antonmedv/fx · error

specify a file as the first argument to be able to save: fx

Error message

specify a file as the first argument to be able to save: fx file.json

What it means

Raised by the __save__ builtin injected into the JS VM when a script calls save() but the tool was launched without a target file path. It is a generic guard on the global FilePath variable: save() needs a real file to write the JSON payload to, so an empty FilePath means there is nowhere to persist the output and the save is rejected before any write is attempted.

Source

Thrown at internal/engine/vm.go:32

	if err := vm.Set("__save__", func(json string) error {
		if FilePath == "" {
			return fmt.Errorf("specify a file as the first argument to be able to save: fx file.json ")
		}
		if info, err := os.Lstat(FilePath); err == nil && info.Mode()&os.ModeSymlink != 0 {
			return fmt.Errorf("cannot save to a symbolic link: %s", FilePath)
		}
		if err := os.WriteFile(FilePath, []byte(json), 0644); err != nil {
			return err
		}
		return nil
	}); err != nil {
		panic(err)
	}

View on GitHub (pinned to 4f31cd3a0c)

Solutions

  1. Run fx with a file argument, e.g. fx file.json
  2. Only call save() when a file argument was supplied
  3. Pass the target path explicitly before invoking save() in the script
Defensive patterns

Strategy: validation

When it happens

Trigger: Thrown at internal/engine/vm.go:32 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/d4b12ccfbfcfe908. Report an issue: GitHub.