Reference

Copy / serialize spectrum

Which tool copies a value — and what you actually get. Companion to lesson 1.

← Lesson 1 Lesson 2 Lesson 3 Lesson 4 Lesson 5 Glossary Type matrix Transfer Resources

Pick by the hard problem

Hard problem Default tool
New outer object; nested refs may stay shared Shallow copy
A JSON document (HTTP body, file, log) JSON — design for loss
Same-realm snapshot; nested identity must break structuredClone()
Value already crossing a platform API that clones Implicit clone — don’t double-copy
Large binary; sender must give up the bytes Transfer
Methods, class identity, versioned wire, revival Custom serializer

JSON vs structured clone (the usual mix-up)

Value JSON round-trip Structured clone
Plain objects / arrays / primitives OK (no symbols) OK (no symbol primitives)
Date ISO string via toJSON Still a Date
Map / Set Become {} (silent) Preserved
Cycles TypeError Preserved
undefined on an object Key dropped Kept
Function / DOM node Dropped or null (silent) DataCloneError
ArrayBuffer / typed arrays Useless / object-shaped Cloned (or transferred)
Class instance methods / prototype Lost Lost (plain data)

JSON behavior: MDN JSON.stringify. Clone types: MDN structured clone.

Implicit surfaces (already cloning)