diff --git a/.eslintignore b/.eslintignore
new file mode 100644
index 0000000..378eac2
--- /dev/null
+++ b/.eslintignore
@@ -0,0 +1 @@
+build
diff --git a/.gitignore b/.gitignore
index 3c3629e..dd87e2d 100644
--- a/.gitignore
+++ b/.gitignore
@@ -1 +1,2 @@
 node_modules
+build
diff --git a/.travis.yml b/.travis.yml
index 21d7f7c..6535747 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -3,4 +3,5 @@ node_js:
 - '6'
 - '8'
 script:
+- npm build
 - npm test
diff --git a/README.md b/README.md
index 0a26ec4..dc4b120 100644
--- a/README.md
+++ b/README.md
@@ -8,6 +8,7 @@ Config is described with a combination of a functions:
 var parser = root(section({
     system: section({
         parallelLimit: option({
+            defaultValue: 0,
             parseEnv: Number,
             parseCli: Number,
             validate: function() {...}
diff --git a/lib/core.ts b/lib/core.ts
index 34f1d0f..73bc74f 100644
--- a/lib/core.ts
+++ b/lib/core.ts
@@ -1,24 +1,36 @@
-const _ = require('lodash');
-const {buildLazyObject, forceParsing} = require('./lazy');
-const {MissingOptionError, UnknownKeysError} = require('./errors');
-const initLocator = require('./locator');
+import _ from 'lodash';
+
+import {MissingOptionError, UnknownKeysError} from './errors';
+import {buildLazyObject, forceParsing} from './lazy';
+import initLocator from './locator';
+
+import type {Rooted, Parser} from './types/common';
+import type {LazyObject} from './types/lazy';
+import type {Locator} from './types/locator';
+import type {MapParser} from './types/map';
+import type {OptionParser, OptionParserConfig} from './types/option';
+import type {RootParser, RootPrefixes, ConfigParser} from './types/root';
+import type {SectionParser, SectionProperties} from './types/section';
+import type {Map} from './types/utils';
 
 /**
  * Single option
  */
-function option({
+export function option<Value, Result, MappedValue = Value>({
     defaultValue,
     parseCli = _.identity,
     parseEnv = _.identity,
     validate = _.noop,
     map: mapFunc = _.identity,
     isDeprecated = false
-}) {
+}: OptionParserConfig<Value, MappedValue, Result> = {}): OptionParser<MappedValue, Result> {
+    const validateFunc: typeof validate = validate;
+
     return (locator, parsed) => {
         const config = parsed.root;
         const currNode = locator.parent ? _.get(config, locator.parent) : config;
 
-        let value, isSetByUser = true;
+        let value: unknown, isSetByUser = true;
         if (locator.cliOption !== undefined) {
             value = parseCli(locator.cliOption);
         } else if (locator.envVar !== undefined) {
@@ -38,7 +50,7 @@ function option({
             console.warn(`Using "${locator.name}" option is deprecated`);
         }
 
-        validate(value, config, currNode, {isSetByUser});
+        validateFunc(value, config, currNode, {isSetByUser});
 
         return mapFunc(value, config, currNode, {isSetByUser});
     };
@@ -48,13 +60,15 @@ function option({
  * Object with fixed properties.
  * Any unknown property will be reported as error.
  */
-function section(properties) {
-    const expectedKeys = _.keys(properties);
+export function section<Config, Result>(properties: SectionProperties<Config, Result>): SectionParser<Config, Result> {
+    const expectedKeys = _.keys(properties) as Array<keyof Config>;
+
     return (locator, config) => {
         const unknownKeys = _.difference(
             _.keys(locator.option),
-            expectedKeys
+            expectedKeys as Array<string>
         );
+
         if (unknownKeys.length > 0) {
             throw new UnknownKeysError(
                 unknownKeys.map((key) => `${locator.name}.${key}`)
@@ -63,6 +77,7 @@ function section(properties) {
 
         const lazyResult = buildLazyObject(expectedKeys, (key) => {
             const parser = properties[key];
+
             return () => parser(locator.nested(key), config);
         });
 
@@ -76,32 +91,34 @@ function section(properties) {
  * Object with user-specified keys and values,
  * parsed by valueParser.
  */
-function map(valueParser, defaultValue) {
+export function map<SubConfig, Result>(
+    valueParser: Parser<SubConfig, Result>,
+    defaultValue: Map<SubConfig>
+): MapParser<Map<SubConfig>, Result> {
     return (locator, config) => {
         if (locator.option === undefined) {
             if (!defaultValue) {
-                return {};
+                return {} as LazyObject<Map<SubConfig>>;
             }
             locator = locator.resetOption(defaultValue);
         }
 
-        const optionsToParse = Object.keys(locator.option);
-        const lazyResult = buildLazyObject(optionsToParse, (key) => {
+        const optionsToParse = Object.keys(locator.option as Map<SubConfig>);
+        const lazyResult = buildLazyObject<Map<SubConfig>>(optionsToParse, (key) => {
             return () => valueParser(locator.nested(key), config);
         });
+
         _.set(config, locator.name, lazyResult);
 
         return lazyResult;
     };
 }
 
-function root(rootParser, {envPrefix, cliPrefix}) {
+export function root<Config, Result = Config>(rootParser: RootParser<Config, Result>, {envPrefix, cliPrefix}: RootPrefixes = {}): ConfigParser<Config> {
     return ({options, env, argv}) => {
         const rootLocator = initLocator({options, env, argv, envPrefix, cliPrefix});
-        const parsed = {};
-        rootParser(rootLocator, parsed);
-        return forceParsing(parsed.root);
+        const parsed = rootParser(rootLocator as Locator<Config>, {} as Rooted<Result>);
+
+        return forceParsing(parsed);
     };
 }
-
-module.exports = {option, section, map, root};
diff --git a/lib/errors.ts b/lib/errors.ts
index 69d1daf..db41cd1 100644
--- a/lib/errors.ts
+++ b/lib/errors.ts
@@ -1,5 +1,7 @@
-class MissingOptionError extends Error {
-    constructor(optionName) {
+export class MissingOptionError extends Error {
+    optionName: string;
+
+    constructor(optionName: string) {
         const message = `${optionName} is required`;
         super(message);
         this.name = 'MissingOptionError';
@@ -10,8 +12,10 @@ class MissingOptionError extends Error {
     }
 }
 
-class UnknownKeysError extends Error {
-    constructor(keys) {
+export class UnknownKeysError extends Error {
+    keys: Array<string>;
+
+    constructor(keys: Array<string>) {
         const message = `Unknown options: ${keys.join(', ')}`;
         super(message);
         this.name = 'UnknownKeysError';
@@ -21,5 +25,3 @@ class UnknownKeysError extends Error {
         Error.captureStackTrace(this, UnknownKeysError);
     }
 }
-
-module.exports = {MissingOptionError, UnknownKeysError};
diff --git a/lib/index.ts b/lib/index.ts
index 76fbcbc..e1ce12a 100644
--- a/lib/index.ts
+++ b/lib/index.ts
@@ -1,7 +1,2 @@
-const {root, section, map, option} = require('./core');
-const {MissingOptionError, UnknownKeysError} = require('./errors');
-
-module.exports = {
-    root, section, map, option,
-    MissingOptionError, UnknownKeysError
-};
+export {root, section, map, option} from './core';
+export {MissingOptionError, UnknownKeysError} from './errors';
diff --git a/lib/lazy.ts b/lib/lazy.ts
index 1af3c3b..d1ee3fc 100644
--- a/lib/lazy.ts
+++ b/lib/lazy.ts
@@ -1,38 +1,50 @@
-const _ = require('lodash');
+import _ from 'lodash';
 
-const isLazy = Symbol('isLazy');
+import type {LazyObject} from './types/lazy';
 
-function buildLazyObject(keys, getKeyGetter) {
+export const isLazy = Symbol('isLazy');
+
+type SimpleOrLazyObject<T> = T | LazyObject<T>;
+
+export function buildLazyObject<T>(keys: Array<keyof T>, getKeyGetter: (key: keyof T) => () => (SimpleOrLazyObject<T[keyof T]>)): LazyObject<T> {
     const target = {
         [isLazy]: true
-    };
+    } as LazyObject<T>;
+
     for (const key of keys) {
         defineLazy(target, key, getKeyGetter(key));
     }
+
     return target;
 }
 
-function forceParsing(lazyObject) {
+export function forceParsing<T>(lazyObject: LazyObject<T>): T {
     return _.cloneDeep(lazyObject);
 }
 
-function defineLazy(object, key, getter) {
+function defineLazy<T>(object: LazyObject<T>, key: keyof T, getter: () => SimpleOrLazyObject<T[keyof T]>): void {
     let defined = false;
-    let value;
+    let value: T[keyof T];
 
     Object.defineProperty(object, key, {
-        get() {
+        get(): T[keyof T] {
             if (!defined) {
                 defined = true;
-                value = getter();
-                if (_.isObject(value) && value[isLazy]) {
-                    value = forceParsing(value);
-                }
+                const val = getter();
+
+                value = isLazyObject(val) ? forceParsing(val) : val;
             }
+
             return value;
         },
         enumerable: true
     });
 }
 
-module.exports = {forceParsing, buildLazyObject};
+function isLazyObject<T>(value: T): value is LazyObject<T> {
+    return _.isObject(value) && hasOwnProperty(value, isLazy) && value[isLazy] === true;
+}
+
+function hasOwnProperty<T extends object, K extends PropertyKey>(obj: T, prop: K): obj is T & Record<K, unknown> {
+    return Object.prototype.hasOwnProperty.call(obj, prop);
+}
diff --git a/lib/locator.ts b/lib/locator.ts
index 63f8207..dfcc7fb 100644
--- a/lib/locator.ts
+++ b/lib/locator.ts
@@ -1,6 +1,8 @@
-const _ = require('lodash');
+import _ from 'lodash';
 
-function parseArgv(argv, cliPrefix) {
+import type {LocatorArg, Locator, Node, Prefixes} from './types/locator';
+
+function parseArgv(argv: Array<string>, cliPrefix: string): Array<string> {
     return argv.reduce(function(argv, arg) {
         if (!arg.startsWith(cliPrefix) || !_.includes(arg, '=')) {
             return argv.concat(arg);
@@ -11,20 +13,22 @@ function parseArgv(argv, cliPrefix) {
         const value = parts.slice(1).join('=');
 
         return argv.concat(option, value);
-    }, []);
+    }, [] as Array<string>);
 }
 
-module.exports = function({options, env, argv, envPrefix = '', cliPrefix = '--'}) {
+export = function initLocator<Options>({options, env, argv, envPrefix = '', cliPrefix = '--'}: LocatorArg<Options>): Locator<Options> {
     const parsedArgv = parseArgv(argv, cliPrefix);
 
-    function getNested(option, {namePrefix, envPrefix, cliPrefix}) {
-        return (subKey) => {
-            const envName = envPrefix + _.snakeCase(subKey);
-            const cliFlag = cliPrefix + _.kebabCase(subKey);
+    function getNested<Options>(option: Options | undefined, {namePrefix, envPrefix, cliPrefix}: Prefixes) {
+        return <Key extends keyof Options>(subKey: Key): Locator<Options[Key]> => {
+            const stringSubKey = subKey.toString();
+
+            const envName = envPrefix + _.snakeCase(stringSubKey);
+            const cliFlag = cliPrefix + _.kebabCase(stringSubKey);
 
             const argIndex = parsedArgv.lastIndexOf(cliFlag);
-            const subOption = _.get(option, subKey);
-            const newName = namePrefix ? `${namePrefix}.${subKey}` : subKey;
+            const subOption: Options[Key] = _.get(option, subKey);
+            const newName = namePrefix ? `${namePrefix}.${stringSubKey}` : stringSubKey;
 
             return mkLocator(
                 {
@@ -43,11 +47,11 @@ module.exports = function({options, env, argv, envPrefix = '', cliPrefix = '--'}
         };
     }
 
-    function mkLocator(base, prefixes) {
+    function mkLocator<Options>(base: Node<Options>, prefixes: Prefixes): Locator<Options> {
         return _.extend(base, {
             nested: getNested(base.option, prefixes),
-            resetOption: function(newOptions) {
-                return _.extend({}, base, {
+            resetOption: function(newOptions: Options): Locator<Options> {
+                return _.extend({}, base as Locator<Options>, {
                     option: newOptions,
                     nested: getNested(newOptions, prefixes)
                 });
diff --git a/lib/types/common.ts b/lib/types/common.ts
new file mode 100644
index 0000000..1c2f130
--- /dev/null
+++ b/lib/types/common.ts
@@ -0,0 +1,12 @@
+import type {LazyObject} from './lazy';
+import type {MapParser} from './map';
+import type {OptionParser} from './option';
+import type {SectionParser} from './section';
+
+export type ParsedConfig<Config> = {[Key in keyof Config]: LazyObject<Config[Key]>};
+
+export type Parser<Config, Result> = OptionParser<Config, Result> | SectionParser<Config, Result> | MapParser<Config, Result>;
+
+export interface Rooted<T> {
+    root: T;
+}
diff --git a/lib/types/lazy.ts b/lib/types/lazy.ts
new file mode 100644
index 0000000..c449d5e
--- /dev/null
+++ b/lib/types/lazy.ts
@@ -0,0 +1,5 @@
+import type {isLazy} from '../lazy';
+
+export type LazyObject<T> = T & {
+    [isLazy]: true;
+};
diff --git a/lib/types/locator.ts b/lib/types/locator.ts
new file mode 100644
index 0000000..338e5f1
--- /dev/null
+++ b/lib/types/locator.ts
@@ -0,0 +1,20 @@
+import type {RootPrefixes, ConfigParserArg} from './root';
+
+export type LocatorArg<Config> = RootPrefixes & ConfigParserArg<Config>;
+
+export type Prefixes = Required<RootPrefixes> & {
+    namePrefix: string;
+};
+
+export interface Node<Options> {
+    name: string;
+    parent: string | null;
+    option: Options;
+    envVar?: string;
+    cliOption?: string;
+}
+
+export interface Locator<Options> extends Node<Options> {
+    nested<Key extends keyof Options>(key: Key): Locator<Options[Key]>;
+    resetOption(newOption: Options): Locator<Options>;
+}
diff --git a/lib/types/map.ts b/lib/types/map.ts
new file mode 100644
index 0000000..af1d0ab
--- /dev/null
+++ b/lib/types/map.ts
@@ -0,0 +1,7 @@
+import type {Rooted} from './common';
+import type {LazyObject} from './lazy';
+import type {Locator} from './locator';
+
+export interface MapParser<Config, Result> {
+    (locator: Locator<Config>, config: Rooted<Result>): LazyObject<Config>;
+}
diff --git a/lib/types/option.ts b/lib/types/option.ts
new file mode 100644
index 0000000..7c5a4e6
--- /dev/null
+++ b/lib/types/option.ts
@@ -0,0 +1,19 @@
+import type {Rooted} from './common';
+import type {Locator} from './locator';
+
+interface MetaInfo {
+    isSetByUser: boolean;
+}
+
+export interface OptionParserConfig<Value, MappedValue, Result> {
+    defaultValue?: Value | ((config: Result, currNode: any) => Value);
+    parseCli?: (input?: string) => Value | undefined;
+    parseEnv?: (input?: string) => Value | undefined;
+    validate?: (value: unknown, config: Result, currNode: any, meta: MetaInfo) => asserts value is Value;
+    map?(value: Value, config: Result, currNode: any, meta: MetaInfo): MappedValue;
+    isDeprecated?: boolean;
+}
+
+export interface OptionParser<Value, Result> {
+    (locator: Locator<Value>, config: Rooted<Result>): Value;
+}
diff --git a/lib/types/root.ts b/lib/types/root.ts
new file mode 100644
index 0000000..5a0d676
--- /dev/null
+++ b/lib/types/root.ts
@@ -0,0 +1,20 @@
+import type {MapParser} from './map';
+import type {SectionParser} from './section';
+import type {DeepPartial} from './utils';
+
+export interface ConfigParserArg<Options> {
+    options: Options;
+    env: NodeJS.ProcessEnv;
+    argv: NodeJS.Process['argv'];
+}
+
+export interface ConfigParser<Config> {
+    (arg: ConfigParserArg<DeepPartial<Config>>): Config;
+}
+
+export interface RootPrefixes {
+    envPrefix?: string;
+    cliPrefix?: string;
+}
+
+export type RootParser<Config, Result> = SectionParser<Config, Result> | MapParser<Config, Result>;
diff --git a/lib/types/section.ts b/lib/types/section.ts
new file mode 100644
index 0000000..5d19867
--- /dev/null
+++ b/lib/types/section.ts
@@ -0,0 +1,8 @@
+import type {Parser, Rooted} from './common';
+import type {LazyObject} from './lazy';
+import type {Locator} from './locator';
+
+export type SectionProperties<Config, Result> = {[Key in keyof Config]: Parser<Config[Key], Result>};
+export interface SectionParser<Config, Result> {
+    (locator: Locator<Config>, config: Rooted<Result>): LazyObject<Config>;
+}
diff --git a/lib/types/utils.ts b/lib/types/utils.ts
new file mode 100644
index 0000000..6da59cb
--- /dev/null
+++ b/lib/types/utils.ts
@@ -0,0 +1,5 @@
+export type DeepPartial<T> = {
+    [K in keyof T]?: T[K] extends object ? DeepPartial<T[K]> : T[K];
+}
+
+export type Map<T> = Record<string, T>;
diff --git a/package-lock.json b/package-lock.json
index 32e3f28..9ef080e 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -4,6 +4,24 @@
   "lockfileVersion": 1,
   "requires": true,
   "dependencies": {
+    "@tsconfig/recommended": {
+      "version": "1.0.1",
+      "resolved": "https://registry.npmjs.org/@tsconfig/recommended/-/recommended-1.0.1.tgz",
+      "integrity": "sha512-2xN+iGTbPBEzGSnVp/Hd64vKJCJWxsi9gfs88x4PPMyEjHJoA3o5BY9r5OLPHIZU2pAQxkSAsJFqn6itClP8mQ==",
+      "dev": true
+    },
+    "@types/lodash": {
+      "version": "4.14.176",
+      "resolved": "https://registry.npmjs.org/@types/lodash/-/lodash-4.14.176.tgz",
+      "integrity": "sha512-xZmuPTa3rlZoIbtDUyJKZQimJV3bxCmzMIO2c9Pz9afyDro6kr7R79GwcB6mRhuoPmV2p1Vb66WOJH7F886WKQ==",
+      "dev": true
+    },
+    "@types/node": {
+      "version": "12.20.55",
+      "resolved": "https://registry.npmjs.org/@types/node/-/node-12.20.55.tgz",
+      "integrity": "sha512-J8xLz7q2OFulZ2cyGTLE1TbbZcjpno7FaN6zdJNrgAdrJ+DZzh/uFR6YrTb4C+nXakvud8Q4+rbhoIWlYQbUFQ==",
+      "dev": true
+    },
     "acorn": {
       "version": "5.1.2",
       "resolved": "https://registry.npmjs.org/acorn/-/acorn-5.1.2.tgz",
@@ -16,7 +34,7 @@
       "integrity": "sha1-r9+UiPsezvyDSPb7IvRk4ypYs2s=",
       "dev": true,
       "requires": {
-        "acorn": "3.3.0"
+        "acorn": "^3.0.4"
       },
       "dependencies": {
         "acorn": {
@@ -33,10 +51,10 @@
       "integrity": "sha1-wG9Zh3jETGsWGrr+NGa4GtGBTtI=",
       "dev": true,
       "requires": {
-        "co": "4.6.0",
-        "fast-deep-equal": "1.0.0",
-        "json-schema-traverse": "0.3.1",
-        "json-stable-stringify": "1.0.1"
+        "co": "^4.6.0",
+        "fast-deep-equal": "^1.0.0",
+        "json-schema-traverse": "^0.3.0",
+        "json-stable-stringify": "^1.0.1"
       }
     },
     "ajv-keywords": {
@@ -69,7 +87,7 @@
       "integrity": "sha1-c9g7wmP4bpf4zE9rrhsOkKfSLIY=",
       "dev": true,
       "requires": {
-        "sprintf-js": "1.0.3"
+        "sprintf-js": "~1.0.2"
       }
     },
     "array-union": {
@@ -78,7 +96,7 @@
       "integrity": "sha1-mjRBDk9OPaI96jdb5b5w8kd47Dk=",
       "dev": true,
       "requires": {
-        "array-uniq": "1.0.3"
+        "array-uniq": "^1.0.1"
       }
     },
     "array-uniq": {
@@ -105,9 +123,9 @@
       "integrity": "sha1-Y/1D99weO7fONZR9uP42mj9Yx0s=",
       "dev": true,
       "requires": {
-        "chalk": "1.1.3",
-        "esutils": "2.0.2",
-        "js-tokens": "3.0.2"
+        "chalk": "^1.1.3",
+        "esutils": "^2.0.2",
+        "js-tokens": "^3.0.2"
       },
       "dependencies": {
         "chalk": {
@@ -116,11 +134,11 @@
           "integrity": "sha1-qBFcVeSnAv5NFQq9OHKCKn4J/Jg=",
           "dev": true,
           "requires": {
-            "ansi-styles": "2.2.1",
-            "escape-string-regexp": "1.0.5",
-            "has-ansi": "2.0.0",
-            "strip-ansi": "3.0.1",
-            "supports-color": "2.0.0"
+            "ansi-styles": "^2.2.1",
+            "escape-string-regexp": "^1.0.2",
+            "has-ansi": "^2.0.0",
+            "strip-ansi": "^3.0.0",
+            "supports-color": "^2.0.0"
           }
         },
         "strip-ansi": {
@@ -129,7 +147,7 @@
           "integrity": "sha1-ajhfuIU9lS1f8F0Oiq+UJ43GPc8=",
           "dev": true,
           "requires": {
-            "ansi-regex": "2.1.1"
+            "ansi-regex": "^2.0.0"
           }
         }
       }
@@ -146,7 +164,7 @@
       "integrity": "sha1-wHshHHyVLsH479Uad+8NHTmQopI=",
       "dev": true,
       "requires": {
-        "balanced-match": "1.0.0",
+        "balanced-match": "^1.0.0",
         "concat-map": "0.0.1"
       }
     },
@@ -162,7 +180,7 @@
       "integrity": "sha1-lAhe9jWB7NPaqSREqP6U6CV3dR8=",
       "dev": true,
       "requires": {
-        "callsites": "0.2.0"
+        "callsites": "^0.2.0"
       }
     },
     "callsites": {
@@ -177,12 +195,12 @@
       "integrity": "sha1-D2RYS6ZC8PKs4oBiefTwbKI61zw=",
       "dev": true,
       "requires": {
-        "assertion-error": "1.0.2",
-        "check-error": "1.0.2",
-        "deep-eql": "3.0.1",
-        "get-func-name": "2.0.0",
-        "pathval": "1.1.0",
-        "type-detect": "4.0.3"
+        "assertion-error": "^1.0.1",
+        "check-error": "^1.0.1",
+        "deep-eql": "^3.0.0",
+        "get-func-name": "^2.0.0",
+        "pathval": "^1.0.0",
+        "type-detect": "^4.0.0"
       }
     },
     "chalk": {
@@ -191,9 +209,9 @@
       "integrity": "sha512-0BMM/2hG3ZaoPfR6F+h/oWpZtsh3b/s62TjSM6MGCJWEbJDN1acqCXvyhhZsDSVFklpebUoQ5O1kKC7lOzrn9g==",
       "dev": true,
       "requires": {
-        "ansi-styles": "3.2.0",
-        "escape-string-regexp": "1.0.5",
-        "supports-color": "4.5.0"
+        "ansi-styles": "^3.1.0",
+        "escape-string-regexp": "^1.0.5",
+        "supports-color": "^4.0.0"
       },
       "dependencies": {
         "ansi-styles": {
@@ -202,7 +220,7 @@
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       }
     },
     "optionator": {
@@ -954,12 +972,12 @@
       "integrity": "sha1-NkxeQJ0/TWMB1sC0wFu6UBgK62Q=",
       "dev": true,
       "requires": {
-        "deep-is": "0.1.3",
-        "fast-levenshtein": "2.0.6",
-        "levn": "0.3.0",
-        "prelude-ls": "1.1.2",
-        "type-check": "0.3.2",
-        "wordwrap": "1.0.0"
+        "deep-is": "~0.1.3",
+        "fast-levenshtein": "~2.0.4",
+        "levn": "~0.3.0",
+        "prelude-ls": "~1.1.2",
+        "type-check": "~0.3.2",
+        "wordwrap": "~1.0.0"
       }
     },
     "os-tmpdir": {
@@ -1021,7 +1039,7 @@
       "integrity": "sha1-ITXW36ejWMBprJsXh3YogihFD/o=",
       "dev": true,
       "requires": {
-        "pinkie": "2.0.4"
+        "pinkie": "^2.0.0"
       }
     },
     "pluralize": {
@@ -1060,13 +1078,13 @@
       "integrity": "sha512-m+qzzcn7KUxEmd1gMbchF+Y2eIUbieUaxkWtptyHywrX0rE8QEYqPC07Vuy4Wm32/xE16NcdBctb8S0Xe/5IeQ==",
       "dev": true,
       "requires": {
-        "core-util-is": "1.0.2",
-        "inherits": "2.0.3",
-        "isarray": "1.0.0",
-        "process-nextick-args": "1.0.7",
-        "safe-buffer": "5.1.1",
-        "string_decoder": "1.0.3",
-        "util-deprecate": "1.0.2"
+        "core-util-is": "~1.0.0",
+        "inherits": "~2.0.3",
+        "isarray": "~1.0.0",
+        "process-nextick-args": "~1.0.6",
+        "safe-buffer": "~5.1.1",
+        "string_decoder": "~1.0.3",
+        "util-deprecate": "~1.0.1"
       }
     },
     "require-uncached": {
@@ -1075,8 +1093,8 @@
       "integrity": "sha1-Tg1W1slmL9MeQwEcS5WqSZVUIdM=",
       "dev": true,
       "requires": {
-        "caller-path": "0.1.0",
-        "resolve-from": "1.0.1"
+        "caller-path": "^0.1.0",
+        "resolve-from": "^1.0.0"
       }
     },
     "resolve-from": {
@@ -1091,8 +1109,8 @@
       "integrity": "sha1-n37ih/gv0ybU/RYpI9YhKe7g368=",
       "dev": true,
       "requires": {
-        "onetime": "2.0.1",
-        "signal-exit": "3.0.2"
+        "onetime": "^2.0.0",
+        "signal-exit": "^3.0.2"
       }
     },
     "rimraf": {
@@ -1101,7 +1119,7 @@
       "integrity": "sha512-lreewLK/BlghmxtfH36YYVg1i8IAce4TI7oao75I1g245+6BctqTVQiBP3YUJ9C6DQOXJmkYR9X9fCLtCOJc5w==",
       "dev": true,
       "requires": {
-        "glob": "7.1.2"
+        "glob": "^7.0.5"
       }
     },
     "run-async": {
@@ -1110,7 +1128,7 @@
       "integrity": "sha1-A3GrSuC91yDUFm19/aZP96RFpsA=",
       "dev": true,
       "requires": {
-        "is-promise": "2.1.0"
+        "is-promise": "^2.1.0"
       }
     },
     "rx-lite": {
@@ -1125,7 +1143,7 @@
       "integrity": "sha1-dTuHqJoRyVRnxKwWJsTvxOBcZ74=",
       "dev": true,
       "requires": {
-        "rx-lite": "4.0.8"
+        "rx-lite": "*"
       }
     },
     "safe-buffer": {
@@ -1152,7 +1170,7 @@
       "integrity": "sha1-RKrGW2lbAzmJaMOfNj/uXer98eo=",
       "dev": true,
       "requires": {
-        "shebang-regex": "1.0.0"
+        "shebang-regex": "^1.0.0"
       }
     },
     "shebang-regex": {
@@ -1173,16 +1191,16 @@
       "integrity": "sha512-4qIY0pCWCvGCJpV/1JkFu9kbsNEZ9O34cG1oru/c7OCDtrEs50Gq/VjkA2ID5ZwLyoNx1i1ws118oh/p6fVeDg==",
       "dev": true,
       "requires": {
-        "diff": "3.3.1",
+        "diff": "^3.1.0",
         "formatio": "1.2.0",
-        "lodash.get": "4.4.2",
-        "lolex": "2.1.3",
-        "native-promise-only": "0.8.1",
-        "nise": "1.1.1",
-        "path-to-regexp": "1.7.0",
-        "samsam": "1.3.0",
+        "lodash.get": "^4.4.2",
+        "lolex": "^2.1.3",
+        "native-promise-only": "^0.8.1",
+        "nise": "^1.1.1",
+        "path-to-regexp": "^1.7.0",
+        "samsam": "^1.1.3",
         "text-encoding": "0.6.4",
-        "type-detect": "4.0.3"
+        "type-detect": "^4.0.0"
       }
     },
     "slice-ansi": {
@@ -1191,7 +1209,7 @@
       "integrity": "sha512-POqxBK6Lb3q6s047D/XsDVNPnF9Dl8JSaqe9h9lURl0OdNqy/ujDrOiIHtsqXMGbWWTIomRzAMaTyawAU//Reg==",
       "dev": true,
       "requires": {
-        "is-fullwidth-code-point": "2.0.0"
+        "is-fullwidth-code-point": "^2.0.0"
       }
     },
     "sprintf-js": {
@@ -1200,23 +1218,23 @@
       "integrity": "sha1-BOaSb2YolTVPPdAVIDYzuFcpfiw=",
       "dev": true
     },
-    "string_decoder": {
-      "version": "1.0.3",
-      "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.0.3.tgz",
-      "integrity": "sha512-4AH6Z5fzNNBcH+6XDMfA/BTt87skxqJlO0lAh3Dker5zThcAxG6mKz+iGu308UKoPPQ8Dcqx/4JhujzltRa+hQ==",
-      "dev": true,
-      "requires": {
-        "safe-buffer": "5.1.1"
-      }
-    },
     "string-width": {
       "version": "2.1.1",
       "resolved": "https://registry.npmjs.org/string-width/-/string-width-2.1.1.tgz",
       "integrity": "sha512-nOqH59deCq9SRHlxq1Aw85Jnt4w6KvLKqWVik6oA9ZklXLNIOlqg4F2yrT1MVaTjAqvVwdfeZ7w7aCvJD7ugkw==",
       "dev": true,
       "requires": {
-        "is-fullwidth-code-point": "2.0.0",
-        "strip-ansi": "4.0.0"
+        "is-fullwidth-code-point": "^2.0.0",
+        "strip-ansi": "^4.0.0"
+      }
+    },
+    "string_decoder": {
+      "version": "1.0.3",
+      "resolved": "https://registry.npmjs.org/string_decoder/-/string_decoder-1.0.3.tgz",
+      "integrity": "sha512-4AH6Z5fzNNBcH+6XDMfA/BTt87skxqJlO0lAh3Dker5zThcAxG6mKz+iGu308UKoPPQ8Dcqx/4JhujzltRa+hQ==",
+      "dev": true,
+      "requires": {
+        "safe-buffer": "~5.1.0"
       }
     },
     "strip-ansi": {
@@ -1225,7 +1243,7 @@
       "integrity": "sha1-qEeQIusaw2iocTibY1JixQXuNo8=",
       "dev": true,
       "requires": {
-        "ansi-regex": "3.0.0"
+        "ansi-regex": "^3.0.0"
       },
       "dependencies": {
         "ansi-regex": {
@@ -1254,12 +1272,12 @@
       "integrity": "sha512-UUkEAPdSGxtRpiV9ozJ5cMTtYiqz7Ni1OGqLXRCynrvzdtR1p+cfOWe2RJLwvUG8hNanaSRjecIqwOjqeatDsA==",
       "dev": true,
       "requires": {
-        "ajv": "5.2.3",
-        "ajv-keywords": "2.1.0",
-        "chalk": "2.2.0",
-        "lodash": "4.17.4",
+        "ajv": "^5.2.3",
+        "ajv-keywords": "^2.1.0",
+        "chalk": "^2.1.0",
+        "lodash": "^4.17.4",
         "slice-ansi": "1.0.0",
-        "string-width": "2.1.1"
+        "string-width": "^2.1.1"
       }
     },
     "text-encoding": {
@@ -1271,7 +1289,7 @@
     "text-table": {
       "version": "0.2.0",
       "resolved": "https://registry.npmjs.org/text-table/-/text-table-0.2.0.tgz",
-      "integrity": "sha1-f17oI66AUgfACvLfSoTsP8+lcLQ=",
+      "integrity": "sha512-N+8UisAXDGk8PFXP4HAzVR9nbfmVJ3zYLAWiTIoqC5v5isinhr+r5uaO8+7r3BMfuNIufIsA7RdpVgacC2cSpw==",
       "dev": true
     },
     "through": {
@@ -1286,7 +1304,7 @@
       "integrity": "sha512-jRCJlojKnZ3addtTOjdIqoRuPEKBvNXcGYqzO6zWZX8KfKEpnGY5jfggJQ3EjKuu8D4bJRr0y+cYJFmYbImXGw==",
       "dev": true,
       "requires": {
-        "os-tmpdir": "1.0.2"
+        "os-tmpdir": "~1.0.2"
       }
     },
     "tryit": {
@@ -1301,7 +1319,7 @@
       "integrity": "sha1-WITKtRLPHTVeP7eE8wgEsrUg23I=",
       "dev": true,
       "requires": {
-        "prelude-ls": "1.1.2"
+        "prelude-ls": "~1.1.2"
       }
     },
     "type-detect": {
@@ -1316,6 +1334,12 @@
       "integrity": "sha1-hnrHTjhkGHsdPUfZlqeOxciDB3c=",
       "dev": true
     },
+    "typescript": {
+      "version": "4.8.3",
+      "resolved": "https://registry.npmjs.org/typescript/-/typescript-4.8.3.tgz",
+      "integrity": "sha512-goMHfm00nWPa8UvR/CPSvykqf6dVV8x/dp0c5mFTMTIu0u0FlGWRioyy7Nn0PGAdHxpJZnuO/ut+PpQ8UiHAig==",
+      "dev": true
+    },
     "util-deprecate": {
       "version": "1.0.2",
       "resolved": "https://registry.npmjs.org/util-deprecate/-/util-deprecate-1.0.2.tgz",
@@ -1328,7 +1352,7 @@
       "integrity": "sha512-xcJpopdamTuY5duC/KnTTNBraPK54YwpenP4lzxU8H91GudWpFv38u0CKjclE1Wi2EH2EDz5LRcHcKbCIzqGyg==",
       "dev": true,
       "requires": {
-        "isexe": "2.0.0"
+        "isexe": "^2.0.0"
       }
     },
     "wordwrap": {
@@ -1349,7 +1373,7 @@
       "integrity": "sha1-X8A4KOJkzqP+kUVUdvejxWbLB1c=",
       "dev": true,
       "requires": {
-        "mkdirp": "0.5.1"
+        "mkdirp": "^0.5.1"
       }
     },
     "yallist": {
diff --git a/package.json b/package.json
index a2e0e73..31ec7d0 100644
--- a/package.json
+++ b/package.json
@@ -2,24 +2,31 @@
   "name": "gemini-configparser",
   "version": "1.3.0",
   "description": "Config parser module for gemini",
-  "main": "lib/index.js",
-  "types": "./typings/index.d.ts",
+  "main": "./build/index.js",
+  "types": "./build/index.d.ts",
   "author": "Sergey Tatarintsev <sevinf@yandex-team.ru> (https://github.com/SevInf)",
   "license": "MIT",
   "engines": {
     "node": ">=12"
   },
   "devDependencies": {
+    "@tsconfig/recommended": "^1.0.1",
+    "@types/lodash": "^4.14.176",
+    "@types/node": "^12.20.55",
     "chai": "^4.1.2",
     "eslint": "^4.9.0",
     "eslint-config-gemini-testing": "^2.0.0",
     "mocha": "^4.0.1",
-    "sinon": "^4.0.1"
+    "sinon": "^4.0.1",
+    "typescript": "^4.8.3"
   },
   "dependencies": {
     "lodash": "^4.17.4"
   },
   "scripts": {
+    "build": "tsc --build",
+    "prepublishOnly": "npm run build",
+    "pretest": "npm run build",
     "test": "npm run lint && npm run test-unit",
     "lint": "eslint .",
     "test-unit": "mocha test"
diff --git a/test/lazy.js b/test/lazy.js
index 51f6378..69576e9 100644
--- a/test/lazy.js
+++ b/test/lazy.js
@@ -1,4 +1,4 @@
-const {buildLazyObject, forceParsing} = require('../lib/lazy');
+const {buildLazyObject, forceParsing} = require('../build/lazy');
 
 describe('build lazy object', () => {
     it('should build an object with given keys', () => {
diff --git a/test/locator.js b/test/locator.js
index 8cd49d5..a979f9f 100644
--- a/test/locator.js
+++ b/test/locator.js
@@ -1,4 +1,4 @@
-const locator = require('../lib/locator');
+const locator = require('../build/locator');
 
 function locatorWithOptions(options) {
     return locator({options, env: {}, argv: []});
diff --git a/test/map.js b/test/map.js
index bc1ff75..7d3dd8c 100644
--- a/test/map.js
+++ b/test/map.js
@@ -1,5 +1,5 @@
-const {map} = require('../lib/core');
-const {forceParsing} = require('../lib/lazy');
+const {map} = require('../build/core');
+const {forceParsing} = require('../build/lazy');
 const _ = require('lodash');
 
 describe('map', () => {
diff --git a/test/option.js b/test/option.js
index e304225..c793e78 100644
--- a/test/option.js
+++ b/test/option.js
@@ -1,5 +1,5 @@
-const {option} = require('../lib/core');
-const {MissingOptionError} = require('../lib/errors');
+const {option} = require('../build/core');
+const {MissingOptionError} = require('../build/errors');
 
 describe('option', () => {
     const LAZY_CONFIG = {
diff --git a/test/section.js b/test/section.js
index e20691a..4854307 100644
--- a/test/section.js
+++ b/test/section.js
@@ -1,5 +1,5 @@
-const {section} = require('../lib/core');
-const {forceParsing} = require('../lib/lazy');
+const {section} = require('../build/core');
+const {forceParsing} = require('../build/lazy');
 
 function stubLocator(locatorKeys) {
     return Object.assign({
diff --git a/tsconfig.json b/tsconfig.json
new file mode 100644
index 0000000..98a7554
--- /dev/null
+++ b/tsconfig.json
@@ -0,0 +1,15 @@
+{
+  "extends": "@tsconfig/recommended/tsconfig.json",
+  "include": ["lib"],
+  "exclude": ["build", "test"],
+  "compilerOptions": {
+    "target": "ES2019",
+    "outDir": "build",
+    "allowJs": true,
+    "noImplicitReturns": true,
+    "noUnusedLocals": true,
+    "noUnusedParameters": true,
+    "declaration": true,
+    "sourceMap": true
+  }
+}
diff --git a/typings/index.d.ts b/typings/index.d.ts
deleted file mode 100644
index 9bfe50e..0000000
--- a/typings/index.d.ts
+++ /dev/null
@@ -1,45 +0,0 @@
-declare module "gemini-configparser" {
-  type PartialConfig = never;
-  type Locator = never;
-  type UnsanitizedRootConfig = any;
-  type UnsanitizedConfigNode = any;
-
-  type Parser<T> = (locator: Locator, config: PartialConfig) => T;
-
-  type RootPrefixes = {
-      envPrefix: string;
-      cliPrefix: string;
-  };
-
-  type MetaInfo = {
-      isSetByUser: boolean;
-  };
-
-  type OptionParserConfig<T> = {
-      defaultValue: T | ((config: UnsanitizedRootConfig, currNode: UnsanitizedConfigNode) => T);
-      parseCli?(input: string): T;
-      parseEnv?(input: string): T;
-      validate?(value: T, config: UnsanitizedRootConfig, currNode: UnsanitizedConfigNode, meta: MetaInfo): void;
-  };
-
-  type MappedOptionParserConfig<S, T> = OptionParserConfig<S> & {
-      map(value: S, config: UnsanitizedRootConfig, currNode: UnsanitizedConfigNode, meta: MetaInfo): T;
-  };
-
-  type SectionProperties<T> = { [name in keyof T]: Parser<T[name]> };
-
-  export function option<T>(description: OptionParserConfig<T>): Parser<T>;
-  export function option<S, T>(description: MappedOptionParserConfig<S, T>): Parser<T>;
-  export function section<T>(properties: SectionProperties<T>): Parser<T>;
-
-  type RootParserArg = {
-      options: UnsanitizedRootConfig;
-      env: NodeJS.ProcessEnv;
-      argv: Array<string>;
-  };
-  type RootParser<T> = (arg: RootParserArg) => T;
-  export function root<T>(rootParser: Parser<T>, prefixes: RootPrefixes): RootParser<T>;
-
-  export class MissingOptionError extends Error {}
-  export class UnknownKeysError extends Error {}
-}