Add Flow type annotations (#23)

* Add flowtype annotations

* Add flow to test script

1 files changed, 15 insertions, 4 deletions

diff --git a/src/index.js b/src/index.js
index ae6bd63..92b19f9 100644
--- a/src/index.js
+++ b/src/index.js
@@ -1,8 +1,19 @@
+// @flow
+// An event handler can take an optional event argument
+// and should not return a value
+type EventHandler = (event?: any) => void;
+// An array of all currently registered event handlers for a type
+type EventHandlerList = Array<EventHandler>;
+// A map of event types and their corresponding event handlers.
+type EventHandlerMap = {
+  [type: string]: EventHandlerList,
+};
+
 /** Mitt: Tiny (~200b) functional event emitter / pubsub.
  *  @name mitt
  *  @returns {Mitt}
  */
-export default function mitt(all) {
+export default function mitt(all: EventHandlerMap) {
 	all = all || Object.create(null);
 
 	return {
@@ -14,7 +25,7 @@ export default function mitt(all) {
 		 * @return {Object} the `mitt` instance for chaining
 		 * @memberOf mitt
 		 */
-		on(type, handler) {
+		on(type: string, handler: EventHandler) {
 			(all[type] || (all[type] = [])).push(handler);
 		},
 
@@ -26,7 +37,7 @@ export default function mitt(all) {
 		 * @return {Object} the `mitt` instance for chaining
 		 * @memberOf mitt
 		 */
-		off(type, handler) {
+		off(type: string, handler: EventHandler) {
 			let e = all[type] || (all[type] = []);
 			e.splice(e.indexOf(handler) >>> 0, 1);
 		},
@@ -40,7 +51,7 @@ export default function mitt(all) {
 		 * @return {Object} the `mitt` instance for chaining
 		 * @memberof mitt
 		 */
-		emit(type, evt) {
+		emit(type: string, evt: EventHandler) {
 			(all[type] || []).map((handler) => { handler(evt); });
 			(all['*'] || []).map((handler) => { handler(type, evt); });
 		}