Require Node.js 8, add TypeScript definition (#14)

.travis.yml

@@ -2,4 +2,3 @@ language: node_js
 node_js:
   - '10'
   - '8'
-  - '6'

index.d.ts

@@ -0,0 +1,13 @@
+import {Primitive, JsonObject} from 'type-fest';
+
+export type ErrorObject = {
+	name?: string;
+	stack?: string;
+	message?: string;
+	code?: string;
+} & JsonObject;
+
+/**
+Serialize an error into a plain object.
+*/
+export default function serializeError<ErrorType>(error: ErrorType): ErrorType extends Primitive ? ErrorType : ErrorObject;

index.js

@@ -5,10 +5,7 @@ const destroyCircular = (from, seen) => {
 
 	seen.push(from);
 
-	// TODO: Use `Object.entries() when targeting Node.js 8
-	for (const key of Object.keys(from)) {
-		const value = from[key];
-
+	for (const [key, value] of Object.entries(from)) {
 		if (typeof value === 'function') {
 			continue;
 		}
@@ -37,7 +34,7 @@ const destroyCircular = (from, seen) => {
 	return to;
 };
 
-module.exports = value => {
+const serializeError = value => {
 	if (typeof value === 'object') {
 		return destroyCircular(value, []);
 	}
@@ -50,3 +47,6 @@ module.exports = value => {
 
 	return value;
 };
+
+module.exports = serializeError;
+module.exports.default = serializeError;

index.test-d.ts

@@ -0,0 +1,7 @@
+import {expectType} from 'tsd-check';
+import serializeError, {ErrorObject} from '.';
+
+const error = new Error('unicorn');
+
+expectType<number>(serializeError(1));
+expectType<ErrorObject>(serializeError(error));

package.json

@@ -10,13 +10,14 @@
 		"url": "sindresorhus.com"
 	},
 	"engines": {
-		"node": ">=6"
+		"node": ">=8"
 	},
 	"scripts": {
-		"test": "xo && ava"
+		"test": "xo && ava && tsd-check"
 	},
 	"files": [
-		"index.js"
+		"index.js",
+		"index.d.ts"
 	],
 	"keywords": [
 		"error",
@@ -28,8 +29,12 @@
 		"process",
 		"send"
 	],
+	"dependencies": {
+		"type-fest": "^0.3.0"
+	},
 	"devDependencies": {
-		"ava": "^0.25.0",
-		"xo": "^0.23.0"
+		"ava": "^1.3.1",
+		"tsd-check": "^0.5.0",
+		"xo": "^0.24.0"
 	}
 }