Add TypeScript definition (#3)

index.d.ts

@@ -0,0 +1,13 @@
+/**
+ * Check if `input` is a ES2015 promise.
+ *
+ * @param input - Value to be checked.
+ *
+ * @example
+ *
+ * import isPromise from 'p-is-promise';
+ *
+ * isPromise(Promise.resolve('🦄'));
+ * //=> true
+ */
+export default function(input: unknown): input is Promise<unknown>;

index.js

@@ -1,5 +1,5 @@
 'use strict';
-module.exports = x => (
+const isPromise = x => (
 	x instanceof Promise ||
 	(
 		x !== null &&
@@ -8,3 +8,6 @@ module.exports = x => (
 		typeof x.catch === 'function'
 	)
 );
+
+module.exports = isPromise;
+module.exports.default = isPromise;

index.test-d.ts

@@ -0,0 +1,11 @@
+import {expectType} from 'tsd-check';
+import isPromise from '.';
+
+expectType<boolean>(isPromise('🦄'));
+
+const unicorn: unknown = Promise.resolve('🦄');
+
+if (isPromise(unicorn)) {
+	expectType<Promise<unknown>>(unicorn);
+	expectType<unknown>(await unicorn);
+}

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": [
 		"promise",
@@ -35,6 +36,7 @@
 	"devDependencies": {
 		"ava": "^0.25.0",
 		"bluebird": "^3.4.6",
+		"tsd-check": "^0.2.1",
 		"xo": "^0.23.0"
 	}
 }