Convert Go map to json

I tried to convert my Go map to a json string with encoding/json Marshal, but it resulted in a empty string.

Here’s my code :

package main

import (

type Foo struct {
    Number int    `json:"number"`
    Title  string `json:"title"`

func main() {
    datas := make(map[int]Foo)

    for i := 0; i < 10; i++ {
        datas[i] = Foo{Number: 1, Title: "test"}

    jsonString, _ := json.Marshal(datas)


My output is :

map[9:{1 test} 2:{1 test} 7:{1 test} 3:{1 test} 4:{1 test} 5:{1 test} 6:{1 test} 8:{1 test} 0:{1 test} 1:{1 test}]


I really don’t know where I’m wrong. Thank you for your help.

1 Like

If you had caught the error, you would have seen this:

jsonString, err := json.Marshal(datas)

// [] json: unsupported type: map[int]main.Foo

The thing is you cannot use integers as keys in JSON; it is forbidden. Instead, you can convert these values to strings beforehand, for instance using strconv.Itoa .

See this post for more details: