Really simple pubsub

pubsub.test.ts

import { create } from './pubsub';

it('`create()` returns object with `publish` and `subscribe` fns', () => {
	const result = create();
	expect(result).toBeInstanceOf(Object);
	expect(result).toHaveProperty('publish');
	expect(result.publish).toBeInstanceOf(Function);
	expect(result).toHaveProperty('subscribe');
	expect(result.subscribe).toBeInstanceOf(Function);
});

it('can publish with no subscribers', () => {
	const { publish } = create<number>();
	expect(() => {
		publish(111);
	}).not.toThrow();
});

it('can publish with one subscriber', () => {
	const { subscribe, publish } = create<number>();
	const callback = jest.fn();
	subscribe(callback);
	publish(222);
	expect(callback).toHaveBeenCalledWith(222);
});

it('can publish with two subscribers', () => {
	const { subscribe, publish } = create<number>();
	const callback1 = jest.fn();
	const callback2 = jest.fn();
	subscribe(callback1);
	subscribe(callback2);
	publish(333);
	expect(callback1).toHaveBeenCalledWith(333);
	expect(callback2).toHaveBeenCalledWith(333);
});

it('can unsubscribe from subscribers', () => {
	const { subscribe, publish } = create<number>();
	const callback1 = jest.fn();
	const callback2 = jest.fn();
	const unsubscribe = subscribe(callback1);
	subscribe(callback2);
	unsubscribe();
	publish(444);
	expect(callback1).not.toHaveBeenCalled();
	expect(callback2).toHaveBeenCalledWith(444);
});

it('can subscribe many times', () => {
	const { subscribe, publish } = create<number>();
	const callback = jest.fn();
	subscribe(callback);
	subscribe(callback);
	publish(555);
	expect(callback).toHaveBeenCalledTimes(2);
	expect(callback).toHaveBeenCalledWith(555);
});

it('can unsubscribe many times', () => {
	const { subscribe, publish } = create<number>();
	const callback1 = jest.fn();
	const callback2 = jest.fn();
	const unsubscribe = subscribe(callback1);
	subscribe(callback2);
	unsubscribe();
	unsubscribe();
	unsubscribe();
	publish(666);
	expect(callback1).not.toHaveBeenCalled();
	expect(callback2).toHaveBeenCalledWith(666);
});

pubsub.ts

export type Callback<T> = (arg: T) => void;

export const create = <T>() => {
	const callbacks: Callback<T>[] = [];
	return {
		publish (value: T) {
			callbacks
				.filter(Boolean)
				.forEach((cb) => cb(value));
		},
		subscribe (callback: Callback<T>) {
			const i = callbacks.push(callback) - 1;
			return () => delete callbacks[i];
		},
	};
};

Creates a pubsub pair.

const { subscribe, publish } = create();
subscribe(value => console.log(`Got ${value}`));
publish(1234); // => "Got 1234"

Subscribe function returns unsubscribe function.

const { subscribe, publish } = create();
const unsubscribe = subscribe(value => console.log(`Got ${value}`));
unsubscribe();
publish(1234); // Does nothing