/backend.js b/ext/bg/js/backend.js
index 529055d2..7b2ec46d 100644
--- a/ext/bg/js/backend.js
+++ b/ext/bg/js/backend.js
@@ -16,6 +16,14 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global optionsSave, utilIsolate
+conditionsTestValue, profileConditionsDescriptor, profileOptionsGetDefaultFieldTemplates
+handlebarsRenderDynamic, handlebarsRenderStatic
+requestText, requestJson, optionsLoad
+dictConfigured, dictTermsSort, dictEnabledSet, dictNoteFormat
+audioGetUrl, audioInject
+jpConvertReading, jpDistributeFuriganaInflected, jpKatakanaToHiragana
+Translator, AnkiConnect, AnkiNull, Mecab, BackendApiForwarder, JsonSchema, ClipboardMonitor*/
 
 class Backend {
     constructor() {
diff --git a/ext/bg/js/clipboard-monitor.js b/ext/bg/js/clipboard-monitor.js
index b4a27fa2..c2f41385 100644
--- a/ext/bg/js/clipboard-monitor.js
+++ b/ext/bg/js/clipboard-monitor.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiClipboardGet, jpIsStringPartiallyJapanese*/
 
 class ClipboardMonitor {
     constructor() {
diff --git a/ext/bg/js/context.js b/ext/bg/js/context.js
index 37adb6b7..bec964fb 100644
--- a/ext/bg/js/context.js
+++ b/ext/bg/js/context.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiCommandExec, apiGetEnvironmentInfo, apiOptionsGet*/
 
 function showExtensionInfo() {
     const node = document.getElementById('extension-info');
diff --git a/ext/bg/js/database.js b/ext/bg/js/database.js
index e87cc64b..b54d832c 100644
--- a/ext/bg/js/database.js
+++ b/ext/bg/js/database.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global dictFieldSplit, dictTagSanitize, JSZip*/
 
 class Database {
     constructor() {
diff --git a/ext/bg/js/dictionary.js b/ext/bg/js/dictionary.js
index 48f65d6c..e03dece0 100644
--- a/ext/bg/js/dictionary.js
+++ b/ext/bg/js/dictionary.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global utilSetEqual, utilSetIntersection, apiTemplateRender*/
 
 function dictEnabledSet(options) {
     const dictionaries = {};
diff --git a/ext/bg/js/handlebars.js b/ext/bg/js/handlebars.js
index 62f89ee4..e8cb67eb 100644
--- a/ext/bg/js/handlebars.js
+++ b/ext/bg/js/handlebars.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global jpIsCharCodeKanji, jpDistributeFurigana, Handlebars*/
 
 function handlebarsEscape(text) {
     return Handlebars.Utils.escapeExpression(text);
diff --git a/ext/bg/js/japanese.js b/ext/bg/js/japanese.js
index c45c0958..e8a6fa08 100644
--- a/ext/bg/js/japanese.js
+++ b/ext/bg/js/japanese.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global wanakana*/
 
 const JP_HALFWIDTH_KATAKANA_MAPPING = new Map([
     ['ｦ', 'ヲヺ-'],
diff --git a/ext/bg/js/options.js b/ext/bg/js/options.js
index 97032660..f9db99a2 100644
--- a/ext/bg/js/options.js
+++ b/ext/bg/js/options.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global utilStringHashCode*/
 
 /*
  * Generic options functions
diff --git a/ext/bg/js/search-frontend.js b/ext/bg/js/search-frontend.js
index e453ccef..509c4009 100644
--- a/ext/bg/js/search-frontend.js
+++ b/ext/bg/js/search-frontend.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiOptionsGet*/
 
 async function searchFrontendSetup() {
     const optionsContext = {
diff --git a/ext/bg/js/search-query-parser-generator.js b/ext/bg/js/search-query-parser-generator.js
index 8d71890b..f842644e 100644
--- a/ext/bg/js/search-query-parser-generator.js
+++ b/ext/bg/js/search-query-parser-generator.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiGetQueryParserTemplatesHtml, TemplateHandler*/
 
 class QueryParserGenerator {
     constructor() {
diff --git a/ext/bg/js/search-query-parser.js b/ext/bg/js/search-query-parser.js
index f648fdd4..3d38e6e8 100644
--- a/ext/bg/js/search-query-parser.js
+++ b/ext/bg/js/search-query-parser.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiTermsFind, apiOptionsSet, apiTextParse, apiTextParseMecab, TextScanner, QueryParserGenerator*/
 
 class QueryParser extends TextScanner {
     constructor(search) {
diff --git a/ext/bg/js/search.js b/ext/bg/js/search.js
index 6641255f..93bcfa53 100644
--- a/ext/bg/js/search.js
+++ b/ext/bg/js/search.js
@@ -16,6 +16,8 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiOptionsSet, apiTermsFind, Display, QueryParser, ClipboardMonitor*/
+
 class DisplaySearch extends Display {
     constructor() {
         super(document.querySelector('#spinner'), document.querySelector('#content'));
diff --git a/ext/bg/js/settings/anki-templates.js b/ext/bg/js/settings/anki-templates.js
index 5e74358f..2e80e334 100644
--- a/ext/bg/js/settings/anki-templates.js
+++ b/ext/bg/js/settings/anki-templates.js
@@ -16,6 +16,9 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global getOptionsContext, getOptionsMutable, settingsSaveOptions
+profileOptionsGetDefaultFieldTemplates, ankiGetFieldMarkers, ankiGetFieldMarkersHtml, dictFieldFormat
+apiOptionsGet, apiTermsFind*/
 
 function onAnkiFieldTemplatesReset(e) {
     e.preventDefault();
diff --git a/ext/bg/js/settings/anki.js b/ext/bg/js/settings/anki.js
index f55989fc..4263fc51 100644
--- a/ext/bg/js/settings/anki.js
+++ b/ext/bg/js/settings/anki.js
@@ -16,6 +16,9 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global getOptionsContext, getOptionsMutable, settingsSaveOptions
+utilBackgroundIsolate, utilAnkiGetDeckNames, utilAnkiGetModelNames, utilAnkiGetModelFieldNames
+onFormOptionsChanged*/
 
 // Private
 
diff --git a/ext/bg/js/settings/audio-ui.js b/ext/bg/js/settings/audio-ui.js
index 711c2291..555380b4 100644
--- a/ext/bg/js/settings/audio-ui.js
+++ b/ext/bg/js/settings/audio-ui.js
@@ -16,7 +16,6 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
-
 class AudioSourceUI {
     static instantiateTemplate(templateSelector) {
         const template = document.querySelector(templateSelector);
diff --git a/ext/bg/js/settings/audio.js b/ext/bg/js/settings/audio.js
index cff3f521..588d9a11 100644
--- a/ext/bg/js/settings/audio.js
+++ b/ext/bg/js/settings/audio.js
@@ -16,6 +16,8 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global getOptionsContext, getOptionsMutable, settingsSaveOptions
+AudioSourceUI, audioGetTextToSpeechVoice*/
 
 let audioSourceUI = null;
 
diff --git a/ext/bg/js/settings/backup.js b/ext/bg/js/settings/backup.js
index becdc568..6d1f28e9 100644
--- a/ext/bg/js/settings/backup.js
+++ b/ext/bg/js/settings/backup.js
@@ -16,6 +16,10 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiOptionsGetFull, apiGetEnvironmentInfo
+utilBackend, utilIsolate, utilBackgroundIsolate, utilReadFileArrayBuffer
+optionsGetDefault, optionsUpdateVersion
+profileOptionsGetDefaultFieldTemplates*/
 
 // Exporting
 
diff --git a/ext/bg/js/settings/conditions-ui.js b/ext/bg/js/settings/conditions-ui.js
index 4d041451..5a271321 100644
--- a/ext/bg/js/settings/conditions-ui.js
+++ b/ext/bg/js/settings/conditions-ui.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global conditionsNormalizeOptionValue*/
 
 class ConditionsUI {
     static instantiateTemplate(templateSelector) {
diff --git a/ext/bg/js/settings/dictionaries.js b/ext/bg/js/settings/dictionaries.js
index 0d827f49..fb459404 100644
--- a/ext/bg/js/settings/dictionaries.js
+++ b/ext/bg/js/settings/dictionaries.js
@@ -16,6 +16,11 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global getOptionsContext, getOptionsMutable, getOptionsFullMutable, settingsSaveOptions, apiOptionsGetFull, apiOptionsGet
+utilBackgroundIsolate, utilDatabaseDeleteDictionary, utilDatabaseGetDictionaryInfo, utilDatabaseGetDictionaryCounts
+utilDatabasePurge, utilDatabaseImport
+storageUpdateStats, storageEstimate
+PageExitPrevention*/
 
 let dictionaryUI = null;
 
diff --git a/ext/bg/js/settings/main.js b/ext/bg/js/settings/main.js
index cf0f08db..1ba4a7ef 100644
--- a/ext/bg/js/settings/main.js
+++ b/ext/bg/js/settings/main.js
@@ -16,6 +16,14 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global getOptionsContext, apiOptionsSave
+utilBackend, utilIsolate, utilBackgroundIsolate
+ankiErrorShown, ankiFieldsToDict
+ankiTemplatesUpdateValue, onAnkiOptionsChanged, onDictionaryOptionsChanged
+appearanceInitialize, audioSettingsInitialize, profileOptionsSetup, dictSettingsInitialize
+ankiInitialize, ankiTemplatesInitialize, storageInfoInitialize
+*/
+
 function getOptionsMutable(optionsContext) {
     return utilBackend().getOptions(
         utilBackgroundIsolate(optionsContext)
diff --git a/ext/bg/js/settings/popup-preview-frame.js b/ext/bg/js/settings/popup-preview-frame.js
index 37a4b416..042f335f 100644
--- a/ext/bg/js/settings/popup-preview-frame.js
+++ b/ext/bg/js/settings/popup-preview-frame.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiOptionsGet, Popup, PopupProxyHost, Frontend, TextSourceRange*/
 
 class SettingsPopupPreview {
     constructor() {
diff --git a/ext/bg/js/settings/profiles.js b/ext/bg/js/settings/profiles.js
index c4e68b53..3e589809 100644
--- a/ext/bg/js/settings/profiles.js
+++ b/ext/bg/js/settings/profiles.js
@@ -16,6 +16,10 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global getOptionsMutable, getOptionsFullMutable, settingsSaveOptions, apiOptionsGetFull
+utilBackgroundIsolate, formWrite
+conditionsClearCaches, ConditionsUI, profileConditionsDescriptor*/
+
 let currentProfileIndex = 0;
 let profileConditionsContainer = null;
 
diff --git a/ext/bg/js/settings/storage.js b/ext/bg/js/settings/storage.js
index 6c10f665..cbe1bb4d 100644
--- a/ext/bg/js/settings/storage.js
+++ b/ext/bg/js/settings/storage.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiGetEnvironmentInfo*/
 
 function storageBytesToLabeledString(size) {
     const base = 1000;
diff --git a/ext/bg/js/translator.js b/ext/bg/js/translator.js
index d32f8c67..81c2464b 100644
--- a/ext/bg/js/translator.js
+++ b/ext/bg/js/translator.js
@@ -16,6 +16,12 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global requestJson
+dictTermsMergeBySequence, dictTagBuildSource, dictTermsMergeByGloss, dictTermsSort, dictTagsSort
+dictEnabledSet, dictTermsGroup, dictTermsCompressTags, dictTermsUndupe, dictTagSanitize
+jpDistributeFurigana, jpConvertHalfWidthKanaToFullWidth, jpConvertNumericTofullWidth
+jpConvertAlphabeticToKana, jpHiraganaToKatakana, jpKatakanaToHiragana, jpIsCharCodeJapanese
+Database, Deinflector*/
 
 class Translator {
     constructor() {
diff --git a/ext/fg/js/document.js b/ext/fg/js/document.js
index 71654b29..7284cdd1 100644
--- a/ext/fg/js/document.js
+++ b/ext/fg/js/document.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global TextSourceElement, TextSourceRange, DOM*/
 
 const REGEX_TRANSPARENT_COLOR = /rgba\s*\([^)]*,\s*0(?:\.0+)?\s*\)/;
 
diff --git a/ext/fg/js/float.js b/ext/fg/js/float.js
index 8d61d8f6..d31b8336 100644
--- a/ext/fg/js/float.js
+++ b/ext/fg/js/float.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global popupNestedInitialize, Display*/
 
 class DisplayFloat extends Display {
     constructor() {
diff --git a/ext/fg/js/frontend-initialize.js b/ext/fg/js/frontend-initialize.js
index 9c923fea..c32e97d4 100644
--- a/ext/fg/js/frontend-initialize.js
+++ b/ext/fg/js/frontend-initialize.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global PopupProxyHost, PopupProxy, Frontend*/
 
 async function main() {
     const data = window.frontendInitializationData || {};
diff --git a/ext/fg/js/frontend.js b/ext/fg/js/frontend.js
index 2286bf19..3611d44e 100644
--- a/ext/fg/js/frontend.js
+++ b/ext/fg/js/frontend.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiGetZoom, apiOptionsGet, apiTermsFind, apiKanjiFind, docSentenceExtract, TextScanner*/
 
 class Frontend extends TextScanner {
     constructor(popup, ignoreNodes) {
diff --git a/ext/fg/js/popup-nested.js b/ext/fg/js/popup-nested.js
index 3f3c945e..3e5f5b80 100644
--- a/ext/fg/js/popup-nested.js
+++ b/ext/fg/js/popup-nested.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiOptionsGet*/
 
 let popupNestedInitialized = false;
 
diff --git a/ext/fg/js/popup-proxy-host.js b/ext/fg/js/popup-proxy-host.js
index 427172c6..98729796 100644
--- a/ext/fg/js/popup-proxy-host.js
+++ b/ext/fg/js/popup-proxy-host.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiFrameInformationGet, FrontendApiReceiver, Popup*/
 
 class PopupProxyHost {
     constructor() {
diff --git a/ext/fg/js/popup-proxy.js b/ext/fg/js/popup-proxy.js
index 63aa6bbe..db6dffb1 100644
--- a/ext/fg/js/popup-proxy.js
+++ b/ext/fg/js/popup-proxy.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global FrontendApiSender*/
 
 class PopupProxy {
     constructor(depth, parentId, parentFrameId, url) {
diff --git a/ext/fg/js/popup.js b/ext/fg/js/popup.js
index e7dae93e..0b142dda 100644
--- a/ext/fg/js/popup.js
+++ b/ext/fg/js/popup.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiInjectStylesheet*/
 
 class Popup {
     constructor(id, depth, frameIdPromise) {
diff --git a/ext/mixed/js/audio.js b/ext/mixed/js/audio.js
index b0c5fa82..76a3e7da 100644
--- a/ext/mixed/js/audio.js
+++ b/ext/mixed/js/audio.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global apiAudioGetUrl*/
 
 class TextToSpeechAudio {
     constructor(text, voice) {
diff --git a/ext/mixed/js/display-generator.js b/ext/mixed/js/display-generator.js
index d5ab9dbc..3617e546 100644
--- a/ext/mixed/js/display-generator.js
+++ b/ext/mixed/js/display-generator.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+/*global apiGetDisplayTemplatesHtml*/
 
 class DisplayGenerator {
     constructor() {
diff --git a/ext/mixed/js/display.js b/ext/mixed/js/display.js
index a6cfe848..b18e275d 100644
--- a/ext/mixed/js/display.js
+++ b/ext/mixed/js/display.js
@@ -16,6 +16,11 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global docRangeFromPoint, docSentenceExtract
+apiKanjiFind, apiTermsFind, apiNoteView, apiOptionsGet, apiDefinitionsAddable, apiDefinitionAdd
+apiScreenshotGet, apiForward
+audioPrepareTextToSpeech, audioGetFromSources
+DisplayGenerator, WindowScroll, DisplayContext, DOM*/
 
 class Display {
     constructor(spinner, container) {
diff --git a/ext/mixed/js/text-scanner.js b/ext/mixed/js/text-scanner.js
index 88f1e27a..e6da1e5e 100644
--- a/ext/mixed/js/text-scanner.js
+++ b/ext/mixed/js/text-scanner.js
@@ -16,6 +16,7 @@
  * along with this program.  If not, see <https://www.gnu.org/licenses/>.
  */
 
+/*global docRangeFromPoint, TextSourceRange, DOM*/
 
 class TextScanner {
     constructor(node, ignoreNodes, ignoreElements, ignorePoints) {