The structured clone algorithm

The structured clone algorithm copies complex JavaScript objects. It is used internally when invoking structuredClone(), to transfer data between Workers via postMessage(), storing objects with IndexedDB, or copying objects for other APIs.

It clones by recursing through the input object while maintaining a map of previously visited references, to avoid infinitely traversing cycles.

Things that don't work with structured clone

Function objects cannot be duplicated by the structured clone algorithm; attempting to throws a DataCloneError exception.
Cloning DOM nodes likewise throws a DataCloneError exception.
Certain object properties are not preserved:
- The lastIndex property of RegExp objects is not preserved.
- Property descriptors, setters, getters, and similar metadata-like features are not duplicated. For example, if an object is marked readonly with a property descriptor, it will be read/write in the duplicate, since that's the default.
- The prototype chain is not walked or duplicated.
- Class private elements are not duplicated. (Although internal fields of built-in types may.)

Supported types

JavaScript types

Array
ArrayBuffer
Boolean
DataView
Date
Error types (but see Error types below).
Map
Number
Object objects: but only plain objects (e.g., from object literals).
Primitive types, except symbol.
RegExp: but note that lastIndex is not preserved.
Set
String
TypedArray

Error types

For Error types, the error name must be one of: Error, EvalError, RangeError, ReferenceError, SyntaxError, TypeError, URIError (or will be set to "Error").

Browsers must serialize the properties name and message, and are expected to serialize other "interesting" properties of the errors such as stack, cause, etc.

AggregateError support is expected to be added to the specification in whatwg/html#5749 (and is already supported in some browsers).