JSON in Go

Learning Focus

Go's encoding/json is fast and safe. Struct tags give you explicit control over the JSON mapping — making serialization predictable and testable.

Struct Mapping

package main

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

type User struct {
    ID        int       `json:"id"`
    Name      string    `json:"name"`
    Email     string    `json:"email"`
    IsActive  bool      `json:"isActive"`
    CreatedAt time.Time `json:"createdAt"`
    Password  string    `json:"-"`               // never serialized
    Notes     string    `json:"notes,omitempty"` // omit if empty
}

Unmarshal (Parse)

raw := []byte(`{"id":1,"name":"Alice","email":"alice@example.com","isActive":true}`)
var u User
if err := json.Unmarshal(raw, &u); err != nil {
    log.Fatal(err)
}
fmt.Println(u.Name) // Alice

Marshal (Serialize)

user := User{ID: 2, Name: "Bob", Email: "bob@example.com", IsActive: true}
out, _ := json.MarshalIndent(user, "", "  ")
fmt.Println(string(out))

Streaming with json.Decoder

resp, _ := http.Get("https://api.example.com/users")
defer resp.Body.Close()

var users []User
if err := json.NewDecoder(resp.Body).Decode(&users); err != nil {
    log.Fatal(err)
}

Dynamic JSON: map[string]interface{}

var dynamic map[string]interface{}
json.Unmarshal(raw, &dynamic)

name := dynamic["name"].(string)  // type assertion required

Struct Tag Reference

Tag	Effect
json:"name"	Use name as JSON key
json:"-"	Exclude field
json:",omitempty"	Omit if zero value
json:",string"	Encode number/bool as string

Concept Map

Concept Flow

JSON bytes  →  json.Unmarshal  →  Go struct / map  →  Business Logic  →  json.MarshalIndent  →  JSON bytes
HTTP Body   →  json.NewDecoder  →  Go struct / map

Common Pitfalls

Pitfall	Consequence	Prevention
Unexported (lowercase) fields	Silently not marshaled	Use exported (capitalized) names
Missing omitempty on pointers	Zero values output as null	Add omitempty or handle explicitly
interface{} type assertions	Panic on wrong type	Check ok: v, ok := d["x"].(string)
Non-pointer to Unmarshal	Fields not populated	Pass &target to json.Unmarshal

What's Next

• Previous: JSON in JavaScript — Compare JS approach.
• Next: JSON in Web APIs — Use JSON as the REST API transport layer.
• Section Overview — Return to the Programming module index.