Convert Go map to json

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

jsonString, err := json.Marshal(datas)
fmt.Println(err)

// [] 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: https://stackoverflow.com/a/24284721/2679935

It actually tells you what's wrong, but you ignored it because you didn't check the error returned from json.Marshal.

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

JSON spec doesn't support anything except strings for object keys, while javascript won't be fussy about it, it's still illegal.

You have two options:

1 Use map[string]Foo and convert the index to string (using fmt.Sprint for example):

datas := make(map[string]Foo, N)

for i := 0; i < 10; i++ {
    datas[fmt.Sprint(i)] = Foo{Number: 1, Title: "test"}
}
j, err := json.Marshal(datas)
fmt.Println(string(j), err)

2 Simply just use a slice (javascript array):

datas2 := make([]Foo, N)
for i := 0; i < 10; i++ {
    datas2[i] = Foo{Number: 1, Title: "test"}
}
j, err = json.Marshal(datas2)
fmt.Println(string(j), err)

playground