Get React key for immutable object without memory leaks.

getWeakKey.js

export const getWeakKey = (() => {
  const map = new WeakMap();
  let idx = 0;
  return element => {
    if (map.has(element)) return map.get(element);
    idx++;
    map.set(element, idx.toString());
    return idx;
  };
})();