diff --git a/index.d.ts b/index.d.ts
new file mode 100644
index 0000000..2500c20
--- /dev/null
+++ b/index.d.ts
@@ -0,0 +1,4 @@
+/**
+Get the visual width of the widest line in a string - the number of columns required to display it.
+*/
+export default function widestLine(input: string): number;
diff --git a/index.js b/index.js
index c98a4ae..f49a3a8 100644
--- a/index.js
+++ b/index.js
@@ -1,7 +1,7 @@
 'use strict';
 const stringWidth = require('string-width');
 
-module.exports = input => {
+const widestLine = input => {
 	let max = 0;
 
 	for (const line of input.split('\n')) {
@@ -10,3 +10,6 @@ module.exports = input => {
 
 	return max;
 };
+
+module.exports = widestLine;
+module.exports.default = widestLine;
diff --git a/index.test-d.ts b/index.test-d.ts
new file mode 100644
index 0000000..e9a5477
--- /dev/null
+++ b/index.test-d.ts
@@ -0,0 +1,4 @@
+import {expectType} from 'tsd-check';
+import widestLine from '.';
+
+expectType<number>(widestLine('古\n\u001B[1m@\u001B[22m'));
diff --git a/package.json b/package.json
index 4578af2..ff99ba2 100644
--- a/package.json
+++ b/package.json
@@ -13,10 +13,11 @@
 		"node": ">=6"
 	},
 	"scripts": {
-		"test": "xo && ava"
+		"test": "xo && ava && tsd-check"
 	},
 	"files": [
-		"index.js"
+		"index.js",
+		"index.d.ts"
 	],
 	"keywords": [
 		"string",
@@ -44,10 +45,11 @@
 		"fixed-width"
 	],
 	"dependencies": {
-		"string-width": "^2.1.1"
+		"string-width": "^3.1.0"
 	},
 	"devDependencies": {
-		"ava": "^0.25.0",
-		"xo": "^0.23.0"
+		"ava": "^1.3.1",
+		"tsd-check": "^0.5.0",
+		"xo": "^0.24.0"
 	}
 }