All 10 JSON & Data Serialization questions
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JavaScript › JSON & Data Serialization

How does defining a toJSON() method on a class customize its JSON.stringify() output?

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Answer

If an object has a toJSON() method, JSON.stringify() calls it and serializes ITS return value instead of the object's own properties directly — a clean way to control exactly how a custom class is represented in JSON without a manual replacer.
class Money {
  constructor(cents) { this.cents = cents; }
  toJSON() { return (this.cents / 100).toFixed(2); }
}
JSON.stringify({ price: new Money(1999) }); // '{"price":"19.99"}'
Real-world example Making a Money or Temperature value object serialize as a simple display-friendly value instead of its internal representation.

Common follow-ups: Does Date's built-in toISOString-based JSON output work the same way, via its own toJSON() method?

Classes & Class Syntax
All 10 JSON & Data Serialization questions