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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? How does defining a toJSON() method on a class customize its JSON.stringify() output? 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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What is structuredClone() and how does it differ from JSON.parse(JSON.stringify(obj)) for deep cloning?

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Answer

structuredClone() is a native deep-cloning function that correctly handles circular references, Maps, Sets, Dates, TypedArrays, and more — all of which the JSON round-trip technique either throws on or silently mangles. It's the modern, correct default choice for deep cloning.
const original = { date: new Date(), map: new Map([['a', 1]]) };
const clone = structuredClone(original);
console.log(clone.date instanceof Date); // true
console.log(clone.map instanceof Map);   // true
Real-world example Deep-cloning complex application state (including Dates and Maps) without the data-loss bugs of the JSON-based hack.

Common follow-ups: What kinds of values can structuredClone() NOT handle, like functions or DOM nodes?

Map Set WeakMap & WeakSet
All 10 JSON & Data Serialization questions