Documenting the pattern I liked in React's useSubscription hook.

subscriber-pattern.ts

// https://github.com/facebook/react/blob/master/packages/use-subscription/src/useSubscription.js

type Subscription<Value> = () => {
  getValue: () => Value;
  subscribe: (callback: Function) => () => void;
};

const subscription: Subscription<string> = () => ({
  getValue: () => something.value,
  subscribe: callback => {
    // Subscribe the subscriber that triggered this function.
    something.subscribe(callback);
    // Return the unsubscription function to the subscriber for use whenever.
    return () => something.unsubscribe(callback);
  },
});

let value = '';

const subscriber = () => {
  const {getValue, subscribe} = subscription();

  value = getValue();

  const callback = () => {
    value = getValue();
  };

  const unsubscribe = subscribe(callback);

  // Whenever you want to unsubscribe:
  unsubscribe();

  // Or even return for external use:
  return () => unsubscribe();
};