package logrus

import (
	"encoding/json"
	"fmt"
	"time"
)

type JSONFormatter struct{}

func (f *JSONFormatter) Format(entry *Entry) ([]byte, error) {
	data := make(Fields, len(entry.Data)+3)
	for k, v := range entry.Data {
		switch v := v.(type) {
		case error:
			// Otherwise errors are ignored by `encoding/json`
			// https://github.com/Sirupsen/logrus/issues/137
			data[k] = v.Error()
		default:
			data[k] = v
		}
	}
	prefixFieldClashes(data)
	data["time"] = entry.Time.Format(time.RFC3339)
	data["msg"] = entry.Message
	data["level"] = entry.Level.String()

	serialized, err := json.Marshal(data)
	if err != nil {
		return nil, fmt.Errorf("Failed to marshal fields to JSON, %v", err)
	}
	return append(serialized, '\n'), nil
}

package logrus

import (
	"encoding/json"
	"fmt"
	"time"
)

type JSONFormatter struct{}

func (f *JSONFormatter) Format(entry *Entry) ([]byte, error) {
	data := make(Fields, len(entry.Data)+3)
	for k, v := range entry.Data {
		// Otherwise errors are ignored by `encoding/json`
		// https://github.com/Sirupsen/logrus/issues/137
		if err, ok := v.(error); ok {
			data[k] = err.Error()
		} else {
			data[k] = v
		}
	}
	prefixFieldClashes(data)
	data["time"] = entry.Time.Format(time.RFC3339)
	data["msg"] = entry.Message
	data["level"] = entry.Level.String()

	serialized, err := json.Marshal(data)
	if err != nil {
		return nil, fmt.Errorf("Failed to marshal fields to JSON, %v", err)
	}
	return append(serialized, '\n'), nil
}

--- 
+++ 
@@ -11,11 +11,12 @@
 func (f *JSONFormatter) Format(entry *Entry) ([]byte, error) {
 	data := make(Fields, len(entry.Data)+3)
 	for k, v := range entry.Data {
-		// Otherwise errors are ignored by `encoding/json`
-		// https://github.com/Sirupsen/logrus/issues/137
-		if err, ok := v.(error); ok {
-			data[k] = err.Error()
-		} else {
+		switch v := v.(type) {
+		case error:
+			// Otherwise errors are ignored by `encoding/json`
+			// https://github.com/Sirupsen/logrus/issues/137
+			data[k] = v.Error()
+		default:
 			data[k] = v
 		}
 	}
[JSON] Use type-switch for error field
