Appendix

Appendix A – Callback ref

React comes with two ways of receiving an element instance via the ref prop. The easiest one is the one we introduced, the object format. But there's another one called a callback ref that takes a functional format:

function commitAttachRef(fiber, instance) {
  var ref = fiber.ref;
  if (ref !== null) {
    if (typeof ref === 'function') {
      ref(instance)
    } else {
      ref.current = instance
    }
  }
}

Similarly, this functional format is supported when the DOM element is unmounted:

function commitDetachRef(fiber) {
  var ref = fiber.ref
  if (ref !== null) {
    if (typeof ref === 'function') {
       ref(null)
    } else {
     ...