JSON custom types serialization/parsing

.js

const OBJECT_TYPE = '[object Object]';
const TYPE_PROP = '__type__';
const VALUE_PROP = '__value__';

const jsParseMap = {
  '[object BigInt]': BigInt,
};

const jsSerializeMap = {
  '[object BigInt]': (v) => `${v}`,
};

const isTypedObject = (v) => {
  const typeTag = {}.toString.call(v);

  return (
    typeTag === OBJECT_TYPE
    && TYPE_PROP in v 
    && VALUE_PROP in v
  ); 
};

const replacer = (_, v) => {
  const typeTag = {}.toString.call(v);

  if (typeTag in jsSerializeMap) {
    return {
      [TYPE_PROP]: typeTag,
      [VALUE_PROP]: jsSerializeMap[typeTag](v),
    };
  } 

  return v;
};

const reviver = (_, v) => {
  if (isTypedObject(v)) {
    return jsParseMap[v[TYPE_PROP]](v[VALUE_PROP]);
  }

  return v;
};

// Example
const obj = {
  foo: 20n,
};

const jsonStr = JSON.stringify(obj, replacer); // {"foo":{"__type__":"[object BigInt]","__value__":"20"}}

typeof JSON.parse(jsonStr, reviver).foo // bigint;