karatelabs/karate · error · JsErrorException

DataCloneError: a could not be cloned

Error message

DataCloneError: a ${type} could not be cloned

What it means

The structured clone algorithm (JsStructuredClone) cannot copy values of type `function` or `symbol`, matching the HTML spec's DataCloneError. When clone() encounters such a value via typeOf it throws "DataCloneError: a ${type} could not be cloned". This occurs in postMessage-style messaging, worker-style APIs, or any structured-clone copy operation.

Solutions

1. Remove functions/symbols from the data being cloned; send plain data (objects, arrays, primitives).
2. Pass function references separately (e.g. by name) and resolve them on the receiving side.
3. Convert symbol-keyed data to string keys before cloning.

Example fix

// before
channel.postMessage({ callback: function () { ... } });

// after
channel.postMessage({ callbackName: 'myHandler' }); // resolve by name on receiver

// before cloning, assert the payload is data-only
function isCloneable(v) {
  return v === null || ['string','number','boolean','undefined'].includes(typeof v)
    || (Array.isArray(v) && v.every(isCloneable))
    || (typeof v === 'object' && Object.values(v).every(isCloneable));
}

function isCloneable(v) {
  if (typeof v === 'function' || typeof v === 'symbol') return false;
  if (v === null || typeof v !== 'object') return true;
  return Object.values(v).every(isCloneable);
}

When it happens

Trigger: Calling the structured-clone entry (clone/call/copyOwn) with an argument that is a function or a symbol, or an object graph containing one (e.g. passing a callback through a message).

Common situations: Passing callbacks or class methods through message passing APIs; including symbols as object keys in payloads intended for cloning; porting Node/browser structuredClone code that hit the same rule.

Related errors

• a class declaration may not be the body of
• a function declaration may not be the body of
• a lexical declaration may not be the body of
• AggregateError requires an iterable of errors
• append() needs at least two arguments

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/0060f4f53d054046. Report an issue: GitHub.

Appendix: source

Thrown at karate-js/src/main/java/io/karatelabs/js/JsStructuredClone.java:65

    private static final byte DATA_ATTRS = JsObject.WRITABLE | JsObject.CONFIGURABLE;

    private JsStructuredClone() {
    }

    static Object call(Context context, Object[] args) {
        if (args.length == 0) {
            throw JsErrorException.typeError("structuredClone requires at least 1 argument");
        }
        // args[1] is the options bag carrying `transfer` — nothing in this
        // engine is transferable, so it is accepted and ignored
        return clone(args[0], new IdentityHashMap<>(), context instanceof CoreContext cc ? cc : null);
    }

    private static Object clone(Object value, IdentityHashMap<Object, Object> memo, CoreContext ctx) {
        String type = Terms.typeOf(value);
        if ("function".equals(type) || "symbol".equals(type)) {
            throw dataCloneError(type);
        }
        Object seen = memo.get(value);
        if (seen != null) {
            return seen;
        }
        if (value instanceof JsPrimitive jp) {
            return remember(memo, value, boxed(jp));
        }
        if (value instanceof JsDate d) {
            return remember(memo, value, new JsDate(d.getTimeValue()));
        }
        if (value instanceof JsRegex r) {
            return remember(memo, value, new JsRegex(r.pattern, r.flags));
        }
        if (value instanceof JsArray a) {
            JsArray copy = new JsArray(new ArrayList<>(a.list.size()));
            memo.put(value, copy);
            // jsEntries is the spec own-key walk: holes are skipped, index

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