callback

A function to be invoked each time a registered target value is garbage collected. Its return value is ignored. The function is called with the following arguments:

heldValue

The value that was passed to the second parameter of the register() method when the target object was registered.

You create the registry passing in the callback:

const registry = new FinalizationRegistry((heldValue) => {
  // …
});

• FinalizationRegistry