How is structuredClone() different from JSON.parse(JSON.stringify(...)) for deep copying?

Direct answer

structuredClone uses the structured clone algorithm and supports cyclic graphs and many built-in data types that JSON loses, but it still cannot clone every JavaScript value and should not be treated as a way to preserve arbitrary executable object behavior.

structuredClone(value) is usually a much better built-in choice than a JSON round-trip when the goal is to duplicate supported in-memory data.

It can preserve cycles and supports many structured data types such as Date, Map, Set, typed arrays, and ArrayBuffer.
Functions and DOM nodes are not general cloneable data values and can cause cloning to fail.
Structured cloning is about data, not preserving arbitrary class behavior, accessors, property descriptors, or every custom prototype relationship exactly as authored.
Transferable objects can sometimes be transferred rather than copied when using APIs/options that support transfer lists.

For immutable application state, you often do not need to deep-clone an entire graph. Copying only the path being changed is usually cheaper and preserves structural sharing.