JSON.stringify and JSON.parse: The Complete Guide

JSON.stringify — Basic Usage

const user = { name: "Alice", age: 30, active: true };
JSON.stringify(user);
// '{"name":"Alice","age":30,"active":true}'

Pretty-printing with indentation

JSON.stringify(user, null, 2);
// {
//   "name": "Alice",
//   "age": 30,
//   "active": true
// }

JSON.stringify(user, null, '\t'); // Tab indentation

What JSON.stringify omits

These values are omitted or converted during serialization:

const obj = {
  fn: () => {},        // omitted (functions)
  sym: Symbol('x'),   // omitted (symbols)
  undef: undefined,   // omitted (undefined)
  nan: NaN,           // becomes null
  inf: Infinity,      // becomes null
  date: new Date(),   // becomes ISO string
  regex: /abc/        // becomes {}
};
JSON.stringify(obj);
// '{"nan":null,"inf":null,"date":"2026-03-05T10:00:00.000Z","regex":{}}'

The Replacer Argument

The second argument to JSON.stringify is a replacer — either an array of allowed keys, or a function.

Array replacer — whitelist specific keys

const user = { id: 1, name: "Alice", password: "secret", email: "a@b.com" };
JSON.stringify(user, ['id', 'name']);
// '{"id":1,"name":"Alice"}'  — password excluded

Function replacer — transform values

JSON.stringify(user, (key, value) => {
  if (key === 'password') return undefined; // exclude
  if (typeof value === 'number') return value * 2; // transform
  return value;
});

Custom toJSON method

class Money {
  constructor(amount, currency) {
    this.amount = amount;
    this.currency = currency;
  }
  toJSON() {
    return `${this.amount} ${this.currency}`;
  }
}
JSON.stringify(new Money(42, 'USD')); // '"42 USD"'

JSON.parse — Basic Usage

const jsonString = '{"name":"Alice","age":30}';
const user = JSON.parse(jsonString);
console.log(user.name); // "Alice"

Always use try-catch

function safeParseJSON(str) {
  try {
    return { data: JSON.parse(str), error: null };
  } catch (e) {
    return { data: null, error: e.message };
  }
}

const { data, error } = safeParseJSON(userInput);
if (error) {
  console.error('Invalid JSON:', error);
}

Reviver — transform values on parse

const json = '{"name":"Alice","createdAt":"2026-03-05T10:00:00.000Z"}';

const obj = JSON.parse(json, (key, value) => {
  // Revive ISO date strings back to Date objects
  if (typeof value === 'string' && /^\d{4}-\d{2}-\d{2}T/.test(value)) {
    return new Date(value);
  }
  return value;
});

console.log(obj.createdAt instanceof Date); // true

Handling Circular References

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

Fix using a WeakSet to track seen objects:

function safeStringify(obj) {
  const seen = new WeakSet();
  return JSON.stringify(obj, (key, value) => {
    if (typeof value === 'object' && value !== null) {
      if (seen.has(value)) return '[Circular]';
      seen.add(value);
    }
    return value;
  });
}
safeStringify(a); // '{"self":"[Circular]"}'

Deep Clone with JSON

A common pattern for deep cloning simple objects:

const original = { a: 1, b: { c: 2 } };
const clone = JSON.parse(JSON.stringify(original));
clone.b.c = 99;
console.log(original.b.c); // 2 — original unchanged

Limitation: This does not clone functions, Dates (loses Date object, gets string), undefined values, symbols, or handle circular references. For production deep cloning, use structuredClone() instead.

// Better: structuredClone (modern browsers + Node 17+)
const clone = structuredClone(original);

Performance Tips

• Avoid stringify in hot loops — JSON.stringify is expensive for large objects. Cache the result if the object does not change.
• Use streaming for large data — For JSON larger than a few MB, use streaming parsers like jsonstream or clarinet in Node.js.
• Skip pretty-printing in production — JSON.stringify(obj, null, 2) is slower than JSON.stringify(obj) and produces larger output. Only pretty-print for logging and debugging.
• Prefer structuredClone over JSON round-trip — Faster and handles more types including Date, Map, Set, ArrayBuffer.

Frequently Asked Questions

What is the difference between JSON.stringify and toString?

toString() on an object returns [object Object] — not useful. JSON.stringify() serializes the full structure of the object to a JSON string. Always use JSON.stringify() for data serialization.

Does JSON.parse validate the JSON?

Yes — JSON.parse() throws a SyntaxError if the string is not valid JSON. This is why wrapping it in try-catch is essential when parsing untrusted input. For detailed error information (line/column), use the JSON Validator tool.

Can JSON.stringify handle BigInt?

No. JSON.stringify(BigInt(9007199254740993)) throws a TypeError: Do not know how to serialize a BigInt. Use a replacer to convert BigInt to string: (key, val) => typeof val === 'bigint' ? val.toString() : val.