diff --git a/.eslintrc b/.eslintrc
index d004ea9..7e815e6 100644
--- a/.eslintrc
+++ b/.eslintrc
@@ -1,8 +1,7 @@
 {
-  "extends": [
-    "eslint-config-unjs"
-  ],
+  "extends": ["eslint-config-unjs"],
   "rules": {
-    "unicorn/prevent-abbreviations": 0
+    "unicorn/prevent-abbreviations": 0,
+    "@typescript-eslint/no-non-null-assertion": 0
   }
 }
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 3db5858..e1ff5e5 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -21,5 +21,6 @@ jobs:
       - run: pnpm install
       - run: pnpm lint
       - run: pnpm build
+      - run: pnpm test:types
       - run: pnpm vitest --coverage
       - uses: codecov/codecov-action@v3
diff --git a/package.json b/package.json
index da1deba..6303352 100644
--- a/package.json
+++ b/package.json
@@ -26,7 +26,8 @@
     "lint:fix": "eslint --ext .ts,.js,.mjs,.cjs . --fix && prettier -w src test",
     "prepack": "unbuild",
     "release": "changelogen --release && npm publish && git push --follow-tags",
-    "test": "vitest run --coverage"
+    "test": "vitest run --coverage && pnpm test:types",
+    "test:types": "tsc --noEmit"
   },
   "dependencies": {
     "defu": "^6.1.2",
@@ -43,6 +44,7 @@
     "changelogen": "^0.5.1",
     "eslint": "^8.36.0",
     "eslint-config-unjs": "^0.1.0",
+    "expect-type": "^0.15.0",
     "prettier": "^2.8.4",
     "typescript": "^4.9.5",
     "unbuild": "^1.1.2",
diff --git a/pnpm-lock.yaml b/pnpm-lock.yaml
index 38144f8..00d67c4 100644
--- a/pnpm-lock.yaml
+++ b/pnpm-lock.yaml
@@ -7,6 +7,7 @@ specifiers:
   dotenv: ^16.0.3
   eslint: ^8.36.0
   eslint-config-unjs: ^0.1.0
+  expect-type: ^0.15.0
   giget: ^1.1.2
   jiti: ^1.17.2
   mlly: ^1.2.0
@@ -33,6 +34,7 @@ devDependencies:
   changelogen: 0.5.1
   eslint: 8.36.0
   eslint-config-unjs: 0.1.0_vgl77cfdswitgr47lm5swmv43m
+  expect-type: 0.15.0
   prettier: 2.8.4
   typescript: 4.9.5
   unbuild: 1.1.2
@@ -2085,6 +2087,10 @@ packages:
       strip-final-newline: 3.0.0
     dev: true
 
+  /expect-type/0.15.0:
+    resolution: {integrity: sha512-yWnriYB4e8G54M5/fAFj7rCIBiKs1HAACaY13kCz6Ku0dezjS9aMcfcdVK2X8Tv2tEV1BPz/wKfQ7WA4S/d8aA==}
+    dev: true
+
   /fast-deep-equal/3.1.3:
     resolution: {integrity: sha512-f3qQ9oQy9j2AhBe/H9VC91wLmKBCCU/gDOnKNAYG5hswO7BLKj09Hc5HYNz9cGI++xlpDCIgDaitVs03ATR84Q==}
     dev: true
diff --git a/src/dotenv.ts b/src/dotenv.ts
index 3b93fff..2d54873 100644
--- a/src/dotenv.ts
+++ b/src/dotenv.ts
@@ -65,7 +65,7 @@ export async function setupDotenv(options: DotenvOptions): Promise<Env> {
 export async function loadDotenv(options: DotenvOptions): Promise<Env> {
   const environment = Object.create(null);
 
-  const dotenvFile = resolve(options.cwd, options.fileName);
+  const dotenvFile = resolve(options.cwd, options.fileName!);
 
   if (existsSync(dotenvFile)) {
     const parsed = dotenv.parse(await fsp.readFile(dotenvFile, "utf8"));
@@ -73,7 +73,7 @@ export async function loadDotenv(options: DotenvOptions): Promise<Env> {
   }
 
   // Apply process.env
-  if (!options.env._applied) {
+  if (!options.env?._applied) {
     Object.assign(environment, options.env);
     environment._applied = true;
   }
@@ -97,7 +97,7 @@ function interpolate(
     return source[key] !== undefined ? source[key] : target[key];
   }
 
-  function interpolate(value: unknown, parents: string[] = []) {
+  function interpolate(value: unknown, parents: string[] = []): any {
     if (typeof value !== "string") {
       return value;
     }
@@ -105,17 +105,17 @@ function interpolate(
     return parse(
       // eslint-disable-next-line unicorn/no-array-reduce
       matches.reduce((newValue, match) => {
-        const parts = /(.?)\${?([\w:]+)?}?/g.exec(match);
+        const parts = /(.?)\${?([\w:]+)?}?/g.exec(match) || [];
         const prefix = parts[1];
 
         let value, replacePart: string;
 
         if (prefix === "\\") {
-          replacePart = parts[0];
+          replacePart = parts[0] || "";
           value = replacePart.replace("\\$", "$");
         } else {
           const key = parts[2];
-          replacePart = parts[0].slice(prefix.length);
+          replacePart = (parts[0] || "").slice(prefix.length);
 
           // Avoid recursion
           if (parents.includes(key)) {
diff --git a/src/index.ts b/src/index.ts
index 645fb3d..a4aa8cb 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -1,2 +1,3 @@
 export * from "./dotenv";
 export * from "./loader";
+export * from "./types";
diff --git a/src/loader.ts b/src/loader.ts
index bf45fc3..223e229 100644
--- a/src/loader.ts
+++ b/src/loader.ts
@@ -2,92 +2,26 @@ import { existsSync } from "node:fs";
 import { rmdir } from "node:fs/promises";
 import { homedir } from "node:os";
 import { resolve, extname, dirname } from "pathe";
-import createJiti, { JITI } from "jiti";
+import createJiti from "jiti";
 import * as rc9 from "rc9";
 import { defu } from "defu";
 import { findWorkspaceDir, readPackageJSON } from "pkg-types";
-import type { JITIOptions } from "jiti/dist/types";
-import { DotenvOptions, setupDotenv } from "./dotenv";
-
-export type UserInputConfig = Record<string, any>;
-
-export interface ConfigLayerMeta {
-  name?: string;
-  [key: string]: any;
-}
-
-export interface C12InputConfig {
-  $test?: UserInputConfig;
-  $development?: UserInputConfig;
-  $production?: UserInputConfig;
-  $env?: Record<string, UserInputConfig>;
-  $meta?: ConfigLayerMeta;
-}
-
-export interface InputConfig extends C12InputConfig, UserInputConfig {}
-
-export interface SourceOptions {
-  meta?: ConfigLayerMeta;
-  overrides?: UserInputConfig;
-  [key: string]: any;
-}
-
-export interface ConfigLayer<T extends InputConfig = InputConfig> {
-  config: T | null;
-  source?: string;
-  sourceOptions?: SourceOptions;
-  meta?: ConfigLayerMeta;
-  cwd?: string;
-  configFile?: string;
-}
-
-export interface ResolvedConfig<T extends InputConfig = InputConfig>
-  extends ConfigLayer<T> {
-  layers?: ConfigLayer<T>[];
-  cwd?: string;
-}
-
-export interface ResolveConfigOptions {
-  cwd: string;
-}
-
-export interface LoadConfigOptions<T extends InputConfig = InputConfig> {
-  name?: string;
-  cwd?: string;
-
-  configFile?: string;
-
-  rcFile?: false | string;
-  globalRc?: boolean;
-
-  dotenv?: boolean | DotenvOptions;
-
-  envName?: string | false;
-
-  packageJson?: boolean | string | string[];
-
-  defaults?: T;
-  defaultConfig?: T;
-  overrides?: T;
-
-  resolve?: (
-    id: string,
-    options: LoadConfigOptions
-  ) => null | ResolvedConfig | Promise<ResolvedConfig | null>;
-
-  jiti?: JITI;
-  jitiOptions?: JITIOptions;
-
-  extend?:
-    | false
-    | {
-        extendKey?: string | string[];
-      };
-}
-
-export async function loadConfig<T extends InputConfig = InputConfig>(
-  options: LoadConfigOptions<T>
-): Promise<ResolvedConfig<T>> {
+import { setupDotenv } from "./dotenv";
+
+import type {
+  UserInputConfig,
+  ConfigLayerMeta,
+  LoadConfigOptions,
+  ResolvedConfig,
+  ConfigLayer,
+  SourceOptions,
+  InputConfig,
+} from "./types";
+
+export async function loadConfig<
+  T extends UserInputConfig = UserInputConfig,
+  MT extends ConfigLayerMeta = ConfigLayerMeta
+>(options: LoadConfigOptions<T, MT>): Promise<ResolvedConfig<T, MT>> {
   // Normalize options
   options.cwd = resolve(process.cwd(), options.cwd || ".");
   options.name = options.name || "config";
@@ -106,7 +40,7 @@ export async function loadConfig<T extends InputConfig = InputConfig>(
   // Create jiti instance
   options.jiti =
     options.jiti ||
-    createJiti(undefined, {
+    createJiti(undefined as unknown as string, {
       interopDefault: true,
       requireCache: false,
       esmResolve: true,
@@ -114,7 +48,7 @@ export async function loadConfig<T extends InputConfig = InputConfig>(
     });
 
   // Create context
-  const r: ResolvedConfig<T> = {
+  const r: ResolvedConfig<T, MT> = {
     config: {} as any,
     cwd: options.cwd,
     configFile: resolve(options.cwd, options.configFile),
@@ -188,7 +122,7 @@ export async function loadConfig<T extends InputConfig = InputConfig>(
     await extendConfig(r.config, options);
     r.layers = r.config._layers;
     delete r.config._layers;
-    r.config = defu(r.config, ...r.layers.map((e) => e.config)) as T;
+    r.config = defu(r.config, ...r.layers!.map((e) => e.config)) as T;
   }
 
   // Preserve unmerged sources as layers
@@ -201,8 +135,9 @@ export async function loadConfig<T extends InputConfig = InputConfig>(
     { config, configFile: options.configFile, cwd: options.cwd },
     options.rcFile && { config: configRC, configFile: options.rcFile },
     options.packageJson && { config: pkgJson, configFile: "package.json" },
-  ].filter((l) => l && l.config) as ConfigLayer<T>[];
-  r.layers = [...baseLayers, ...r.layers];
+  ].filter((l) => l && l.config) as ConfigLayer<T, MT>[];
+
+  r.layers = [...baseLayers, ...r.layers!];
 
   // Apply defaults
   if (options.defaults) {
@@ -213,8 +148,11 @@ export async function loadConfig<T extends InputConfig = InputConfig>(
   return r;
 }
 
-async function extendConfig(config, options: LoadConfigOptions) {
-  config._layers = config._layers || [];
+async function extendConfig<
+  T extends UserInputConfig = UserInputConfig,
+  MT extends ConfigLayerMeta = ConfigLayerMeta
+>(config: InputConfig<T, MT>, options: LoadConfigOptions<T, MT>) {
+  (config as any)._layers = config._layers || [];
   if (!options.extend) {
     return;
   }
@@ -223,7 +161,7 @@ async function extendConfig(config, options: LoadConfigOptions) {
     keys = [keys];
   }
   const extendSources = [];
-  for (const key of keys) {
+  for (const key of keys as string[]) {
     extendSources.push(
       ...(Array.isArray(config[key]) ? config[key] : [config[key]]).filter(
         Boolean
@@ -276,11 +214,14 @@ const GIT_PREFIXES = ["github:", "gitlab:", "bitbucket:", "https://"];
 const NPM_PACKAGE_RE =
   /^(@[\da-z~-][\d._a-z~-]*\/)?[\da-z~-][\d._a-z~-]*($|\/.*)/;
 
-async function resolveConfig(
+async function resolveConfig<
+  T extends UserInputConfig = UserInputConfig,
+  MT extends ConfigLayerMeta = ConfigLayerMeta
+>(
   source: string,
-  options: LoadConfigOptions,
-  sourceOptions: SourceOptions = {}
-): Promise<ResolvedConfig> {
+  options: LoadConfigOptions<T, MT>,
+  sourceOptions: SourceOptions<T, MT> = {}
+): Promise<ResolvedConfig<T, MT>> {
   // Custom user resolver
   if (options.resolve) {
     const res = await options.resolve(source, options);
@@ -309,26 +250,31 @@ async function resolveConfig(
   // Try resolving as npm package
   if (NPM_PACKAGE_RE.test(source)) {
     try {
-      source = options.jiti.resolve(source, { paths: [options.cwd] });
+      source = options.jiti!.resolve(source, { paths: [options.cwd!] });
     } catch {}
   }
 
   // Import from local fs
   const isDir = !extname(source);
-  const cwd = resolve(options.cwd, isDir ? source : dirname(source));
+  const cwd = resolve(options.cwd!, isDir ? source : dirname(source));
   if (isDir) {
-    source = options.configFile;
+    source = options.configFile!;
   }
-  const res: ResolvedConfig = { config: undefined, cwd, source, sourceOptions };
+  const res: ResolvedConfig<T, MT> = {
+    config: undefined as unknown as T,
+    cwd,
+    source,
+    sourceOptions,
+  };
   try {
-    res.configFile = options.jiti.resolve(resolve(cwd, source), {
+    res.configFile = options.jiti!.resolve(resolve(cwd, source), {
       paths: [cwd],
     });
   } catch {}
-  if (!existsSync(res.configFile)) {
+  if (!existsSync(res.configFile!)) {
     return res;
   }
-  res.config = options.jiti(res.configFile);
+  res.config = options.jiti!(res.configFile!);
   if (res.config instanceof Function) {
     res.config = await res.config();
   }
@@ -336,8 +282,8 @@ async function resolveConfig(
   // Extend env specific config
   if (options.envName) {
     const envConfig = {
-      ...res.config["$" + options.envName],
-      ...res.config.$env?.[options.envName],
+      ...res.config!["$" + options.envName],
+      ...res.config!.$env?.[options.envName],
     };
     if (Object.keys(envConfig).length > 0) {
       res.config = defu(envConfig, res.config);
@@ -345,12 +291,12 @@ async function resolveConfig(
   }
 
   // Meta
-  res.meta = defu(res.sourceOptions.meta, res.config.$meta);
-  delete res.config.$meta;
+  res.meta = defu(res.sourceOptions!.meta, res.config!.$meta) as MT;
+  delete res.config!.$meta;
 
   // Overrides
-  if (res.sourceOptions.overrides) {
-    res.config = defu(res.sourceOptions.overrides, res.config);
+  if (res.sourceOptions!.overrides) {
+    res.config = defu(res.sourceOptions!.overrides, res.config) as T;
   }
 
   return res;
diff --git a/src/types.ts b/src/types.ts
new file mode 100644
index 0000000..8ad888d
--- /dev/null
+++ b/src/types.ts
@@ -0,0 +1,108 @@
+import type { JITI } from "jiti";
+import type { JITIOptions } from "jiti/dist/types";
+import type { DotenvOptions } from "./dotenv";
+
+export interface ConfigLayerMeta {
+  name?: string;
+  [key: string]: any;
+}
+
+export type UserInputConfig = Record<string, any>;
+
+export interface C12InputConfig<
+  T extends UserInputConfig = UserInputConfig,
+  MT extends ConfigLayerMeta = ConfigLayerMeta
+> {
+  $test?: T;
+  $development?: T;
+  $production?: T;
+  $env?: Record<string, T>;
+  $meta?: MT;
+}
+
+export type InputConfig<
+  T extends UserInputConfig = UserInputConfig,
+  MT extends ConfigLayerMeta = ConfigLayerMeta
+> = C12InputConfig<T, MT> & T;
+
+export interface SourceOptions<
+  T extends UserInputConfig = UserInputConfig,
+  MT extends ConfigLayerMeta = ConfigLayerMeta
+> {
+  meta?: MT;
+  overrides?: T;
+  [key: string]: any;
+}
+
+export interface ConfigLayer<
+  T extends UserInputConfig = UserInputConfig,
+  MT extends ConfigLayerMeta = ConfigLayerMeta
+> {
+  config: T | null;
+  source?: string;
+  sourceOptions?: SourceOptions<T, MT>;
+  meta?: MT;
+  cwd?: string;
+  configFile?: string;
+}
+
+export interface ResolvedConfig<
+  T extends UserInputConfig = UserInputConfig,
+  MT extends ConfigLayerMeta = ConfigLayerMeta
+> extends ConfigLayer<T, MT> {
+  layers?: ConfigLayer<T, MT>[];
+  cwd?: string;
+}
+
+export interface LoadConfigOptions<
+  T extends UserInputConfig = UserInputConfig,
+  MT extends ConfigLayerMeta = ConfigLayerMeta
+> {
+  name?: string;
+  cwd?: string;
+
+  configFile?: string;
+
+  rcFile?: false | string;
+  globalRc?: boolean;
+
+  dotenv?: boolean | DotenvOptions;
+
+  envName?: string | false;
+
+  packageJson?: boolean | string | string[];
+
+  defaults?: T;
+  defaultConfig?: T;
+  overrides?: T;
+
+  resolve?: (
+    id: string,
+    options: LoadConfigOptions<T, MT>
+  ) =>
+    | null
+    | undefined
+    | ResolvedConfig<T, MT>
+    | Promise<ResolvedConfig<T, MT> | undefined | null>;
+
+  jiti?: JITI;
+  jitiOptions?: JITIOptions;
+
+  extend?:
+    | false
+    | {
+        extendKey?: string | string[];
+      };
+}
+
+export type DefineConfig<
+  T extends UserInputConfig = UserInputConfig,
+  MT extends ConfigLayerMeta = ConfigLayerMeta
+> = (input: InputConfig<T, MT>) => InputConfig<T, MT>;
+
+export function createDefineConfig<
+  T extends UserInputConfig = UserInputConfig,
+  MT extends ConfigLayerMeta = ConfigLayerMeta
+>(): DefineConfig<T, MT> {
+  return (input: InputConfig<T, MT>) => input;
+}
diff --git a/test/index.test.ts b/test/index.test.ts
index b250eff..05bee0f 100644
--- a/test/index.test.ts
+++ b/test/index.test.ts
@@ -2,13 +2,19 @@ import { fileURLToPath } from "node:url";
 import { expect, it, describe } from "vitest";
 import { loadConfig } from "../src";
 
-const r = (path) => fileURLToPath(new URL(path, import.meta.url));
-const transformPaths = (object) =>
+const r = (path: string) => fileURLToPath(new URL(path, import.meta.url));
+const transformPaths = (object: object) =>
   JSON.parse(JSON.stringify(object).replaceAll(r("."), "<path>/"));
 
 describe("c12", () => {
   it("load fixture config", async () => {
-    const { config, layers } = await loadConfig({
+    type UserConfig = Partial<{
+      virtual: boolean;
+      overriden: boolean;
+      defaultConfig: boolean;
+      extends: string[];
+    }>;
+    const { config, layers } = await loadConfig<UserConfig>({
       cwd: r("./fixture"),
       dotenv: true,
       packageJson: ["c12", "c12-alt"],
@@ -33,7 +39,7 @@ describe("c12", () => {
       },
     });
 
-    expect(transformPaths(config)).toMatchInlineSnapshot(`
+    expect(transformPaths(config!)).toMatchInlineSnapshot(`
       {
         "$env": {
           "test": {
@@ -71,7 +77,7 @@ describe("c12", () => {
       }
     `);
 
-    expect(transformPaths(layers)).toMatchInlineSnapshot(`
+    expect(transformPaths(layers!)).toMatchInlineSnapshot(`
       [
         {
           "config": {
@@ -201,7 +207,7 @@ describe("c12", () => {
       },
     });
 
-    expect(transformPaths(config)).toMatchInlineSnapshot(`
+    expect(transformPaths(config!)).toMatchInlineSnapshot(`
       {
         "$test": {
           "envConfig": true,
diff --git a/test/test.ts b/test/test.ts
index 7024d5b..a46cabf 100644
--- a/test/test.ts
+++ b/test/test.ts
@@ -1,7 +1,7 @@
 import { fileURLToPath } from "node:url";
 import { loadConfig } from "../src";
 
-const r = (path) => fileURLToPath(new URL(path, import.meta.url));
+const r = (path: string) => fileURLToPath(new URL(path, import.meta.url));
 
 async function main() {
   const fixtureDir = r("./fixture");
diff --git a/test/types.ts b/test/types.ts
new file mode 100644
index 0000000..4832bb1
--- /dev/null
+++ b/test/types.ts
@@ -0,0 +1,31 @@
+import { expectTypeOf } from "expect-type";
+import { loadConfig, InputConfig, createDefineConfig } from "../src";
+
+interface MyConfig {
+  foo: string;
+}
+
+interface MyMeta {
+  metaFoo: string;
+}
+
+const defineMyConfig = createDefineConfig<MyConfig, MyMeta>();
+
+const userConfig = defineMyConfig({
+  foo: "bar",
+  $meta: {
+    metaFoo: "bar",
+  },
+  $development: {
+    foo: "bar",
+  },
+});
+
+expectTypeOf(userConfig.$production!.foo).toEqualTypeOf<string>();
+expectTypeOf(userConfig.$meta!.metaFoo).toEqualTypeOf<string>();
+
+async function main() {
+  const config = await loadConfig<MyConfig, MyMeta>({});
+  expectTypeOf(config.config!.foo).toEqualTypeOf<string>();
+  expectTypeOf(config.meta!.metaFoo).toEqualTypeOf<string>();
+}
diff --git a/tsconfig.json b/tsconfig.json
index 3f2c847..7d08257 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -3,10 +3,9 @@
     "target": "ESNext",
     "module": "ESNext",
     "moduleResolution": "Node",
-    "esModuleInterop": true
+    "esModuleInterop": true,
+    "strict": true
   },
-  "include": [
-    "src",
-    "test"
-  ]
+  "include": ["src", "test"],
+  "exclude": ["node_modules"]
 }