What does the toJSON() method do?

JavaScriptJunior~2 min read

toJSON() is a special method that JavaScript calls automatically when an object is serialized via JSON.stringify().

It lets you control exactly what ends up in the JSON string.

In short

If an object has a toJSON() method, then calling JSON.stringify(obj) serializes the result of calling that method instead of the object itself.

Example - basic

javascript

const user = {
  name: 'Alex',
  age: 25,
  password: 'secret',
  toJSON() {
    // choose what ends up in the JSON
    return { name: this.name, age: this.age };
  }
};

console.log(JSON.stringify(user));
// '{"name":"Alex","age":25}'

password did not end up in the JSON, because toJSON() returns only name and age.

How it works under the hood

When you call:

javascript

JSON.stringify(obj)

JavaScript does the following:

Checks: does obj have a toJSON() method? → if it does - calls it:

javascript

const value = obj.toJSON();

Serializes the result of the call, not the object itself.

Example with nested objects

toJSON() works recursively for all nested objects.

javascript

const user = {
  name: 'Alex',
  stats: {
    score: 42,
    toJSON() {
      return 'Top Secret'; // replace the object with a string
    }
  }
};

console.log(JSON.stringify(user));
// '{"name":"Alex","stats":"Top Secret"}'

The toJSON() method inside stats was called automatically and replaced the nested object.

Example with classes

You can use toJSON() in your own classes to serialize instances nicely:

javascript

class User {
  constructor(name, age) {
    this.name = name;
    this.age = age;
  }

  toJSON() {
    return {
      user: this.name,
      age: this.age
    };
  }
}

const alex = new User('Alex', 25);
console.log(JSON.stringify(alex));
// '{"user":"Alex","age":25}'

This is convenient for defining a "clean" representation of data, for example when sending it to a server.

Example: built-in types have toJSON() too

Type	What `toJSON()` does	Example
`Date`	Returns an ISO string	`new Date().toJSON()` → `"2025-10-14T17:00:00.000Z"`
`Map` / `Set`	Not serialized (by default)	`JSON.stringify(new Map())` → `{}`
`BigInt`	Not supported	Error: TypeError: Do not know how to serialize a BigInt

Example: when toJSON() returns something other than an object

You can return any value - a string, a number, even null.

javascript

const product = {
  name: 'T-shirt',
  price: 1500,
  toJSON() {
    return `${this.name}: ${this.price}$`;
  }
};

console.log(JSON.stringify(product));
// "\"T-shirt: 1500$\""

The returned string becomes the serialization result.

Interaction with replacer

If an object has both toJSON() and a replacer, toJSON() is called first, and then the result is passed to the replacer.

javascript

const obj = {
  name: 'Alex',
  toJSON() {
    return { custom: true };
  }
};

console.log(JSON.stringify(obj, (key, value) => {
  if (key === 'custom') return 'done';
  return value;
}));
// '{"custom":"done"}'

SUMMARY

What it does	Determines what gets serialized by `JSON.stringify()`
Where it is called	Automatically during `JSON.stringify()`
What it returns	Any value (object, string, number, etc.)
Works recursively	Yes (for nested objects)
Usable in classes	Yes
Ignored by `JSON.parse()`	Yes (only affects serialization)

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