JSON Formatter Go — คู่มือ MarshalIndent()

package main

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

type ServiceConfig struct {
	Host       string   `json:"host"`
	Port       int      `json:"port"`
	Workers    int      `json:"workers"`
	TLSEnabled bool     `json:"tls_enabled"`
	AllowedIPs []string `json:"allowed_ips"`
}

func main() {
	cfg := ServiceConfig{
		Host:       "payments-api.internal",
		Port:       8443,
		Workers:    8,
		TLSEnabled: true,
		AllowedIPs: []string{"10.0.0.0/8", "172.16.0.0/12"},
	}

	data, err := json.MarshalIndent(cfg, "", "	")
	if err != nil {
		log.Fatalf("marshal config: %v", err)
	}
	fmt.Println(string(data))
}
// {
// 	"host": "payments-api.internal",
// 	"port": 8443,
// 	"workers": 8,
// 	"tls_enabled": true,
// 	"allowed_ips": [
// 		"10.0.0.0/8",
// 		"172.16.0.0/12"
// 	]
// }

// Tab indent — นิยมใน Go-native tooling
data, _ := json.MarshalIndent(cfg, "", "	")
// {
// 	"host": "payments-api.internal",
// 	"port": 8443
// }

// Two-space indent — พบบ่อยสำหรับ API ที่มี consumer JS/Python
data, _ = json.MarshalIndent(cfg, "", "  ")
// {
//   "host": "payments-api.internal",
//   "port": 8443
// }

// Four-space indent
data, _ = json.MarshalIndent(cfg, "", "    ")
// {
//     "host": "payments-api.internal",
//     "port": 8443
// }

type UserProfile struct {
	ID          string  `json:"id"`
	Email       string  `json:"email"`
	DisplayName string  `json:"display_name,omitempty"`  // ละเว้นถ้าเป็น string ว่าง
	AvatarURL   *string `json:"avatar_url,omitempty"`    // ละเว้นถ้าเป็น nil pointer
	IsAdmin     bool    `json:"is_admin,omitempty"`      // ละเว้นถ้าเป็น false
	passwordHash string                                   // unexported — ยกเว้นอัตโนมัติ
}

// ผู้ใช้ที่มี optional field ทั้งหมดถูกกรอก
full := UserProfile{
	ID: "usr_7b3c", Email: "somchai.j@example.th",
	DisplayName: "สมชาย ใจดี", IsAdmin: true,
}
// {
//   "id": "usr_7b3c",
//   "email": "somchai.j@example.th",
//   "display_name": "สมชาย ใจดี",
//   "is_admin": true
// }

// ผู้ใช้ที่ไม่มี optional field — ถูกละเว้นทั้งหมด
minimal := UserProfile{ID: "usr_2a91", Email: "somying.r@example.th"}
// {
//   "id": "usr_2a91",
//   "email": "somying.r@example.th"
// }

type AuthToken struct {
	TokenID      string `json:"token_id"`
	Subject      string `json:"sub"`
	IssuedAt     int64  `json:"iat"`
	ExpiresAt    int64  `json:"exp"`
	SigningKey    []byte `json:"-"`   // ไม่เคย serialize
	RefreshToken string `json:"-"`   // ไม่เคย serialize
}

tok := AuthToken{
	TokenID: "tok_8f2a", Subject: "usr_7b3c",
	IssuedAt: 1741614120, ExpiresAt: 1741617720,
	SigningKey: []byte("ลับ"), RefreshToken: "rt_9e4f",
}
data, _ := json.MarshalIndent(tok, "", "  ")
// {
//   "token_id": "tok_8f2a",
//   "sub": "usr_7b3c",
//   "iat": 1741614120,
//   "exp": 1741617720
// }
// SigningKey และ RefreshToken ไม่ปรากฏเลย

package main

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

type Money struct {
	Amount   int64  // เก็บเป็นสตางค์เพื่อหลีกเลี่ยง floating-point drift
	Currency string
}

func (m Money) MarshalJSON() ([]byte, error) {
	return json.Marshal(struct {
		Amount   float64 `json:"amount"`
		Currency string  `json:"currency"`
		Display  string  `json:"display"`
	}{
		Amount:   float64(m.Amount) / 100,
		Currency: m.Currency,
		Display:  fmt.Sprintf("%s %.2f", m.Currency, float64(m.Amount)/100),
	})
}

type Invoice struct {
	ID       string `json:"id"`
	Subtotal Money  `json:"subtotal"`
	Tax      Money  `json:"tax"`
	Total    Money  `json:"total"`
}

func main() {
	inv := Invoice{
		ID:       "inv_9a2f91bc",
		Subtotal: Money{Amount: 199000, Currency: "THB"},
		Tax:      Money{Amount: 15920, Currency: "THB"},
		Total:    Money{Amount: 214920, Currency: "THB"},
	}
	data, err := json.MarshalIndent(inv, "", "  ")
	if err != nil {
		log.Fatalf("marshal invoice: %v", err)
	}
	fmt.Println(string(data))
}
// {
//   "id": "inv_9a2f91bc",
//   "subtotal": { "amount": 1990, "currency": "THB", "display": "THB 1990.00" },
//   "tax":      { "amount": 159.2, "currency": "THB", "display": "THB 159.20" },
//   "total":    { "amount": 2149.2, "currency": "THB", "display": "THB 2149.20" }
// }

type OrderStatus int

const (
	StatusPending OrderStatus = iota
	StatusPaid
	StatusShipped
	StatusCancelled
)

var orderStatusNames = map[OrderStatus]string{
	StatusPending:   "pending",
	StatusPaid:      "paid",
	StatusShipped:   "shipped",
	StatusCancelled: "cancelled",
}

func (s OrderStatus) MarshalJSON() ([]byte, error) {
	name, ok := orderStatusNames[s]
	if !ok {
		return nil, fmt.Errorf("สถานะคำสั่งซื้อที่ไม่รู้จัก: %d", s)
	}
	return json.Marshal(name)
}

type Order struct {
	ID     string      `json:"id"`
	Status OrderStatus `json:"status"`
}

o := Order{ID: "ord_3c7f", Status: StatusShipped}
data, _ := json.MarshalIndent(o, "", "  ")
// {
//   "id": "ord_3c7f",
//   "status": "shipped"
// }

import (
    "encoding/json"
    "fmt"

    "github.com/google/uuid"
)

type AuditEvent struct {
    EventID   uuid.UUID `json:"event_id"`   // serialize เป็น "550e8400-e29b-41d4-a716-446655440000"
    SessionID uuid.UUID `json:"session_id"`
    Action    string    `json:"action"`
    ActorID   string    `json:"actor_id"`
    OccuredAt string    `json:"occurred_at"`
}

event := AuditEvent{
    EventID:   uuid.New(),
    SessionID: uuid.MustParse("550e8400-e29b-41d4-a716-446655440000"),
    Action:    "user.password_changed",
    ActorID:   "usr_7f3a91bc",
    OccuredAt: "2026-03-10T14:22:00Z",
}

data, _ := json.MarshalIndent(event, "", "  ")
fmt.Println(string(data))
// {
//   "event_id": "a4b2c1d0-...",
//   "session_id": "550e8400-e29b-41d4-a716-446655440000",
//   "action": "user.password_changed",
//   "actor_id": "usr_7f3a91bc",
//   "occurred_at": "2026-03-10T14:22:00Z"
// }

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"log"
)

func main() {
	// จำลอง raw JSON payload จาก upstream service
	raw := []byte(`{"trace_id":"tr_9a2f","service":"checkout","latency_ms":342,"error":null}`)

	var buf bytes.Buffer
	if err := json.Indent(&buf, raw, "", "	"); err != nil {
		log.Fatalf("indent: %v", err)
	}
	fmt.Println(buf.String())
}
// {
// 	"trace_id": "tr_9a2f",
// 	"service": "checkout",
// 	"latency_ms": 342,
// 	"error": null
// }

func logResponse(logger *slog.Logger, statusCode int, body []byte) {
	var pretty bytes.Buffer
	if err := json.Indent(&pretty, body, "", "  "); err != nil {
		// Body ไม่ใช่ JSON ที่ valid — log แบบ raw
		logger.Debug("upstream response", "status", statusCode, "body", string(body))
		return
	}
	logger.Debug("upstream response", "status", statusCode, "body", pretty.String())
}

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"log"
	"os"
)

func formatJSONFile(path string) error {
	data, err := os.ReadFile(path)
	if err != nil {
		return fmt.Errorf("อ่าน %s: %w", path, err)
	}

	if !json.Valid(data) {
		return fmt.Errorf("JSON ไม่ valid ใน %s", path)
	}

	var buf bytes.Buffer
	if err := json.Indent(&buf, data, "", "	"); err != nil {
		return fmt.Errorf("indent %s: %w", path, err)
	}

	if err := os.WriteFile(path, buf.Bytes(), 0644); err != nil {
		return fmt.Errorf("เขียน %s: %w", path, err)
	}
	return nil
}

func main() {
	if err := formatJSONFile("config/database.json"); err != nil {
		log.Fatalf("format config: %v", err)
	}
	fmt.Println("format config/database.json สำเร็จ")
}

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"net/http"
)

type HealthResponse struct {
	Status    string            `json:"status"`
	Version   string            `json:"version"`
	Checks    map[string]string `json:"checks"`
	UptimeSec int64             `json:"uptime_seconds"`
}

func main() {
	resp, err := http.Get("https://api.payments.internal/v2/health")
	if err != nil {
		log.Fatalf("health check: %v", err)
	}
	defer resp.Body.Close()

	var result HealthResponse
	if err := json.NewDecoder(resp.Body).Decode(&result); err != nil {
		log.Fatalf("decode health response: %v", err)
	}

	pretty, err := json.MarshalIndent(result, "", "  ")
	if err != nil {
		log.Fatalf("marshal health response: %v", err)
	}
	fmt.Printf("Health check (%d):
%s
", resp.StatusCode, string(pretty))
}
// Health check (200):
// {
//   "status": "ok",
//   "version": "1.4.2",
//   "checks": {
//     "database": "ok",
//     "cache": "ok",
//     "queue": "degraded"
//   },
//   "uptime_seconds": 172800
// }

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
	"io"
	"log"
	"net/http"
)

func main() {
	resp, err := http.Get("https://api.payments.internal/v2/health")
	if err != nil {
		log.Fatalf("request: %v", err)
	}
	defer resp.Body.Close()

	body, err := io.ReadAll(resp.Body)
	if err != nil {
		log.Fatalf("อ่าน body: %v", err)
	}

	var buf bytes.Buffer
	if err := json.Indent(&buf, body, "", "  "); err != nil {
		log.Fatalf("indent: %v", err)
	}
	fmt.Println(buf.String())
}

// Pattern A: decode มี type → MarshalIndent
// ใช้เมื่อต้องการตรวจสอบหรือ validate field เฉพาะ
var result map[string]any
json.NewDecoder(resp.Body).Decode(&result)
pretty, _ := json.MarshalIndent(result, "", "  ")
fmt.Println(string(pretty))

// Pattern B: raw bytes → json.Indent
// ใช้สำหรับ debug log แบบเร็ว — ไม่ต้องนิยาม struct
body, _ := io.ReadAll(resp.Body)
var buf bytes.Buffer
json.Indent(&buf, body, "", "  ")
fmt.Println(buf.String())

echo '{"service":"payments","port":8443,"workers":4}' | python3 -m json.tool
# {
#     "service": "payments",
#     "port": 8443,
#     "workers": 4
# }

# Format เท่านั้น
cat api-response.json | jq .

# ดึง nested field
cat api-response.json | jq '.checks.database'

# Filter array
cat audit-log.json | jq '.[] | select(.severity == "error")'

# main.go: อ่าน stdin, format, เขียน stdout
cat <<'EOF' > /tmp/fmt.go
package main
import ("bytes";"encoding/json";"io";"os")
func main() {
    b,_:=io.ReadAll(os.Stdin)
    var buf bytes.Buffer
    json.Indent(&buf,b,"","  ")
    os.Stdout.Write(buf.Bytes())
}
EOF
echo '{"port":8080,"debug":true}' | go run /tmp/fmt.go

go get github.com/goccy/go-json

package main

import (
	// แทนที่สิ่งนี้:
	// "encoding/json"

	// ด้วยสิ่งนี้ — API เหมือนกัน ไม่ต้องเปลี่ยน code อื่น:
	json "github.com/goccy/go-json"

	"fmt"
	"log"
)

type AuditEvent struct {
	RequestID string `json:"request_id"`
	UserID    string `json:"user_id"`
	Action    string `json:"action"`
	ResourceID string `json:"resource_id"`
	IPAddress  string `json:"ip_address"`
	DurationMs int    `json:"duration_ms"`
}

func main() {
	event := AuditEvent{
		RequestID: "req_7d2e91", UserID: "usr_4421",
		Action: "invoice.download", ResourceID: "inv_9a2f",
		IPAddress: "203.0.113.45", DurationMs: 23,
	}
	data, err := json.MarshalIndent(event, "", "  ")
	if err != nil {
		log.Fatalf("marshal: %v", err)
	}
	fmt.Println(string(data))
	// Output เหมือนกับ encoding/json — แค่ความเร็วต่างกัน
}

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"os"
)

type AuditEvent struct {
	RequestID  string `json:"request_id"`
	UserID     string `json:"user_id"`
	Action     string `json:"action"`
	Severity   string `json:"severity"`
	DurationMs int    `json:"duration_ms"`
}

func processAuditLog(path string) error {
	file, err := os.Open(path)
	if err != nil {
		return fmt.Errorf("เปิด %s: %w", path, err)
	}
	defer file.Close()

	dec := json.NewDecoder(file)

	// อ่าน opening token '['
	if _, err := dec.Token(); err != nil {
		return fmt.Errorf("อ่าน opening token: %w", err)
	}

	var processed int
	for dec.More() {
		var event AuditEvent
		if err := dec.Decode(&event); err != nil {
			return fmt.Errorf("decode event %d: %w", processed, err)
		}
		// ประมวลผล event ทีละตัว — การใช้ memory คงที่
		if event.Severity == "error" {
			fmt.Printf("[ERROR] %s: %s (%dms)
", event.UserID, event.Action, event.DurationMs)
		}
		processed++
	}
	fmt.Printf("ประมวลผล audit event %d รายการ
", processed)
	return nil
}

func main() {
	if err := processAuditLog("audit-2026-03.json"); err != nil {
		log.Fatalf("ประมวลผล audit log: %v", err)
	}
}

package main

import (
	"bufio"
	"encoding/json"
	"fmt"
	"log"
	"os"
)

type LogLine struct {
	Timestamp  string `json:"ts"`
	Level      string `json:"level"`
	Service    string `json:"service"`
	Message    string `json:"msg"`
	TraceID    string `json:"trace_id"`
	DurationMs int    `json:"duration_ms,omitempty"`
}

func main() {
	file, err := os.Open("service-2026-03.ndjson")
	if err != nil {
		log.Fatalf("เปิด log: %v", err)
	}
	defer file.Close()

	scanner := bufio.NewScanner(file)
	scanner.Buffer(make([]byte, 1024*1024), 1024*1024) // 1 MB ต่อบรรทัด

	for scanner.Scan() {
		var line LogLine
		if err := json.Unmarshal(scanner.Bytes(), &line); err != nil {
			continue // ข้ามบรรทัดที่ผิดรูปแบบ
		}
		if line.Level == "error" {
			fmt.Printf("%s [%s] %s trace=%s
",
				line.Timestamp, line.Service, line.Message, line.TraceID)
		}
	}
	if err := scanner.Err(); err != nil {
		log.Fatalf("scan: %v", err)
	}
}