Add TypeScript definition (#4)

index.d.ts

@@ -0,0 +1,29 @@
+/**
+Check if a value is a plain object.
+
+An object is plain if it's created by either `{}`, `new Object()`, or `Object.create(null)`.
+
+@example
+```
+import isPlainObject = require('is-plain-obj');
+
+isPlainObject({foo: 'bar'});
+//=> true
+
+isPlainObject(new Object());
+//=> true
+
+isPlainObject(Object.create(null));
+//=> true
+
+isPlainObject([1, 2, 3]);
+//=> false
+
+class Unicorn {}
+isPlainObject(new Unicorn());
+//=> false
+```
+*/
+declare function isPlainObj(candidate: unknown): candidate is object;
+
+export = isPlainObj;

index.js

@@ -1,10 +1,10 @@
 'use strict';
 
-module.exports = input => {
-	if (Object.prototype.toString.call(input) !== '[object Object]') {
+module.exports = candidate => {
+	if (Object.prototype.toString.call(candidate) !== '[object Object]') {
 		return false;
 	}
 
-	const prototype = Object.getPrototypeOf(input);
+	const prototype = Object.getPrototypeOf(candidate);
 	return prototype === null || prototype === Object.getPrototypeOf({});
 };

index.test-d.ts

@@ -0,0 +1,8 @@
+import {expectType} from 'tsd';
+import isPlainObj = require('.');
+
+const foo = 'foo';
+
+if (isPlainObj(foo)) {
+	expectType<object>(foo);
+}

package.json

@@ -13,10 +13,11 @@
 		"node": ">=6"
 	},
 	"scripts": {
-		"test": "xo && ava"
+		"test": "xo && ava && tsd"
 	},
 	"files": [
-		"index.js"
+		"index.js",
+		"index.d.ts"
 	],
 	"keywords": [
 		"obj",
@@ -31,7 +32,8 @@
 		"simple"
 	],
 	"devDependencies": {
-		"ava": "^1.0.1",
-		"xo": "^0.23.0"
+		"ava": "^1.4.1",
+		"tsd": "^0.7.2",
+		"xo": "^0.24.0"
 	}
 }