grafana/k6 · error

failed to create Blob: %w

Error message

failed to create Blob: %w

What it means

Wrap of a Sobek runtime constructor failure while materializing an incoming binary WebSocket message: when binaryType is "blob", the raw bytes are passed to the script's Blob constructor (new Blob([data])). The wrapped error fires when that constructor call throws in the JS runtime — commonly because the Blob global is shadowed, its arguments are invalid, or the runtime environment lacks the expected built-in.

Source

Thrown at internal/js/modules/k6/websockets/websockets.go:503

				Metric: w.builtinMetrics.WSMessagesReceived,
				Tags:   w.tagsAndMeta.Tags,
			},
			Time:     msg.t,
			Metadata: w.tagsAndMeta.Metadata,
			Value:    1,
		})

		rt := w.vu.Runtime()
		ev := w.newEvent(events.MESSAGE, msg.t)

		if msg.mtype == websocket.BinaryMessage {
			var data any
			switch w.binaryType {
			case blobBinaryType:
				var err error
				data, err = rt.New(w.blobConstructor, rt.ToValue([]any{msg.data}))
				if err != nil {
					return fmt.Errorf("failed to create Blob: %w", err)
				}
			case arraybufferBinaryType:
				data = rt.NewArrayBuffer(msg.data)
			default:
				return fmt.Errorf(`unknown binaryType %q, the supported ones are "blob" and "arraybuffer"`, w.binaryType)
			}
			must(rt, ev.DefineDataProperty("data", rt.ToValue(data), sobek.FLAG_FALSE, sobek.FLAG_FALSE, sobek.FLAG_TRUE))
		} else {
			must(
				rt,
				ev.DefineDataProperty("data", rt.ToValue(string(msg.data)), sobek.FLAG_FALSE, sobek.FLAG_FALSE, sobek.FLAG_TRUE),
			)
		}
		must(
			rt,
			ev.DefineDataProperty("origin", rt.ToValue(w.url.String()), sobek.FLAG_FALSE, sobek.FLAG_FALSE, sobek.FLAG_TRUE),
		)

View on GitHub (pinned to 165bb3c1a4)

Solutions

1. Avoid shadowing or replacing the global Blob constructor in the script scope
2. Switch ws.binaryType to "arraybuffer" to receive binary data without constructing a Blob
3. Update k6 to a version whose runtime reliably provides the Blob constructor

When it happens

Trigger: Thrown at internal/js/modules/k6/websockets/websockets.go:478 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of grafana/k6@165bb3c1a4 (2026-08-18). Data as JSON: /api/errors/bffb4ee170f369a1. Report an issue: GitHub.