Go Academy · Lesson

JSON Tags

Control marshaling.

JSON Tags is a free Go Academy lesson on CoddyKit — lesson 2 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Go Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

JSON Tags Control Marshaling

The encoding/json package uses json tags to decide the key names and rules when converting structs to and from JSON.

Default Field Names

Without tags, JSON keys match the exported field names exactly, including capitalization.

package main

import (
	"encoding/json"
	"fmt"
)

type User struct {
	Name string
	Age  int
}

func main() {
	b, _ := json.Marshal(User{"Ann", 30})
	fmt.Println(string(b))
}

Renaming Keys

A json tag changes the key name in the output. Here Name becomes the lowercase name.

package main

import (
	"encoding/json"
	"fmt"
)

type User struct {
	Name string "json:\"name\""
	Age  int    "json:\"age\""
}

func main() {
	b, _ := json.Marshal(User{"Ann", 30})
	fmt.Println(string(b))
}

The omitempty Option

Add ,omitempty to skip a field in the output when it holds its zero value (empty string, 0, nil, etc).

package main

import (
	"encoding/json"
	"fmt"
)

type User struct {
	Name  string "json:\"name\""
	Email string "json:\"email,omitempty\""
}

func main() {
	b, _ := json.Marshal(User{Name: "Ann"})
	fmt.Println(string(b))
}

Ignoring a Field

A tag of - tells the encoder to skip the field completely, useful for secrets like passwords.

package main

import (
	"encoding/json"
	"fmt"
)

type User struct {
	Name     string "json:\"name\""
	Password string "json:\"-\""
}

func main() {
	b, _ := json.Marshal(User{"Ann", "secret"})
	fmt.Println(string(b))
}

Unmarshaling with Tags

Tags work both ways. When decoding JSON, the tag tells the decoder which JSON key fills which field.

package main

import (
	"encoding/json"
	"fmt"
)

type User struct {
	Name string "json:\"name\""
}

func main() {
	var u User
	json.Unmarshal([]byte("{\"name\":\"Bob\"}"), &u)
	fmt.Println(u.Name)
}

Case-Insensitive Matching

When unmarshaling, Go matches JSON keys to tags case-insensitively as a fallback. Still, defining tags explicitly is the safe, clear choice.

String Option

The ,string option encodes a numeric or bool field as a JSON string. Useful for systems that expect numbers quoted.

package main

import (
	"encoding/json"
	"fmt"
)

type Item struct {
	ID int "json:\"id,string\""
}

func main() {
	b, _ := json.Marshal(Item{ID: 42})
	fmt.Println(string(b))
}

Combining Options

You can combine a name with options: json:"email,omitempty". The name comes first, then comma-separated options.

Unexported Fields Skipped

Only exported (capitalized) fields are marshaled. A lowercase field is invisible to encoding/json no matter what tag it has.

package main

import (
	"encoding/json"
	"fmt"
)

type User struct {
	Name string "json:\"name\""
	age  int
}

func main() {
	b, _ := json.Marshal(User{Name: "Ann", age: 30})
	fmt.Println(string(b))
}

Pretty Output

json.MarshalIndent formats JSON with indentation, handy for readable logs while still honoring tags.

package main

import (
	"encoding/json"
	"fmt"
)

type User struct {
	Name string "json:\"name\""
}

func main() {
	b, _ := json.MarshalIndent(User{"Ann"}, "", "  ")
	fmt.Println(string(b))
}

Quick Check

What does the ,omitempty option do in a json tag?

Recap

JSON tags control marshaling:

Rename keys with json:"name"
,omitempty skips zero values
- ignores a field; ,string quotes numbers
Only exported fields are encoded

Frequently asked questions

Is the “JSON Tags” lesson free?

Yes — the full text of “JSON Tags” is free to read here on the web, and the Go Academy course includes 4 lessons in total. To practise it interactively (a built-in code editor and a 24/7 AI tutor) and unlock the rest of the Go Academy course, upgrade to CoddyKit PRO.

What will I learn in “JSON Tags”?

Control marshaling. You practise Go Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

Do I need any experience to start Go Academy?

No prior experience is required. Go Academy on CoddyKit is structured for beginners through advanced learners; this is — lesson 2 of 4, so you can start here or from the beginning and move at your own pace.

How long does the “JSON Tags” lesson take?

Most CoddyKit lessons take about 5–10 minutes. Each one is bite-sized and interactive, so you make steady progress and pick up exactly where you left off across the web and the app.

Can I write and run code in this Go Academy lesson?

Yes. Every Go Academy lesson includes a built-in code editor, so you write and run real code right in your browser and get instant AI feedback — no local setup required.

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