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

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