antonmedv/fx · error

Unsupported value type: %v

Error message

Unsupported value type: %v

What it means

Terminal panic in Stringify: the goja value's reflected kind is none of the supported types (object/array/etc.), so no JSON rendering branch applies. In practice it fires for exotic or uninitialized reflected values reaching the printer, indicating an unsupported value rather than malformed user JSON.

Source

Thrown at internal/engine/stringify.go:112

		var out strings.Builder
		out.WriteString("[")
		out.WriteString("\n")

		for i, key := range keys {
			item := arr.Get(key)
			out.WriteString(strings.Repeat("  ", depth+1))
			out.WriteString(Stringify(item, vm, depth+1))
			if i < len(keys)-1 {
				out.WriteString(",")
			}
			out.WriteString("\n")
		}

		out.WriteString(strings.Repeat("  ", depth))
		out.WriteString("]")
		return out.String()
	}
	panic(fmt.Sprintf("Unsupported value type: %v", rtype.Kind()))
}

View on GitHub (pinned to 4f31cd3a0c)

Solutions

1. Avoid piping values of the unsupported type into Stringify
2. Convert the value to a plain object/array/string before printing
3. Extend Stringify with a branch for the reported reflect.Kind

When it happens

Trigger: Thrown at internal/engine/stringify.go:112 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of antonmedv/fx@4f31cd3a0c (2026-09-02). Data as JSON: /api/errors/334f1a5999187918. Report an issue: GitHub.