A typescript implementation from the "Building a Reactive Library from Scratch" blog https://dev.to/ryansolid/building-a-reactive-library-from-scratch-1i0p

signals.ts

type Getter<T> = () => T;
type Setter<T> = (nextValue: T) => void;
type Signal = {
  execute: () => void;
  dependencies: Set<Set<Signal>>;
};

const context: Signal[] = [];

function subscribe(running: Signal, subscriptions: Set<Signal>) {
  subscriptions.add(running);
  running.dependencies.add(subscriptions);
}

export function createSignal<T>(value: T): [Getter<T>, Setter<T>] {
  const subscriptions: Set<Signal> = new Set();

  const read = () => {
    const running = context[context.length - 1];
    if (running) subscribe(running, subscriptions);
    return value;
  };

  const write = (nextValue: T) => {
    value = nextValue;

    for (const sub of [...subscriptions]) {
      sub.execute();
    }
  };
  return [read, write];
}

function cleanup(running: Signal) {
  for (const dep of running.dependencies) {
    dep.delete(running);
  }
  running.dependencies.clear();
}

export function createEffect(fn: () => void) {
  let running: Signal;

  const execute = () => {
    cleanup(running);
    context.push(running);
    try {
      fn();
    } finally {
      context.pop();
    }
  };

  running = {
    execute,
    dependencies: new Set(),
  };

  execute();
}

export function createMemo<T>(fn: () => T) {
  const [s, setS] = createSignal<T>(null as unknown as T);
  createEffect(() => setS(fn()));
  return s;
}

console.log("1. Create");
const [firstName, setFirstName] = createSignal("John");
const [lastName, setLastName] = createSignal("Smith");
const [showFullName, setShowFullName] = createSignal(true);

const displayName = createMemo(() => {
  if (!showFullName()) return firstName();
  return `${firstName()} ${lastName()}`;
});

createEffect(() => console.log("My name is", displayName()));

console.log("2. Set showFullName: false ");
setShowFullName(false);

console.log("3. Change lastName");
setLastName("Legend");

console.log("4. Set showFullName: true");
setShowFullName(true);