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Example of implementing MarshalJSON and UnmarshalJSON to serialize and deserialize custom types to JSON in Go. Playground:
package main
import (
func main() {
a := make(Stuff)
a[1] = "asdf"
a[-1] = "qwer"
fmt.Println("Initial: ",a)
stuff, err := json.Marshal(a)
fmt.Println("Serialized: ", string(stuff), err)
b := make(Stuff)
err = json.Unmarshal(stuff, &b)
fmt.Println("Deserialized:", b, err)
type Stuff map[int]string
func (this Stuff) MarshalJSON() ([]byte, error) {
buffer := bytes.NewBufferString("{")
length := len(this)
count := 0
for key, value := range this {
jsonValue, err := json.Marshal(value)
if err != nil {
return nil, err
buffer.WriteString(fmt.Sprintf("\"%d\":%s", key, string(jsonValue)))
if count < length {
return buffer.Bytes(), nil
func (this Stuff) UnmarshalJSON(b []byte) error {
var stuff map[string]string
err := json.Unmarshal(b, &stuff)
if err != nil {
return err
for key, value := range stuff {
numericKey, err := strconv.Atoi(key)
if err != nil {
return err
this[numericKey] = value
return nil

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