Poor mans version of S.js

s.js

const effects = [];

let isCollectingEffects = false;

const data = (init) => {
  const deps = [];

  let oldValue = init;

  return (value) => {
    if (value === undefined) {
      if (isCollectingEffects) {
        effects.push(deps);
      }
      return oldValue;
    } else {
      oldValue = value;
      deps.forEach((cb) => cb());
    }
  };
};

const S = (callback) => {
  isCollectingEffects = true;

  effects.splice(0);

  callback();

  isCollectingEffects = false;

  effects.forEach((effect) => {
    effect.push(callback);
  });
};

const name = data("mendes");

S(() => {
  document.body.innerText = name();
});

S(() => {
  console.log(name());
});

setTimeout(() => {
  name("Phellipe");
}, 3000);