`JSON.stringify` and `JSON.parse` that preserves nested undefined values

serializePOJO.js

/**
 * Serialize a POJO while preserving `undefined` values.
 */
function serializePOJO(value, undefinedPlaceholder = "[undefined]") {
  const replacer = (key, value) => (value === undefined ? undefinedPlaceholder : value);
  return JSON.stringify(value, replacer);
}

/**
 * Deserialize a POJO while preserving `undefined` values.
 */
function deserializePOJO(value, undefinedPlaceholder = "[undefined]") {
  const pojo = JSON.parse(value);
  if (pojo === undefinedPlaceholder) {
    return undefined;
  }

  // Function that walks through nested values
  function deepIterate(value, callback, parent, key) {
    if (typeof value === "object" && value !== null) {
      Object.entries(value).forEach(([entryKey, entryValue]) => deepIterate(entryValue, callback, value, entryKey));
    } else if (Array.isArray(value)) {
      value.forEach((itemValue, itemIndex) => deepIterate(itemValue, callback, value, itemIndex));
    } else if (parent !== undefined) {
      callback(value, parent, key);
    }
  }
	
  // https://jsfiddle.net/76y0mudk/5/
  deepIterate(pojo, (value, parent, key) => {
    if (value === undefinedPlaceholder) {
      parent[key] = undefined;
    }
  });

  return pojo;
}

const source = {
  foo : undefined,
  bar : {
   baz : undefined,
   qux : [1, undefined, 2]
  }
};

const serialized = serializePOJO(source);
console.log("Serialized", serialized);

const deserialized = deserializePOJO(serialized);
console.log("Deserialized", deserialized);

// See also https://stackoverflow.com/a/72216472/1480391