Add TypeScript definition (#8)

Author: BendingBender

index.d.ts

@@ -0,0 +1,14 @@
+/**
+ * Create an error from multiple errors.
+ */
+export default class AggregateError extends Error implements Iterable<Error> {
+	readonly name: 'AggregateError';
+
+	/**
+	 * @param errors - If a string, a new `Error` is created with the string as the error message. If a non-Error object, a new `Error` is created with all properties from the object copied over.
+	 * @returns An Error that is also an [`Iterable`](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Iterators_and_Generators#Iterables) for the individual errors.
+	 */
+	constructor(errors: ReadonlyArray<Error | {[key: string]: unknown} | string>);
+
+	[Symbol.iterator](): IterableIterator<Error>;
+}

index.js

@@ -45,3 +45,4 @@ class AggregateError extends Error {
 }
 
 module.exports = AggregateError;
+module.exports.default = AggregateError;

index.test-d.ts

@@ -0,0 +1,14 @@
+import {expectType} from 'tsd-check';
+import AggregateError from '.';
+
+const aggregateError = new AggregateError([
+	new Error('foo'),
+	{foo: 'bar'},
+	'bar'
+]);
+expectType<Iterable<Error>>(aggregateError);
+expectType<IterableIterator<Error>>(aggregateError[Symbol.iterator]());
+
+for (const error of aggregateError) {
+	expectType<Error>(error);
+}

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": [
 		"aggregate",
@@ -35,6 +36,7 @@
 	},
 	"devDependencies": {
 		"ava": "^1.2.1",
+		"tsd-check": "^0.3.0",
 		"xo": "^0.24.0"
 	}
 }