You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: format_json.go
    path of file: ./repos/goldfish/vendor/github.com/hashicorp/vault/audit
    
    the code of the file until where you have to start completion: package audit

import (
	"encoding/json"
	"fmt"
	"io"

	"github.com/hashicorp/vault/helper/salt"
)

// JSONFormatWriter is an AuditFormatWriter implementation that structures data into
// a JSON format.
type JSONFormatWriter struct {
	Prefix   string
	SaltFunc func
    
func (f *JSONFormatWriter) WriteResponse(w io.Writer, resp *AuditResponseEntry) error {
	if resp == nil {
		return fmt.Errorf("response entry was nil, cannot encode")
	}

	if len(f.Prefix) > 0 {
		_, err := w.Write([]byte(f.Prefix))
		if err != nil {
			return err
		}
	}

	enc := json.NewEncoder(w)
	return enc.Encode(resp)
}