SharedArray constructor does not support async functions as…

Error message

SharedArray constructor does not support async functions as second argument

What it means

The second argument to new SharedArray is a builder function that k6 calls synchronously once, in the init context, to produce the shared data. Because the result must be available immediately for cross-VU serialization, the constructor explicitly rejects async functions via common.IsAsyncFunction before doing anything else.

Solutions

Remove the async keyword - the builder must return the array synchronously: () => [...].
If the builder awaits I/O, restructure it: load bytes synchronously with open() at init time (open() is synchronous in k6) and parse them inside a sync builder.
For remote data, fetch it before the test (e.g. in a separate pre-step) and embed it in a local JSON file consumed by a sync builder.

Example fix

// before
new SharedArray('data', async () => JSON.parse(open('./data.json'))); // throws

// after
new SharedArray('data', () => JSON.parse(open('./data.json')));

Defensive patterns

Strategy: type-guard

Validate before calling

const isAsyncFn = (fn) => fn?.constructor?.name === 'AsyncFunction';
if (isAsyncFn(builder)) {
  throw new Error('SharedArray builder must be synchronous; remove the async keyword');
}

Type guard

const isSyncFunction = (fn) => typeof fn === 'function' && fn.constructor.name === 'Function';

Prevention

Never write async in a SharedArray builder - k6 cannot await it in init.
Do file I/O with the synchronous open() instead of fetch inside builders.

When it happens

Trigger: 'new SharedArray("name", async () => [...])' - an async function declared with the async keyword or an AsyncFunction constructor result. The error is thrown before the function is ever called.

Common situations: Refactoring data loaders that were originally async (fetch from a file, DB) into SharedArray builders; copy-pasting async open()/http code into the builder; IDE auto-adding async to the arrow function.

Related errors

AI-assisted analysis of grafana/k6@01ffac6f24 (2026-08-18). Data as JSON: /api/errors/6a7c6a3ce7ad90eb. Report an issue: GitHub.

Appendix: source

Thrown at internal/js/modules/k6/data/data.go:88

const asyncFunctionNotSupportedMsg = "SharedArray constructor does not support async functions as second argument"

// sharedArray is a constructor returning a shareable read-only array
// indentified by the name and having their contents be whatever the call returns
func (d *Data) sharedArray(call sobek.ConstructorCall) *sobek.Object {
	rt := d.vu.Runtime()

	if d.vu.State() != nil {
		common.Throw(rt, errors.New("new SharedArray must be called in the init context"))
	}

	name := call.Argument(0).String()
	if name == "" {
		common.Throw(rt, errors.New("empty name provided to SharedArray's constructor"))
	}
	val := call.Argument(1)

	if common.IsAsyncFunction(rt, val) {
		common.Throw(rt, errors.New(asyncFunctionNotSupportedMsg))
	}

	fn, ok := sobek.AssertFunction(val)
	if !ok {
		common.Throw(rt, errors.New("a function is expected as the second argument of SharedArray's constructor"))
	}

	builder := func() (sharedArray, error) { return getSharedArrayFromCall(rt, fn) }
	array, err := d.shared.loadOrStore(name, builder)
	if err != nil {
		common.Throw(rt, err)
	}

	return array.wrap(rt).ToObject(rt)
}

// RecordReader is the interface that wraps the action of reading records from a resource.
//

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