Add TypeScript definition (#14)

index.d.ts

@@ -0,0 +1,15 @@
+/**
+ * Make a function mimic another one. It will copy over the properties `name`, `length`, `displayName`, and any custom properties you may have set.
+ *
+ * @param to - Mimicking function.
+ * @param from - Function to mimic.
+ * @returns The modified `to`.
+ */
+export default function mimicFn<
+	ArgumentsType extends unknown[],
+	ReturnType,
+	FunctionType extends (...arguments: ArgumentsType) => ReturnType
+>(
+	to: (...arguments: ArgumentsType) => ReturnType,
+	from: FunctionType
+): FunctionType;

index.js

@@ -1,8 +1,11 @@
 'use strict';
-module.exports = (to, from) => {
+const mimicFn = (to, from) => {
 	for (const prop of Reflect.ownKeys(from)) {
 		Object.defineProperty(to, prop, Object.getOwnPropertyDescriptor(from, prop));
 	}
 
 	return to;
 };
+
+module.exports = mimicFn;
+module.exports.default = mimicFn;

index.test-d.ts

@@ -0,0 +1,18 @@
+import {expectType} from 'tsd-check';
+import mimicFn from '.';
+
+function foo(string: string) {
+	return false;
+}
+foo.unicorn = '🦄';
+
+function wrapper(string: string) {
+	return foo(string);
+}
+
+const mimickedFn = mimicFn(wrapper, foo);
+
+expectType<typeof foo & {unicorn: string}>(mimickedFn);
+
+expectType<boolean>(mimickedFn('bar'));
+expectType<string>(mimickedFn.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": [
 		"function",
@@ -34,7 +35,8 @@
 		"change"
 	],
 	"devDependencies": {
-		"ava": "*",
-		"xo": "*"
+		"ava": "^1.3.1",
+		"tsd-check": "^0.3.0",
+		"xo": "^0.24.0"
 	}
 }