Add generic types and update tests (#107)

* Add generic types and update tests

* Add generic types to `on`, `off` and `emit` to enable some nicer TS
  usage if you specify what type you're expecting from the `EventData`.
* Move the tests to be TypeScript source. This will help catch errors in
  the tests if there are any type errors.
* Create a new test to test the generic types and make sure they pass
  and error when expected.
* Upgrade to Mocha 8.
* Did some tidying up of the package.json scripts.

* Tweak TS setup to validate tests

* Fix d.ts generation and tests

Co-authored-by: Jason Miller <[email protected]>

9 files changed, 86 insertions, 44 deletions

diff --git a/.editorconfig b/.editorconfig
index ac0adb7..04d7ef9 100644
--- a/.editorconfig
+++ b/.editorconfig
@@ -10,6 +10,7 @@ insert_final_newline = true
 [{package.json,.*rc,*.yml}]
 indent_style = space
 indent_size = 2
+insert_final_newline = false
 
 [*.md]
 trim_trailing_whitespace = false
diff --git a/.gitignore b/.gitignore
index ecdffab..eb570d3 100644
--- a/.gitignore
+++ b/.gitignore
@@ -2,6 +2,7 @@
 /test-reports
 /node_modules
 /npm-debug.log
+/index.d.ts
 package-lock.json
 .DS_Store
 .idea
diff --git a/README.md b/README.md
index 0796e5f..b6a8022 100644
--- a/README.md
+++ b/README.md
@@ -110,7 +110,7 @@ const emitter: mitt.Emitter = mitt();
 
 Mitt: Tiny (~200b) functional event emitter / pubsub.
 
-Returns **Mitt**
+Returns **Mitt** 
 
 ### on
 
diff --git a/package.json b/package.json
index 90b7b74..a2c2b58 100644
--- a/package.json
+++ b/package.json
@@ -8,12 +8,13 @@
   "esmodules": "dist/mitt.modern.js",
   "main": "dist/mitt.js",
   "umd:main": "dist/mitt.umd.js",
-  "typings": "dist/index.d.ts",
+  "typings": "index.d.ts",
   "scripts": {
-    "test": "npm-run-all --silent typecheck lint testonly",
-    "testonly": "mocha --require esm test/**/*.js",
+    "test": "npm-run-all --silent typecheck lint mocha test-types",
+    "mocha": "mocha test",
+    "test-types": "tsc test/test-types-compilation.ts --noEmit",
     "lint": "eslint src test --ext ts --ext js",
-    "typecheck": "tsc **/*.ts --noEmit",
+    "typecheck": "tsc --noEmit",
     "bundle": "microbundle",
     "build": "npm-run-all --silent clean -p bundle -s docs",
     "clean": "rimraf dist",
@@ -34,8 +35,21 @@
   "license": "MIT",
   "files": [
     "src",
-    "dist"
+    "dist",
+    "index.d.ts"
   ],
+  "mocha": {
+    "extension": [
+      "ts"
+    ],
+    "require": [
+      "ts-node/register",
+      "esm"
+    ],
+    "spec": [
+      "test/*_test.ts"
+    ]
+  },
   "eslintConfig": {
     "extends": [
       "developit",
@@ -68,7 +82,8 @@
     }
   },
   "eslintIgnore": [
-    "dist"
+    "dist",
+    "index.d.ts"
   ],
   "devDependencies": {
     "@types/chai": "^4.2.11",
@@ -82,13 +97,13 @@
     "eslint": "^7.1.0",
     "eslint-config-developit": "^1.2.0",
     "esm": "^3.2.25",
-    "microbundle": "^0.12.0",
-    "mocha": "^7.2.0",
+    "microbundle": "^0.12.3",
+    "mocha": "^8.0.1",
     "npm-run-all": "^4.1.5",
     "rimraf": "^3.0.2",
     "sinon": "^9.0.2",
     "sinon-chai": "^3.5.0",
-    "ts-node": "^8.10.1",
+    "ts-node": "^8.10.2",
     "typescript": "^3.9.3"
   }
-}
+}
\ No newline at end of file
diff --git a/src/index.ts b/src/index.ts
index e640292..c681fd0 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -2,8 +2,8 @@ export type EventType = string | symbol;
 
 // An event handler can take an optional event argument
 // and should not return a value
-export type Handler = (event?: any) => void;
-export type WildcardHandler = (type: EventType, event?: any) => void
+export type Handler<T = any> = (event?: T) => void;
+export type WildcardHandler = (type: EventType, event?: any) => void;
 
 // An array of all currently registered event handlers for a type
 export type EventHandlerList = Array<Handler>;
@@ -13,10 +13,10 @@ export type WildCardEventHandlerList = Array<WildcardHandler>;
 export type EventHandlerMap = Map<EventType, EventHandlerList | WildCardEventHandlerList>;
 
 export interface Emitter {
-	on(type: EventType, handler: Handler): void;
+	on<T = any>(type: EventType, handler: Handler<T>): void;
 	on(type: '*', handler: WildcardHandler): void;
 
-	off(type: EventType, handler: Handler): void;
+	off<T = any>(type: EventType, handler: Handler<T>): void;
 	off(type: '*', handler: WildcardHandler): void;
 
 	emit<T = any>(type: EventType, event?: T): void;
@@ -38,7 +38,7 @@ export default function mitt(all?: EventHandlerMap): Emitter {
 		 * @param {Function} handler Function to call in response to given event
 		 * @memberOf mitt
 		 */
-		on(type: EventType, handler: Handler) {
+		on<T = any>(type: EventType, handler: Handler<T>) {
 			const handlers = all.get(type);
 			const added = handlers && handlers.push(handler);
 			if (!added) {
@@ -53,7 +53,7 @@ export default function mitt(all?: EventHandlerMap): Emitter {
 		 * @param {Function} handler Handler function to remove
 		 * @memberOf mitt
 		 */
-		off(type: EventType, handler: Handler) {
+		off<T = any>(type: EventType, handler: Handler<T>) {
 			const handlers = all.get(type);
 			if (handlers) {
 				handlers.splice(handlers.indexOf(handler) >>> 0, 1);
@@ -70,7 +70,7 @@ export default function mitt(all?: EventHandlerMap): Emitter {
 		 * @param {Any} [evt] Any value (object is recommended and powerful), passed to each handler
 		 * @memberOf mitt
 		 */
-		emit(type: EventType, evt: any) {
+		emit<T = any>(type: EventType, evt: T) {
 			((all.get(type) || []) as EventHandlerList).slice().map((handler) => { handler(evt); });
 			((all.get('*') || []) as WildCardEventHandlerList).slice().map((handler) => { handler(type, evt); });
 		}
diff --git a/test/index.js b/test/index_test.ts
index a837d36..1f1d9bb 100644
--- a/test/index.js
+++ b/test/index_test.ts
@@ -1,4 +1,4 @@
-import mitt from '..';
+import mitt, { Emitter } from '..';
 import chai, { expect } from 'chai';
 import { spy } from 'sinon';
 import sinonChai from 'sinon-chai';
@@ -23,7 +23,7 @@ describe('mitt', () => {
 });
 
 describe('mitt#', () => {
-	let events, inst;
+	let events, inst: Emitter;
 
 	beforeEach( () => {
 		events = new Map();
@@ -143,7 +143,7 @@ describe('mitt#', () => {
 		it('should invoke handler for type', () => {
 			const event = { a: 'b' };
 
-			inst.on('foo', (one, two) => {
+			inst.on('foo', (one, two?) => {
 				expect(one).to.deep.equal(event);
 				expect(two).to.be.an('undefined');
 			});
diff --git a/test/test-types-compilation.ts b/test/test-types-compilation.ts
new file mode 100644
index 0000000..00510da
--- /dev/null
+++ b/test/test-types-compilation.ts
@@ -0,0 +1,43 @@
+/* eslint-disable @typescript-eslint/ban-ts-comment, @typescript-eslint/no-unused-vars */
+
+import mitt from '..';
+
+const emitter = mitt();
+
+/*
+ * Check that if on is provided a generic, it only accepts handlers of that type
+ */
+{
+	const badHandler = (x: number) => {};
+	const goodHandler = (x: string) => {};
+
+	// @ts-expect-error
+	emitter.on<string>('foo', badHandler);
+	emitter.on<string>('foo', goodHandler);
+}
+
+/*
+ * Check that if off is provided a generic, it only accepts handlers of that type
+ */
+{
+	const badHandler = (x: number) => {};
+	const goodHandler = (x: string) => {};
+
+	// @ts-expect-error
+	emitter.off<string>('foo', badHandler);
+	emitter.off<string>('foo', goodHandler);
+}
+
+
+/*
+ * Check that if emitt is provided a generic, it only accepts event data of that type
+ */
+{
+	interface SomeEventData {
+    name: string;
+  }
+  // @ts-expect-error
+	emitter.emit<SomeEventData>('foo', 'NOT VALID');
+	emitter.emit<SomeEventData>('foo', { name: 'jack' });
+}
+
diff --git a/test/types.ts b/test/types.ts
deleted file mode 100644
index 23334bb..0000000
--- a/test/types.ts
+++ /dev/null
@@ -1,20 +0,0 @@
-import mitt, { EventHandlerList, EventHandlerMap } from '..';
-
-const events = mitt();
-function foo() {}
-events.on('foo', foo);
-events.emit('foo', 'hello');
-
-// handler return type should be ignored:
-events.on('foo', async e => e * 42);
-
-// event map type
-const map = new Map<string, EventHandlerList>([
-	['foo', [foo]]
-]);
-const events2 = mitt(map);
-events2.emit('foo', 'hello');
-
-// event map type & iterables
-const map2 : EventHandlerMap = new Map(Object.entries(({ foo: [foo] })));
-mitt(map2);
diff --git a/tsconfig.json b/tsconfig.json
index 2610831..acab4f5 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -3,9 +3,11 @@
 	"compilerOptions": {
 		"noEmit": true,
 		"declaration": true,
-		"moduleResolution": "node"
+		"moduleResolution": "node",
+		"esModuleInterop": true
 	},
-	"exclude": [
-		"test"
+	"include": [
+		"src/*.ts",
+		"test/*.ts",
 	]
 }