Explain shallow vs deep copy. What are the ways to deep copy, and what are the pitfalls of JSON.parse(JSON.stringify(obj))?

A shallow copy makes a new top-level object, but nested objects are still shared. A deep copy copies every level, so nothing is shared.

Shallow copy:

• Spread ({ ...obj }, [...arr]), Object.assign, Array.from and slice are all shallow.
• If you change a nested property through the copy, the original changes too.

Deep copy with structuredClone(obj): this is the modern built-in.

• It handles nested objects, cyclic references (objects that point back to themselves), Date, Map, Set, typed arrays and more.
• It throws an error on functions and DOM nodes.
• It does not keep class prototypes, getters or symbol keys.

Pitfalls of JSON.parse(JSON.stringify(obj)):

• It drops undefined, functions and symbol values.
• It turns Date into a string.
• It turns NaN and Infinity into null.
• It turns Map and Set into {}.
• It throws on cycles and on BigInt.
• It loses prototypes.