fix(SequentialEvent.Emit): Copy handlers array on event emission

Avoids the error of `undefined` handler on race condition with `once` handlers

src/sequential-event.ts

@@ -57,7 +57,7 @@ export class SequentialEvent {
 			return Promise.resolve();
 		}
 
-		const retPromise = emitHandlers(handler, this, args);
+		const retPromise = emitHandlers(handler.slice(0), this, args);
 
 		return retPromise;
 	}

src/utils.ts

@@ -59,11 +59,6 @@ export const getNextPromise = (
 	return (resolve: Function, reject: (reason: any) => any) => {
 		const _getNextPromise = (prevResolve?: any) => {
 			// Handle if an event handler disapeared during event dispatching
-			const handlersLength2 = handlers.length;
-			if (handlersLength2 !== handlersLength) {
-				i -= handlersLength - handlersLength2;
-				handlersLength = handlersLength2;
-			}
 			if (i < handlersLength) {
 				const stepArgs =
 					'undefined' !== typeof prevResolve

test/sequential-event.test.ts

@@ -217,6 +217,20 @@ describe('Once & remove listeners', () => {
 
 		expect(test).toHaveBeenCalledTimes(1);
 	});
+	it('Paralllel execution of "once" should be prevented inside the handler function', async () => {
+		const tests = [jest.fn(), jest.fn()];
+		const mySequentialEvent = new SequentialEvent();
+
+		mySequentialEvent.once('a', tests);
+		await Promise.all([
+			mySequentialEvent.emit('a', 'FOO', 1),
+			mySequentialEvent.emit('a', 'BAR', 42),
+		]);
+		expect(tests[0]).toHaveBeenCalledTimes(1);
+		expect(tests[0]).toHaveBeenCalledWith('FOO', 1);
+		expect(tests[1]).toHaveBeenCalledTimes(1);
+		expect(tests[1]).toHaveBeenCalledWith('BAR', 42);
+	});
 	it('Remove all listeners', async () => {
 		const tests = [jest.fn(), jest.fn()];
 		const mySequentialEvent = new SequentialEvent();