diff --git a/index.d.ts b/index.d.ts
index 4735eba..0dfdf09 100644
--- a/index.d.ts
+++ b/index.d.ts
@@ -1,6 +1,6 @@
-export type Replacer = (key: string, value: any) => void;
+export type Replacer = (key: string, value: any) => number | string | boolean | object | null | undefined;
 export type SortKeys = (a: string, b: string) => number;
-export type JSONStringifyable = object | number | string;
+export type JSONStringifyable = object | number | string | boolean;
 
 export interface Options {
 	/**
@@ -39,6 +39,7 @@ export interface Options {
  * Creates directories for you as needed.
  *
  * @example
+ *
  * import * as writeJsonFile from 'write-json-file';
  *
  * writeJsonFile.sync('foo.json', {foo: true});
@@ -51,6 +52,7 @@ export function sync(filepath: string, data: JSONStringifyable, options?: Option
  * Creates directories for you as needed.
  *
  * @example
+ *
  * import writeJsonFile from 'write-json-file';
  *
  * (async () => {
diff --git a/index.test-d.ts b/index.test-d.ts
new file mode 100644
index 0000000..697bd43
--- /dev/null
+++ b/index.test-d.ts
@@ -0,0 +1,52 @@
+import {expectType} from 'tsd-check';
+import writeJsonFile, {sync, Replacer, SortKeys, JSONStringifyable} from '.';
+
+(async () => {
+	expectType<JSONStringifyable>('🦄');
+	expectType<JSONStringifyable>(1);
+	expectType<JSONStringifyable>(true);
+	expectType<JSONStringifyable>(new Date());
+	expectType<JSONStringifyable>(['hello', 'world']);
+	expectType<JSONStringifyable>({unicorn: '🦄'});
+
+	expectType<SortKeys>(() => 1);
+	expectType<SortKeys>((a: string) => a.length);
+	expectType<SortKeys>((a: string, b: string) => a.length - b.length);
+
+	expectType<Replacer>(() => 1);
+	expectType<Replacer>(() => 'unicorn');
+	expectType<Replacer>(() => true);
+	expectType<Replacer>(() => null);
+	expectType<Replacer>(() => undefined);
+	expectType<Replacer>(() => ({unicorn: '🦄'}));
+	expectType<Replacer>(() => ['unicorn', 1]);
+	expectType<Replacer>(() => () => 'foo');
+	expectType<Replacer>((key: string) => key.toUpperCase());
+	expectType<Replacer>((key: string, value: string) => (key + value).toUpperCase());
+
+	expectType<void>(await writeJsonFile('unicorn.json', {unicorn: '🦄'}));
+	expectType<void>(await writeJsonFile('unicorn.json', '🦄'));
+	expectType<void>(await writeJsonFile('date.json', new Date()));
+	expectType<void>(await writeJsonFile('unicorn.json', {unicorn: '🦄'}, {detectIndent: true}));
+	expectType<void>(await writeJsonFile('unicorn.json', {unicorn: '🦄'}, {indent: ' '}));
+	expectType<void>(await writeJsonFile('unicorn.json', {unicorn: '🦄'}, {indent: 4}));
+	expectType<void>(await writeJsonFile('unicorn.json', {unicorn: '🦄'}, {mode: 0o666}));
+	expectType<void>(await writeJsonFile('unicorn.json', {unicorn: '🦄'}, {replacer: ['unicorn', 1]}));
+	expectType<void>(await writeJsonFile('unicorn.json', {unicorn: '🦄'}, {replacer: () => 'unicorn'}));
+	expectType<void>(await writeJsonFile('unicorn.json', {unicorn: '🦄'}, {sortKeys: () => -1}));
+	expectType<void>(await writeJsonFile('unicorn.json', {unicorn: '🦄'}, {sortKeys: (a: string, b: string) => a.length - b.length}));
+	expectType<void>(await writeJsonFile('unicorn.json', {unicorn: '🦄'}, {sortKeys: true}));
+
+	expectType<void>(sync('unicorn.json', {unicorn: '🦄'}));
+	expectType<void>(sync('unicorn.json', '🦄'));
+	expectType<void>(sync('date.json', new Date()));
+	expectType<void>(sync('unicorn.json', {unicorn: '🦄'}, {detectIndent: true}));
+	expectType<void>(sync('unicorn.json', {unicorn: '🦄'}, {indent: ' '}));
+	expectType<void>(sync('unicorn.json', {unicorn: '🦄'}, {indent: 4}));
+	expectType<void>(sync('unicorn.json', {unicorn: '🦄'}, {mode: 0o666}));
+	expectType<void>(sync('unicorn.json', {unicorn: '🦄'}, {replacer: ['unicorn', 1]}));
+	expectType<void>(sync('unicorn.json', {unicorn: '🦄'}, {replacer: () => 'unicorn'}));
+	expectType<void>(sync('unicorn.json', {unicorn: '🦄'}, {sortKeys: () => -1}));
+	expectType<void>(sync('unicorn.json', {unicorn: '🦄'}, {sortKeys: (a: string, b: string) => a.length - b.length}));
+	expectType<void>(sync('unicorn.json', {unicorn: '🦄'}, {sortKeys: true}));
+})();
diff --git a/package.json b/package.json
index 4b22bb3..927a891 100644
--- a/package.json
+++ b/package.json
@@ -13,7 +13,7 @@
 		"node": ">=6"
 	},
 	"scripts": {
-		"test": "xo && ava"
+		"test": "xo && ava && tsd-check"
 	},
 	"files": [
 		"index.js",
@@ -44,11 +44,12 @@
 	"devDependencies": {
 		"ava": "*",
 		"tempfile": "^2.0.0",
+		"tsd-check": "^0.1.0",
 		"xo": "*"
 	},
 	"xo": {
 		"ignores": [
-			"index.d.ts"
+			"*.ts"
 		]
 	}
 }