In JavaScript, the JSON.stringify() function looks for functions named toJSON in the object being serialized. If an object has a toJSON function, JSON.stringify() calls toJSON() and serializes the return value from toJSON() instead.

For example, the below script prints the same thing as JSON.stringify({ answer: 42 }).

const json = JSON.stringify({
  answer: { toJSON: () => 42 }
});

console.log(json); // {"answer":42}

With ES6 Classes

The toJSON() function is useful for making sure ES6 classes get serialized correctly. For example, suppose you have a custom JavaScript error class.

class HTTPError extends Error {
  constructor(message, status) {
    super(message);
    this.status = status;
  }
}

By default, JavaScript isn’t great with serializing errors. The below script prints {"status":404}, no error message or stack trace.

class HTTPError extends Error {
  constructor(message, status) {
    super(message);
    this.status = status;
  }
}

const e = new HTTPError('Fail', 404);
console.log(JSON.stringify(e)); // {"status":404}

However, if you add a toJSON() method to your HTTPError class, you can configure how JavaScript serializes instances of HTTPError.

class HTTPError extends Error {
  constructor(message, status) {
    super(message);
    this.status = status;
  }

  toJSON() {
    return { message: this.message, status: this.status };
  }
}

const e = new HTTPError('Fail', 404);
console.log(JSON.stringify(e)); // {"message":"Fail","status":404}

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