JSON.stringify() in JavaScript — complete guide with examples

JSON.stringify() converts a JavaScript value into a JSON string. It is the standard way to serialize data before sending it to a server, writing to localStorage, or logging structured output.

Basic syntax

JSON.stringify(value)
JSON.stringify(value, replacer)
JSON.stringify(value, replacer, space)

Simple examples

// Stringify an object
JSON.stringify({ name: "Ada", age: 30 });
// '{"name":"Ada","age":30}'

// Stringify an array
JSON.stringify([1, "two", true, null]);
// '[1,"two",true,null]'

// Stringify a primitive
JSON.stringify(42);    // '42'
JSON.stringify(true);  // 'true'
JSON.stringify(null);  // 'null'

Pretty-printing with the space parameter

Pass a number (indent size) or a string (custom indent character) as the third argument.

const obj = { id: 1, tags: ["json", "api"] };

JSON.stringify(obj, null, 2);
// {
//   "id": 1,
//   "tags": [
//     "json",
//     "api"
//   ]
// }

JSON.stringify(obj, null, '	'); // tab-indented

The replacer parameter

The replacer controls which properties are included. Pass an array to allowlist keys, or a function for full control.

const user = { id: 1, name: "Ada", password: "secret", role: "admin" };

// Array replacer — allowlist specific keys
JSON.stringify(user, ['id', 'name']);
// '{"id":1,"name":"Ada"}'

// Function replacer — transform or omit values
JSON.stringify(user, (key, value) => {
  if (key === 'password') return undefined; // omit
  if (typeof value === 'string') return value.toUpperCase();
  return value;
});
// '{"id":1,"NAME":"ADA","ROLE":"ADMIN"}'

What JSON.stringify omits

Several JavaScript value types are not valid JSON and are silently dropped or converted:

const obj = {
  name: "Ada",
  greet: () => "hello",  // functions → omitted
  sym: Symbol("x"),      // symbols → omitted
  x: undefined,          // undefined → omitted
  score: NaN,            // NaN → null
  ratio: Infinity,       // Infinity → null
  now: new Date(),       // Date → ISO string
};

JSON.stringify(obj);
// '{"name":"Ada","score":null,"ratio":null,"now":"2026-04-27T..."}'

Customising serialization with toJSON()

Add a toJSON() method to any object and JSON.stringify will call it to get the serialised value.

class Money {
  constructor(amount, currency) {
    this.amount = amount;
    this.currency = currency;
  }
  toJSON() {
    return `${this.amount} ${this.currency}`;
  }
}

JSON.stringify(new Money(9.99, 'USD')); // '"9.99 USD"'

Handling circular references

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

// Fix: use a replacer that tracks 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;
  });
}

Sending JSON to a server

await fetch('/api/users', {
  method: 'POST',
  headers: { 'Content-Type': 'application/json' },
  body: JSON.stringify({ name: 'Ada', role: 'engineer' }),
});