foo.js

const a = new FinalizationRegistry(v => {
  console.log('a');
  console.log('b', v);
  for (const val in v) {
    console.log(val);
  }
});

function f() {
  const obj = {a: 3};
  a.register(obj, { b: 3});
}
f();
console.log('before gc');
global.gc();
console.log('after gc');