karatelabs/karate · error · JsErrorException

iterator.throw is not a function

Error message

iterator.throw is not a function

What it means

When a throw completion is delegated to an inner iterator and its 'throw' property is present but not callable (e.g. null, a number, an object), the engine throws 'iterator.throw is not a function'. Per GetMethod semantics this throws immediately without closing the iterator.

Solutions

1. Ensure 'throw' is a function on the iterator object, or remove the property entirely so the missing-method path applies
2. Convert the iterator to a generator to inherit correct throw semantics
3. Check for accidental overwrites of iterator methods when extending the object

Example fix

// before
const it = { next() {...}, throw: null };
// after
const it = { next() {...}, throw(e) { return { done: true }; } };

if ('throw' in it && typeof it.throw !== 'function') delete it.throw; // or fix to a function

function isCallableProp(obj, name) { return obj == null || !(name in obj) || typeof obj[name] === 'function'; }

try { gen.throw(e); } catch (err) { if (String(err).includes('iterator.throw is not a function')) { /* repair iterator then rethrow */ } }

When it happens

Trigger: A custom iterator object has a non-function 'throw' property (typo, wrong assignment, or a placeholder value) and the consumer calls generator.throw() while suspended in yield* over that iterator.

Common situations: Assigning throw: true or throw: null as a placeholder; property-name collisions overwriting the method; partial iterator implementations copied from examples.

Understand the failure class

Background: Type mismatch errors: IllegalArgumentException, TypeError and type guards across 150 open-source libraries — this error's family across 150 libraries.

Related errors

• iterator result is not an object
• The iterator does not provide a 'throw' method
• Generator is already running
• Result of the Symbol.iterator method is not an object
• iterator. is not a function

AI-assisted analysis of karatelabs/karate@a22eb90246 (2026-09-12). Data as JSON: /api/errors/a730357fe6b02fbb. Report an issue: GitHub.

Appendix: source

Thrown at karate-js/src/main/java/io/karatelabs/js/Interpreter.java:2981

                        return Terms.UNDEFINED; // a throwing `throw` getter wins outright
                    }
                    if (throwFn == null || throwFn == Terms.UNDEFINED) {
                        // ABSENT throw: close the delegate (validating
                        // return()'s result when present), then TypeError
                        Object closed = invokeIteratorMethod(iterObj, "return", null, context, false);
                        if (context.isStopped()) {
                            return Terms.UNDEFINED;
                        }
                        if (closed != null && !(closed instanceof ObjectLike)) {
                            throw JsErrorException.typeError("iterator result is not an object");
                        }
                        throw JsErrorException.typeError(
                                "The iterator does not provide a 'throw' method");
                    }
                    if (!(throwFn instanceof JsCallable throwCallable)) {
                        // GetMethod: a PRESENT non-callable throws immediately,
                        // with no iterator close
                        throw JsErrorException.typeError("iterator.throw is not a function");
                    }
                    step = invokeIteratorCallable(iterObj, throwCallable, sent, context);
                    if (context.isStopped()) {
                        return Terms.UNDEFINED;
                    }
                    if (!(step instanceof ObjectLike)) {
                        throw JsErrorException.typeError("iterator result is not an object");
                    }
                }
                case RETURN -> {
                    Object returned = invokeIteratorMethod(iterObj, "return", sent, context, false);
                    if (context.isStopped()) {
                        return Terms.UNDEFINED;
                    }
                    if (returned == null) {
                        // no return method: propagate the return completion
                        context.stopAndReturn(sent);
                        return Terms.UNDEFINED;

View on GitHub (pinned to a22eb90246)