diff --git a/.eslintrc b/.eslintrc
index 18aacdab..807816fb 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -1,7 +1,22 @@
 {
-  "extends": [
-    "oclif",
-    "oclif-typescript",
-    "prettier"
-  ]
+  "extends": ["oclif", "oclif-typescript", "prettier"],
+  "rules": {
+    "unicorn/prefer-node-protocol": "off",
+    "node/no-extraneous-import": "off",
+    "unicorn/prefer-module": "off",
+    "unicorn/import-style": [
+      "error",
+      {
+        "styles": {
+          "path": {
+            "namespace": true
+          }
+        }
+      }
+    ],
+    "no-warning-comments": [
+      "warn",
+      { "terms": ["fixme", "xxx"], "location": "start" }
+    ]
+  }
 }
diff --git a/package.json b/package.json
index 36f3d9ed..b3131db4 100644
--- a/package.json
+++ b/package.json
@@ -20,14 +20,12 @@
     "update-gitmoji-config": "node ./scripts/update-gitmoji-config.js"
   },
   "devDependencies": {
-    "@types/jest-when": "^2.7.2",
     "chai": "^4.2.0",
     "eslint": "^7.32.0",
     "eslint-config-oclif": "^4.0.0",
     "eslint-config-oclif-typescript": "^1.0.2",
     "eslint-config-prettier": "^8.3.0",
     "husky": "^4.3.0",
-    "jest-when": "^3.0.1",
     "lint-staged": "^10.5.1",
     "prettier": "^2.1.1",
     "typescript": "^3.9"
diff --git a/packages/cli/package.json b/packages/cli/package.json
index e97d6627..6e6a4e58 100644
--- a/packages/cli/package.json
+++ b/packages/cli/package.json
@@ -36,7 +36,9 @@
   "devDependencies": {
     "@oclif/dev-cli": "^1",
     "@oclif/test": "^1",
-    "@types/node": "^10"
+    "@types/node": "^10",
+    "@types/jest-when": "^2.7.2",
+    "jest-when": "^3.0.1"
   },
   "engines": {
     "node": ">=8.0.0"
diff --git a/packages/cli/src/base.ts b/packages/cli/src/base.ts
index cd7c4295..94472b0c 100644
--- a/packages/cli/src/base.ts
+++ b/packages/cli/src/base.ts
@@ -8,7 +8,7 @@ const pkg = require("../package.json");
 export default abstract class Base extends Command {
   protected conf: undefined | null | ConfigType;
 
-  async init() {
+  async init(): Promise<void> {
     const notifier = updateNotifier({
       pkg,
       updateCheckInterval: 1000,
diff --git a/packages/cli/src/commands/__tests__/update.test.ts b/packages/cli/src/commands/__tests__/update.test.ts
index e17c1ae4..57c67aca 100644
--- a/packages/cli/src/commands/__tests__/update.test.ts
+++ b/packages/cli/src/commands/__tests__/update.test.ts
@@ -78,7 +78,6 @@ describe("update command", () => {
   const mockSheetLines3 = [{ key: "sheet3.line_1" }, { key: "sheet3.line_2" }];
 
   const test = oclifTest.register("setupMocks", () => ({
-    /* eslint-disable no-console */
     run() {
       ReaderMock.mockClear();
       WorksheetReaderMock.mockClear();
diff --git a/packages/cli/src/commands/open.ts b/packages/cli/src/commands/open.ts
index 5e11d630..a97741d8 100644
--- a/packages/cli/src/commands/open.ts
+++ b/packages/cli/src/commands/open.ts
@@ -17,7 +17,7 @@ class Open extends Base {
     id: cliFlags.id.flag(),
   };
 
-  async run() {
+  async run(): Promise<void> {
     const { flags } = this.parse(Open);
 
     const sheetId = flags.id;
diff --git a/packages/cli/src/commands/update.ts b/packages/cli/src/commands/update.ts
index c66ff241..1b4860f5 100644
--- a/packages/cli/src/commands/update.ts
+++ b/packages/cli/src/commands/update.ts
@@ -90,26 +90,23 @@ class Update extends Base {
       const allWorksheetLines = Object.values(linesByWorkshet).flat();
       const OTHER_DOMAIN = "__other__";
 
-      const linesByDomain = allWorksheetLines.reduce(
-        (domainLines: { [domain: string]: Line[] }, line: Line) => {
-          const domain =
-            domains.find((d) => line.key.startsWith(`${d}.`)) || OTHER_DOMAIN;
+      const linesByDomain: { [domain: string]: Line[] } = {};
 
-          domainLines[domain] = domainLines[domain] ?? [];
-          domainLines[domain].push(line);
+      for (const line of allWorksheetLines) {
+        const domain =
+          domains.find((d) => line.key.startsWith(`${d}.`)) || OTHER_DOMAIN;
 
-          return domainLines;
-        },
-        {}
-      );
+        linesByDomain[domain] = linesByDomain[domain] ?? [];
+        linesByDomain[domain].push(line);
+      }
 
-      domains
-        .filter((domain) => linesByDomain[domain].length === 0)
-        .forEach((domain) => {
+      for (const domain of domains) {
+        if (linesByDomain[domain].length === 0) {
           this.warn(
             `😐 Received no lines for language ${langName} and domain ${domain}`
           );
-        });
+        }
+      }
 
       return (
         Object.keys(linesByDomain)
@@ -128,7 +125,7 @@ class Update extends Base {
   }
 
   // eslint-disable-next-line complexity
-  async run() {
+  async run(): Promise<void> {
     const { flags } = this.parse(Update);
 
     const sheetId = flags.id ?? "";
@@ -161,14 +158,19 @@ class Update extends Base {
     try {
       worksheetReader = new WorksheetReader(sheets, { logger });
     } catch (error) {
-      if (error instanceof InvalidFilterError) {
-        throw new IncorrectFlagValue(error.message);
-      } else {
-        throw error;
-      }
+      const normalizedError =
+        error instanceof InvalidFilterError
+          ? new IncorrectFlagValue(error.message)
+          : error;
+
+      throw normalizedError;
     }
 
-    const plugins = loadPlugins(this.conf?.plugins, { logger }, { languages });
+    const plugins = loadPlugins(
+      this.conf?.plugins ?? [],
+      { logger },
+      { languages }
+    );
     const outputTransformer = transformersByFormat[format];
     const reader = new Reader(sheetId, worksheetReader, plugins, {
       logger,
diff --git a/packages/cli/src/flags/id.ts b/packages/cli/src/flags/id.ts
index 1a95b191..d452e648 100644
--- a/packages/cli/src/flags/id.ts
+++ b/packages/cli/src/flags/id.ts
@@ -14,7 +14,7 @@ export const flag = flags.build({
   },
 });
 
-export const invariant = (id?: string) => {
+export const invariant = (id?: string): void => {
   if (!id) {
     throw new MissingFlagValue("Sheet id");
   }
diff --git a/packages/cli/src/invariants.ts b/packages/cli/src/invariants.ts
index c557357c..f7c6dd57 100644
--- a/packages/cli/src/invariants.ts
+++ b/packages/cli/src/invariants.ts
@@ -1,14 +1,16 @@
 import { CLIError } from "@oclif/errors";
 
 export const cliInvariant = (
-  expression: any,
+  expression: unknown,
   message: string,
-  options: object = {}
-) => {
+  options: Record<string, any> = {}
+): void => {
   if (!expression) {
     throw new CLIError(message, options);
   }
 };
 
-export const noExitCliInvariant = (expression: any, message: string) =>
-  cliInvariant(expression, message, { exit: false });
+export const noExitCliInvariant = (
+  expression: unknown,
+  message: string
+): void => cliInvariant(expression, message, { exit: false });
diff --git a/packages/core/src/__tests__/writer.test.ts b/packages/core/src/__tests__/writer.test.ts
index a7e3a2b0..a793b114 100644
--- a/packages/core/src/__tests__/writer.test.ts
+++ b/packages/core/src/__tests__/writer.test.ts
@@ -2,9 +2,9 @@ import { EOL } from "os";
 import * as mkdirp from "mkdirp";
 
 const fs = {
-  accessAsync: jest.fn().mockResolvedValue(undefined),
+  accessAsync: jest.fn(),
   readFileAsync: jest.fn().mockResolvedValue(""),
-  writeFileAsync: jest.fn().mockResolvedValue(undefined),
+  writeFileAsync: jest.fn(),
 };
 
 jest.mock("fs");
@@ -29,6 +29,7 @@ const plugin = {
   pluginName: "test",
   transformFullOutput: jest.fn(),
   transformLine: jest.fn(),
+  readTranslation: jest.fn(),
 };
 const plugins = new PluginsRunner([plugin], { logger });
 
diff --git a/packages/core/src/line.ts b/packages/core/src/line.ts
index c03f9d9c..09c63fa6 100644
--- a/packages/core/src/line.ts
+++ b/packages/core/src/line.ts
@@ -1,7 +1,6 @@
-/* eslint-disable unicorn/filename-case */
 const COMMENT_STARTERS = ["//", "#"];
-const PLURAL_KEY_RE = /.+##\{(zero|one|two|few|many|other)\}/;
-const PLURAL_POSTFIX_RE = /##\{(zero|one|two|few|many|other)\}/;
+const PLURAL_KEY_RE = /.+##{(zero|one|two|few|many|other)}/;
+const PLURAL_POSTFIX_RE = /##{(zero|one|two|few|many|other)}/;
 
 enum Type {
   COMMENT,
@@ -45,71 +44,72 @@ class Line {
     this.value = value ?? "";
   }
 
-  static checkIsComment(val: any): boolean {
+  static checkIsComment(val: string): boolean {
     for (const commentStarter of COMMENT_STARTERS) {
       if (val.indexOf(commentStarter) === 0) {
         return true;
       }
     }
+
     return false;
   }
 
-  static checkIsPlural = function (val: any): boolean {
-    if (val.match(PLURAL_KEY_RE)) {
+  static checkIsPlural = function (val: string): boolean {
+    if (PLURAL_KEY_RE.test(val)) {
       return true;
     }
+
     return false;
   };
 
-  static parseKeysFromPlural = function (val: string) {
+  static parseKeysFromPlural = function (val: string): [string, string] {
     const match = val.match(PLURAL_POSTFIX_RE);
     if (match) {
       return [val.replace(match[0], ""), match[1]];
     }
+
     return [val, ""];
   };
 
-  static normalizeComment(val: string) {
+  static normalizeComment(val: string): string {
     for (const commentStarter of COMMENT_STARTERS) {
       const index = val.indexOf(commentStarter);
 
       if (index === 0) {
-        const normalized = val.substr(
-          commentStarter.length,
-          val.length - commentStarter.length
-        );
+        const normalized = val.slice(commentStarter.length);
 
         return normalized.trim();
       }
     }
+
     return val;
   }
 
-  isEmpty() {
+  isEmpty(): boolean {
     return !this.isComment() && !(this.key && this.value);
   }
 
-  isComment() {
+  isComment(): boolean {
     return this.type === Type.COMMENT;
   }
 
-  isPlural() {
+  isPlural(): boolean {
     return this.type === Type.PLURAL;
   }
 
-  getComment() {
+  getComment(): string {
     return this.key;
   }
 
-  getPluralKey() {
+  getPluralKey(): string {
     return this.pluralKey;
   }
 
-  setKey(nextKey: string | ((key: string) => string)) {
+  setKey(nextKey: string | ((key: string) => string)): void {
     this.key = typeof nextKey === "function" ? nextKey(this.key) : nextKey;
   }
 
-  setValue(nextValue: string | ((value: string) => string)) {
+  setValue(nextValue: string | ((value: string) => string)): void {
     this.value =
       typeof nextValue === "function" ? nextValue(this.value) : nextValue;
   }
diff --git a/packages/core/src/plugins/create.ts b/packages/core/src/plugins/create.ts
index 0061b80d..cb441063 100644
--- a/packages/core/src/plugins/create.ts
+++ b/packages/core/src/plugins/create.ts
@@ -57,7 +57,7 @@ const pluginDefaults: LoksePlugin = {
   readTranslation: identity,
 };
 
-export function createPlugin(plugin: Partial<LoksePlugin>) {
+export function createPlugin(plugin: Partial<LoksePlugin>): LoksePlugin {
   return {
     ...pluginDefaults,
     ...plugin,
diff --git a/packages/core/src/plugins/load.ts b/packages/core/src/plugins/load.ts
index 16df9a96..589aa97a 100644
--- a/packages/core/src/plugins/load.ts
+++ b/packages/core/src/plugins/load.ts
@@ -52,14 +52,14 @@ function loadPlugin(
 export type PluginName = string;
 export interface PluginDefinition {
   name: PluginName;
-  options: object;
+  options: Record<string, any>;
 }
 
 export function loadPlugins(
-  plugins: (PluginName | PluginDefinition)[] | unknown = [],
+  plugins: (PluginName | PluginDefinition)[] | unknown,
   options: GeneralPluginOptions,
   meta: GeneralPluginMeta
-) {
+): PluginsRunner {
   let loadedPlugins: NamedLoksePlugin[];
 
   if (Array.isArray(plugins)) {
diff --git a/packages/core/src/reader/__tests__/spreadsheet-reader.test.ts b/packages/core/src/reader/__tests__/spreadsheet-reader.test.ts
index 1ff36a16..9c5c864b 100644
--- a/packages/core/src/reader/__tests__/spreadsheet-reader.test.ts
+++ b/packages/core/src/reader/__tests__/spreadsheet-reader.test.ts
@@ -1,3 +1,4 @@
+/* eslint-disable camelcase */
 import { GoogleSpreadsheet } from "google-spreadsheet";
 import SpreadsheetReader from "../spreadsheet-reader";
 import WorksheetReader from "../worksheet-reader";
@@ -14,6 +15,18 @@ const GoogleSpreadsheetMock = GoogleSpreadsheet as jest.Mock;
 
 jest.mock("google-spreadsheet");
 
+const makeFakeLine = (id: string) => {
+  return { id: `line_${id}` } as unknown as Line;
+};
+
+const makeFakeWorksheet = (title: string, lines: Line[]) => {
+  const fakeWorksheet = {
+    title,
+    extractLines: jest.fn().mockReturnValue(lines),
+  };
+  return fakeWorksheet as unknown as Worksheet;
+};
+
 describe("SpreadsheetReader", () => {
   const testLogger = {
     log: jest.fn(),
@@ -90,18 +103,6 @@ describe("SpreadsheetReader", () => {
   });
 
   describe("read", () => {
-    const makeFakeLine = (id: string) => {
-      return { id: `line_${id}` } as unknown as Line;
-    };
-
-    const makeFakeWorksheet = (title: string, lines: Line[]) => {
-      const fakeWorksheet = {
-        title,
-        extractLines: jest.fn().mockReturnValue(lines),
-      };
-      return fakeWorksheet as unknown as Worksheet;
-    };
-
     const linesSet1 = [makeFakeLine("1_1"), makeFakeLine("1_2")];
 
     const linesSet2 = [
@@ -204,7 +205,7 @@ describe("SpreadsheetReader", () => {
       jest.spyOn(reader, "fetchSheets").mockResolvedValue(sheetsList);
 
       await expect(reader.read("key", "en-gb")).resolves.toEqual({
-        fakeSheet1: linesSet1.concat(linesSet3),
+        fakeSheet1: [...linesSet1, ...linesSet3],
         fakeSheet2: linesSet2,
       });
       expect(testLogger.warn).toHaveBeenCalledTimes(1);
diff --git a/packages/core/src/reader/__tests__/worksheet.test.ts b/packages/core/src/reader/__tests__/worksheet.test.ts
index 3a2be6cc..5d7bb75f 100644
--- a/packages/core/src/reader/__tests__/worksheet.test.ts
+++ b/packages/core/src/reader/__tests__/worksheet.test.ts
@@ -3,32 +3,32 @@ import { KeyColumnNotFound, LangColumnNotFound } from "../../errors";
 import { PluginsRunner } from "../../plugins";
 import Worksheet from "../worksheet";
 
+const createRow = (rowIndex: number, values: { [key: string]: any }) =>
+  ({
+    rowIndex,
+    ...values,
+    save: () => null,
+    delete: () => null,
+  } as unknown as GoogleSpreadsheetRow);
+
 describe("Worksheet", () => {
   const logger = { warn: jest.fn(), log: jest.fn() };
   const noPlugins = new PluginsRunner([], { logger });
 
-  const createRow = (rowIndex: number, values: { [key: string]: any }) =>
-    ({
-      rowIndex,
-      ...values,
-      save: () => null,
-      delete: () => null,
-    } as unknown as GoogleSpreadsheetRow);
-
   it("should extract lines", async () => {
     const worksheet = new Worksheet(
       "Worksheet1",
-      ["Key", "Value_fr", "Value_nl"],
+      ["Key", "Value-fr", "Value-nl"],
       [
         createRow(1, {
           Key: "MaClé1",
-          Value_fr: "La valeur 1",
-          Value_nl: "De valuue 1",
+          "Value-fr": "La valeur 1",
+          "Value-nl": "De valuue 1",
         }),
         createRow(2, {
           Key: "MaClé2",
-          Value_fr: "La vale de la clé 2",
-          Value_nl: "De valuee van key 2",
+          "Value-fr": "La vale de la clé 2",
+          "Value-nl": "De valuee van key 2",
         }),
         createRow(3, { Key: "// un commentaire" }),
         createRow(4, { Key: "une clée" }),
@@ -37,7 +37,7 @@ describe("Worksheet", () => {
       ]
     );
 
-    const lines = await worksheet.extractLines("Key", "Value_fr", noPlugins);
+    const lines = await worksheet.extractLines("Key", "Value-fr", noPlugins);
 
     expect(lines.length).toEqual(6);
     expect(lines[0].key).toEqual("MaClé1");
@@ -50,30 +50,30 @@ describe("Worksheet", () => {
   it("should throw when key column doesnt exist ", async () => {
     const worksheet = new Worksheet(
       "Worksheet2",
-      ["Key", "Value_fr", "Value_nl"],
+      ["Key", "Value-fr", "Value-nl"],
       [
         createRow(1, {
           Key: "MaClé1",
-          Value_fr: "La valeur 1",
-          Value_nl: "De valuue 1",
+          "Value-fr": "La valeur 1",
+          "Value-nl": "De valuue 1",
         }),
       ]
     );
 
     await expect(() =>
-      worksheet.extractLines("Wrong_Key", "Value_fr", noPlugins)
+      worksheet.extractLines("Wrong_Key", "Value-fr", noPlugins)
     ).rejects.toThrow(new KeyColumnNotFound("Wrong_Key", "Worksheet2").message);
   });
 
   it("should throw when lang column doesnt exist ", async () => {
     const worksheet = new Worksheet(
       "Worksheet2",
-      ["Key", "Value_fr", "Value_nl"],
+      ["Key", "Value-fr", "Value-nl"],
       [
         createRow(1, {
           Key: "MaClé1",
-          Value_fr: "La valeur 1",
-          Value_nl: "De valuue 1",
+          "Value-fr": "La valeur 1",
+          "Value-nl": "De valuue 1",
         }),
       ]
     );
@@ -86,18 +86,18 @@ describe("Worksheet", () => {
   it("should keep empty lines", async () => {
     const worksheet = new Worksheet(
       "Worksheet3",
-      ["Key", "Value_fr", "Value_nl"],
+      ["Key", "Value-fr", "Value-nl"],
       [
         createRow(1, {}),
         createRow(2, {
           Key: "MaClé1",
-          Value_fr: "La valeur 1",
-          Value_nl: "De valuue 1",
+          "Value-fr": "La valeur 1",
+          "Value-nl": "De valuue 1",
         }),
       ]
     );
 
-    const result = await worksheet.extractLines("Key", "Value_fr", noPlugins);
+    const result = await worksheet.extractLines("Key", "Value-fr", noPlugins);
 
     expect(result.length).toEqual(2);
     expect(result[0].isEmpty()).toEqual(true);
@@ -107,11 +107,11 @@ describe("Worksheet", () => {
   it("should match column names case insensitively", async () => {
     let worksheet = new Worksheet(
       "Worksheet4",
-      ["Key", "Value_FR"],
-      [createRow(1, { Key: "MaClé1", Value_FR: "La valeur 1" })]
+      ["Key", "Value-FR"],
+      [createRow(1, { Key: "MaClé1", "Value-FR": "La valeur 1" })]
     );
 
-    let result = await worksheet.extractLines("key", "value_fr", noPlugins);
+    let result = await worksheet.extractLines("key", "value-fr", noPlugins);
 
     expect(result.length).toEqual(1);
     expect(result[0].key).toEqual("MaClé1");
@@ -119,11 +119,11 @@ describe("Worksheet", () => {
 
     worksheet = new Worksheet(
       "Worksheet5",
-      ["key", "value_fr"],
-      [createRow(1, { key: "MaClé2", value_fr: "La valeur 2" })]
+      ["key", "value-fr"],
+      [createRow(1, { key: "MaClé2", "value-fr": "La valeur 2" })]
     );
 
-    result = await worksheet.extractLines("Key", "VALUE_FR", noPlugins);
+    result = await worksheet.extractLines("Key", "VALUE-FR", noPlugins);
 
     expect(result.length).toEqual(1);
     expect(result[0].key).toEqual("MaClé2");
diff --git a/packages/core/src/reader/spreadsheet-reader.ts b/packages/core/src/reader/spreadsheet-reader.ts
index 47c999fd..cfdb1d2b 100644
--- a/packages/core/src/reader/spreadsheet-reader.ts
+++ b/packages/core/src/reader/spreadsheet-reader.ts
@@ -35,14 +35,16 @@ export class SpreadsheetReader {
     this.worksheets = null;
   }
 
-  async authenticate() {
+  async authenticate(): Promise<void> {
     const { LOKSE_API_KEY, LOKSE_SERVICE_ACCOUNT_EMAIL, LOKSE_PRIVATE_KEY } =
       process.env;
 
     if (LOKSE_SERVICE_ACCOUNT_EMAIL && LOKSE_PRIVATE_KEY) {
       await this.spreadsheet.useServiceAccountAuth({
+        // eslint-disable-next-line camelcase
         client_email: LOKSE_SERVICE_ACCOUNT_EMAIL,
         // Treat new lines properly - https://stackoverflow.com/a/36439803/7051731
+        // eslint-disable-next-line camelcase
         private_key: LOKSE_PRIVATE_KEY.replace(/\\n/g, "\n"),
       });
     } else if (LOKSE_API_KEY) {
@@ -52,7 +54,7 @@ export class SpreadsheetReader {
     }
   }
 
-  async fetchSheets() {
+  async fetchSheets(): Promise<Worksheet[]> {
     if (!this.worksheets) {
       await this.authenticate();
 
@@ -68,44 +70,39 @@ export class SpreadsheetReader {
     return this.worksheets;
   }
 
-  async read(keyColumn: string, valueColumn: string) {
+  async read(
+    keyColumn: string,
+    valueColumn: string
+  ): Promise<WorksheetLinesByTitle> {
     const worksheets = await this.fetchSheets();
     const plugins = this.plugins;
 
-    const worksheetsLines = await worksheets.reduce<
-      Promise<WorksheetLinesByTitle>
-    >(
-      async (
-        worksheetLinesPromise: Promise<WorksheetLinesByTitle>,
-        worksheet
-      ) => {
-        const worksheetLines = await worksheetLinesPromise;
-        const { title } = worksheet;
-
-        try {
-          const lines = await worksheet.extractLines(
-            keyColumn,
-            valueColumn,
-            plugins
+    const worksheetsLines: WorksheetLinesByTitle = {};
+    const processWorksheetsPromises = worksheets.map(async (worksheet) => {
+      const { title } = worksheet;
+
+      try {
+        const lines = await worksheet.extractLines(
+          keyColumn,
+          valueColumn,
+          plugins
+        );
+
+        if (worksheetsLines[title]) {
+          this.logger.warn(
+            `🔀 Found two sheets with same title ${title}. We're gonna concat the data.`
           );
 
-          if (worksheetLines[title]) {
-            this.logger.warn(
-              `🔀 Found two sheets with same title ${title}. We're gonna concat the data.`
-            );
-
-            worksheetLines[title] = worksheetLines[title].concat(lines);
-          } else {
-            worksheetLines[title] = lines;
-          }
-        } catch (error) {
-          this.logger.warn(error.message);
+          worksheetsLines[title] = [...worksheetsLines[title], ...lines];
+        } else {
+          worksheetsLines[title] = lines;
         }
+      } catch (error) {
+        this.logger.warn(error.message);
+      }
+    });
 
-        return worksheetLines;
-      },
-      Promise.resolve({})
-    );
+    await Promise.all(processWorksheetsPromises);
 
     return worksheetsLines;
   }
diff --git a/packages/core/src/reader/worksheet-reader.ts b/packages/core/src/reader/worksheet-reader.ts
index b124a95e..a7df3714 100644
--- a/packages/core/src/reader/worksheet-reader.ts
+++ b/packages/core/src/reader/worksheet-reader.ts
@@ -12,7 +12,7 @@ import type { Logger } from "../logger";
 export class InvalidFilterError extends Error {
   public filterStringified: string;
 
-  constructor(filter: any) {
+  constructor(filter: unknown) {
     const filterStringified = JSON.stringify(filter);
     super(
       `💥 Invalid sheets filter provided: ${filterStringified}. Look at the supported filter format reference.`
@@ -89,7 +89,7 @@ class WorksheetReader {
   static isSheetInTheList(
     worksheet: GoogleSpreadsheetWorksheet,
     list: SheetIndexOrTitle[]
-  ) {
+  ): boolean {
     return list.some((sheetFilter: string | number) => {
       if (sheetFilter === WorksheetReader.ALL_SHEETS_FILTER) {
         return true;
@@ -119,13 +119,13 @@ class WorksheetReader {
     return isIncluded && !isExcluded;
   }
 
-  async loadSheet(worksheet: GoogleSpreadsheetWorksheet) {
+  async loadSheet(worksheet: GoogleSpreadsheetWorksheet): Promise<Worksheet> {
     const rows = await worksheet.getRows();
 
     return new Worksheet(worksheet.title, worksheet.headerValues, rows);
   }
 
-  async read(spreadsheet: GoogleSpreadsheet) {
+  async read(spreadsheet: GoogleSpreadsheet): Promise<Worksheet[]> {
     const worksheets = spreadsheet.sheetsByIndex.filter((worksheet) =>
       this.shouldUseWorksheet(worksheet)
     );
@@ -148,7 +148,9 @@ class WorksheetReader {
       this.logger.warn(`${message}. `);
     }
 
-    const sheets = await all(worksheets.map(this.loadSheet));
+    const sheets = await all(
+      worksheets.map((worksheet) => this.loadSheet(worksheet))
+    );
 
     return sheets;
   }
diff --git a/packages/core/src/reader/worksheet.ts b/packages/core/src/reader/worksheet.ts
index d4ea049f..c2c15fb8 100644
--- a/packages/core/src/reader/worksheet.ts
+++ b/packages/core/src/reader/worksheet.ts
@@ -22,7 +22,7 @@ export default class Worksheet {
     keyColumn: string,
     langColumn: string,
     plugins: PluginsRunner
-  ) {
+  ): Promise<Line[]> {
     let keyColumnId = "";
     let langColumnId = "";
 
@@ -30,6 +30,7 @@ export default class Worksheet {
       if (isEqualCaseInsensitive(headerKey, keyColumn)) {
         keyColumnId = headerKey;
       }
+
       if (isEqualCaseInsensitive(headerKey, langColumn)) {
         langColumnId = headerKey;
       }
diff --git a/packages/core/src/transformer/android.ts b/packages/core/src/transformer/android.ts
index be80d40e..ffdbfc55 100644
--- a/packages/core/src/transformer/android.ts
+++ b/packages/core/src/transformer/android.ts
@@ -4,7 +4,7 @@ import { Transformer } from "./transformer";
 
 function setCharAt(str: string, index: number, chr: number) {
   if (index > str.length - 1) return str;
-  return str.substr(0, index) + chr + str.substr(index + 1);
+  return str.slice(0, index) + chr + str.slice(index + 1);
 }
 
 const androidTransformer: Transformer = {
@@ -15,7 +15,7 @@ const androidTransformer: Transformer = {
   transformKeyValue(key, value) {
     let normalizedValue = value.replace(/%newline%/gi, "\\n");
     normalizedValue = normalizedValue.replace(/'/gi, "\\'");
-    normalizedValue = normalizedValue.replace(/%([sdf])/gi, "%#$$$1");
+    normalizedValue = normalizedValue.replace(/%([dfs])/gi, "%#$$$1");
     normalizedValue = normalizedValue.replace(/&/gi, "&amp;");
     normalizedValue = normalizedValue.replace(/\u00A0/gi, "\\u00A0");
     normalizedValue = normalizedValue.replace(
@@ -40,10 +40,10 @@ const androidTransformer: Transformer = {
   transformPluralsValues(key, values) {
     let ouput = '    <plurals name="' + key + '">' + EOL;
 
-    for (let i = 0; i < values.length; i++) {
-      let normalizedValue = values[i].value.replace(/%newline%/gi, "\\n");
+    for (const value of values) {
+      let normalizedValue = value.value.replace(/%newline%/gi, "\\n");
       normalizedValue = normalizedValue.replace(/'/gi, "\\'");
-      normalizedValue = normalizedValue.replace(/%([sdf])/gi, "%#$$$1");
+      normalizedValue = normalizedValue.replace(/%([dfs])/gi, "%#$$$1");
       normalizedValue = normalizedValue.replace(/&/gi, "&amp;");
       normalizedValue = normalizedValue.replace(/\u00A0/gi, "\\u00A0");
       normalizedValue = normalizedValue.replace(
@@ -53,7 +53,7 @@ const androidTransformer: Transformer = {
 
       ouput +=
         '        <item quantity="' +
-        values[i].getPluralKey() +
+        value.getPluralKey() +
         '">' +
         normalizedValue +
         "</item>" +
@@ -88,11 +88,10 @@ const androidTransformer: Transformer = {
         '<?xml version="1.0" encoding="utf-8"?>' + EOL + "<resources>" + EOL;
     } else {
       const autoGeneratedIndex = input.indexOf(AUTOGENERATED_TAG as string);
-      if (autoGeneratedIndex >= 0) {
-        output = input.substr(0, autoGeneratedIndex);
-      } else {
-        output = input.substr(0, closeTagIndex);
-      }
+      output = input.slice(
+        0,
+        autoGeneratedIndex >= 0 ? autoGeneratedIndex : closeTagIndex
+      );
     }
 
     output += AUTOGENERATED_TAG + EOL + newValues + EOL + "</resources>";
diff --git a/packages/core/src/transformer/ios.ts b/packages/core/src/transformer/ios.ts
index fe13bf8a..fd93ffdd 100644
--- a/packages/core/src/transformer/ios.ts
+++ b/packages/core/src/transformer/ios.ts
@@ -25,7 +25,7 @@ const iOSTransformer: Transformer = {
       iOSTransformer.AUTOGENERATED_TAG as string
     );
     if (generatedIndex >= 0) {
-      input = input.substr(0, generatedIndex);
+      input = input.slice(0, generatedIndex);
     }
 
     const output = input + iOSTransformer.AUTOGENERATED_TAG + EOL + newValues;
diff --git a/packages/core/src/transformer/json.ts b/packages/core/src/transformer/json.ts
index cdfac44c..03f8c6f9 100644
--- a/packages/core/src/transformer/json.ts
+++ b/packages/core/src/transformer/json.ts
@@ -18,7 +18,7 @@ const jsonTransformer: Transformer = {
     return `  "${key}" : "${normalizedValue}",`;
   },
   async insert(_, newValues) {
-    newValues = newValues.substring(0, newValues.length - 1);
+    newValues = newValues.slice(0, -1);
 
     return `${EOL}{${EOL}${newValues}${EOL}}`;
   },
diff --git a/packages/core/src/utils.ts b/packages/core/src/utils.ts
index 9395eedc..e27ced26 100644
--- a/packages/core/src/utils.ts
+++ b/packages/core/src/utils.ts
@@ -1,8 +1,10 @@
-export const forceArray = <T>(val: T | T[] | null) => {
+export const forceArray = <T>(val: T | T[] | null): T[] => {
   if (Array.isArray(val)) return val;
   if (!val) return [];
   return [val];
 };
 
-export const isEqualCaseInsensitive = (string1: string, string2: string) =>
-  string1.toLowerCase() === string2.toLowerCase();
+export const isEqualCaseInsensitive = (
+  string1: string,
+  string2: string
+): boolean => string1.toLowerCase() === string2.toLowerCase();
diff --git a/packages/core/src/writer.ts b/packages/core/src/writer.ts
index 79d4baaa..7416819e 100644
--- a/packages/core/src/writer.ts
+++ b/packages/core/src/writer.ts
@@ -24,7 +24,7 @@ class FileWriter {
     lines: Line[],
     transformer: Transformer,
     encoding = "utf8"
-  ) {
+  ): Promise<string> {
     let fileContent = "";
     const { language, domain, outputDir } = fileInfo;
     const fileName = transformer.getFileName(language, domain);
@@ -62,7 +62,7 @@ class FileWriter {
     lines: Line[],
     transformer: Transformer,
     meta: TransformLineMeta
-  ) {
+  ): Promise<string> {
     let valueToInsert = "";
 
     const plurals: { [pluralKey: string]: Line[] } = {};
@@ -88,6 +88,7 @@ class FileWriter {
         if (!plurals[line.key]) {
           plurals[line.key] = [];
         }
+
         plurals[line.key].push(line);
       } else {
         valueToInsert += transformer.transformKeyValue(line.key, line.value);
@@ -106,6 +107,7 @@ class FileWriter {
         if (typeof transformer.transformPluralsValues === "function") {
           return transformer.transformPluralsValues(key, plural);
         }
+
         return "";
       })
       .join(EOL);
diff --git a/packages/plugin-fallback/src/__tests__/index.test.ts b/packages/plugin-fallback/src/__tests__/index.test.ts
index bfd5dd54..a2fed528 100644
--- a/packages/plugin-fallback/src/__tests__/index.test.ts
+++ b/packages/plugin-fallback/src/__tests__/index.test.ts
@@ -1,4 +1,4 @@
-import { GoogleSpreadsheetRow } from "google-spreadsheet";
+import type { GoogleSpreadsheetRow } from "google-spreadsheet";
 import { Line } from "@lokse/core";
 
 import fallbackPluginFactory from "..";
diff --git a/packages/plugin-fallback/src/index.ts b/packages/plugin-fallback/src/index.ts
index 98e14609..d27d2578 100644
--- a/packages/plugin-fallback/src/index.ts
+++ b/packages/plugin-fallback/src/index.ts
@@ -16,7 +16,7 @@ export default function (
   options: PluginOptions,
   { languages }: GeneralPluginMeta
 ): LoksePlugin {
-  const { defaultLanguage } = options;
+  const { defaultLanguage, logger } = options;
 
   if (!defaultLanguage) {
     throw new PluginError("Default language must be supplied");
@@ -39,14 +39,13 @@ export default function (
     async readTranslation(line, meta) {
       if (line.key && !line.value) {
         const defaultLanguageKey =
-          Object.keys(meta.row).find(isDefaultLang) ?? NOT_FOUND_KEY;
+          Object.keys(meta.row).find((key) => isDefaultLang(key)) ??
+          NOT_FOUND_KEY;
 
         const fallbackLanguageValue = meta.row[defaultLanguageKey] ?? "";
 
         if (logMissingFallback && !fallbackLanguageValue) {
-          options.logger.warn(
-            `Fallback translation of key "${meta.key}" not found`
-          );
+          logger.warn(`Fallback translation of key "${meta.key}" not found`);
         }
 
         line.setValue(fallbackLanguageValue);
diff --git a/packages/plugin-non-breaking-spaces/package.json b/packages/plugin-non-breaking-spaces/package.json
index d66a523b..f17c50e2 100644
--- a/packages/plugin-non-breaking-spaces/package.json
+++ b/packages/plugin-non-breaking-spaces/package.json
@@ -8,7 +8,8 @@
   },
   "bugs": "https://github.com/AckeeCZ/lokse/issues",
   "dependencies": {
-    "@lokse/core": "^2.1.3"
+    "@lokse/core": "^2.1.3",
+    "lodash": "^4.17.20"
   },
   "engines": {
     "node": ">=8.0.0"
diff --git a/packages/plugin-non-breaking-spaces/src/index.ts b/packages/plugin-non-breaking-spaces/src/index.ts
index b502d7ec..e14690d0 100644
--- a/packages/plugin-non-breaking-spaces/src/index.ts
+++ b/packages/plugin-non-breaking-spaces/src/index.ts
@@ -11,8 +11,8 @@ export interface CustomPatterns {
 }
 
 const defaultPatterns: Patterns = {
-  cs: /(\s|^)(a|i|k|o|s|u|v|z)(\s+)/gim,
-  "cs-cz": /(\s|^)(a|i|k|o|s|u|v|z)(\s+)/gim,
+  cs: /(\s|^)([aikosuvz])(\s+)/gim,
+  "cs-cz": /(\s|^)([aikosuvz])(\s+)/gim,
 };
 
 export interface PluginOptions extends GeneralPluginOptions {
diff --git a/packages/plugin-non-breaking-spaces/src/utils.ts b/packages/plugin-non-breaking-spaces/src/utils.ts
index 9372b84a..d99f742d 100644
--- a/packages/plugin-non-breaking-spaces/src/utils.ts
+++ b/packages/plugin-non-breaking-spaces/src/utils.ts
@@ -2,11 +2,14 @@ import { mapKeys, mapValues } from "lodash";
 
 import { CustomPatterns } from ".";
 
-export const lowerCaseKeys = (patterns: CustomPatterns) =>
-  mapKeys(patterns, (_, key) => key.toLowerCase());
+export const lowerCaseKeys = (
+  patterns: CustomPatterns
+): Record<string, string> => mapKeys(patterns, (_, key) => key.toLowerCase());
 
 const convertStringToRegex = (string: string) =>
   new RegExp(`(\\s|^)${string}(\\s+)`, "gim");
 
-export const regexifyValues = (patterns: CustomPatterns) =>
+export const regexifyValues = (
+  patterns: CustomPatterns
+): Record<string, RegExp> =>
   mapValues(patterns, (value) => convertStringToRegex(value));