All 10 JSON & Data Serialization questions
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What values does JSON.stringify() silently drop or convert, and why does this matter?

Intermediate

Answer

Functions, undefined, and Symbols are omitted entirely from objects (or converted to null inside arrays); Dates are converted to ISO strings; NaN and Infinity become null — meaning a round trip through JSON can silently lose or alter data types you didn't expect.
JSON.stringify({ fn: () => {}, val: undefined, date: new Date(), num: NaN });
// '{"date":"2026-08-08T00:00:00.000Z","num":null}'
// fn and val were dropped entirely
Real-world example A bug where a Date field silently becomes a plain string after being sent through an API, breaking date methods on the receiving end.

Common follow-ups: How would you restore a Date object after parsing JSON that stored it as a string?

Date Time & Internationalization (Intl API)
All 10 JSON & Data Serialization questions