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Go 1.27 reference · Slices, Maps, Iterators and JSON

Go encoding/json Marshal and Unmarshal Function

Convert Go values to JSON and back using struct tags to name fields, omit empty values and skip fields.

Go 1.27

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Syntax

func Marshal(v any) ([]byte, error)   func Unmarshal(data []byte, v any) error

Parameters

ParameterDescription
`json:"name"` Field name in JSON.
`json:",omitempty"` Omit zero values.
`json:"-"` Never encode.
MarshalIndent Pretty-printed output.

Examples

Example 1
package main

import (
	"encoding/json"
	"fmt"
)

func main() {
	type User struct {
		ID       int      `json:"id"`
		Name     string   `json:"name"`
		Email    string   `json:"email,omitempty"`
		Password string   `json:"-"`
		Tags     []string `json:"tags"`
	}
	b, _ := json.Marshal(User{ID: 1, Name: "Ana", Password: "secret", Tags: []string{"admin"}})
	fmt.Println(string(b))
}

Output

{"id":1,"name":"Ana","tags":["admin"]}

Run this on your computer — see “Get Started” for setup.

Decoding, and numbers in map[string]any
package main

import (
	"encoding/json"
	"fmt"
)

func main() {
	var order struct {
		ID    int     `json:"id"`
		Total float64 `json:"total"`
	}
	err := json.Unmarshal([]byte(`{"id": 42, "total": 19.5, "extra": true}`), &order)
	fmt.Println(order.ID, order.Total, err)
	var anything map[string]any
	json.Unmarshal([]byte(`{"n": 1, "ok": true}`), &anything)
	fmt.Printf("%v %T\n", anything["n"], anything["n"])
}

Output

42 19.5 <nil>
1 float64

Run this on your computer — see “Get Started” for setup.

Only exported (capitalised) fields are encoded. Decoding into any turns numbers into float64. For request bodies, use json.NewDecoder with DisallowUnknownFields.

  • fmt.Printf · Print formatted output using verbs like %v, %+v, %d, %s, %q, %.2f and %T, with width and alignment options.
  • Go JSON · tutorial

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