All 10 JSON & Data Serialization questions
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What values does JSON.stringify() silently drop or convert, and why does this matter? How do you use the replacer parameter of JSON.stringify() to control what gets serialized? How does the reviver function in JSON.parse() work? Why does JSON.stringify() throw a TypeError on objects with circular references, and how do you handle it? What is structuredClone() and how does it differ from JSON.parse(JSON.stringify(obj)) for deep cloning? How do you safely parse potentially untrusted JSON without exposing your app to prototype pollution? How would you stream-parse a very large JSON file without loading it entirely into memory?
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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