What are the pitfalls of JSON.parse and JSON.stringify?

JSON is a data format, not a JavaScript object serializer. Values that exist in JavaScript but not in JSON are either omitted, converted or rejected.

const source = {
  name: "Asha",
  missing: undefined,
  greet() {},
  when: new Date("2026-01-01"),
};

console.log(JSON.stringify(source));
// {"name":"Asha","when":"2026-01-01T00:00:00.000Z"}

JSON.stringify({ value: 10n }); // TypeError

const circular = {};
circular.self = circular;
// JSON.stringify(circular); // TypeError: circular structure

What gets lost

• undefined, functions and symbols are omitted from objects
• In arrays, those values become null
• Date is converted to an ISO string, not a Date
• BigInt throws
• Circular references throw
• Class prototypes and non-enumerable properties are not preserved

JSON.parse accepts a reviver function for transforming values, and JSON.stringify accepts a replacer function or array for filtering. A toJSON method lets an object control its own serialization.

const parsed = JSON.parse('{"when":"2026-01-01T00:00:00.000Z"}', (key, value) => {
  return key === "when" ? new Date(value) : value;
});

console.log(parsed.when instanceof Date); // true