JSON and map
- Go course
- Intermediate
- Lesson 63/80
- 4 min
Learn map[string]interface{} for dynamic JSON.
JSON and maps
For JSON whose structure isn't known in advance, map[string]any can be used. The decoded value types are fixed: object → map[string]any, array → []any, number → float64, text → string.
var m map[string]any
json.Unmarshal([]byte(`{"ad":"Ada","yas":36}`), &m)
yas := m["yas"].(float64) // not int!Since type assertions are needed everywhere, a struct is preferred when possible.
json.RawMessage
To postpone decoding a field, use json.RawMessage; for example, events decoded into different structs depending on a tur field:
type Olay struct {
Tur string `json:"tur"`
Veri json.RawMessage `json:"veri"`
}
switch o.Tur {
case "siparis":
var s Siparis
json.Unmarshal(o.Veri, &s)
}This builds a structure that is both flexible and type-safe.
Practice Dynamic JSON
Decode the text {"ad":"Ada","diller":["go","sql"],"yas":36} into a map[string]any. Assert yas as float64 and diller as []any, then print the age converted to int and the number of languages.
Starter code
package main
import (
"encoding/json"
"fmt"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"encoding/json"
"fmt"
)
func main() {
var m map[string]any
if err := json.Unmarshal([]byte(`{"ad":"Ada","diller":["go","sql"],"yas":36}`), &m); err != nil {
fmt.Println(err)
return
}
yas := m["yas"].(float64)
diller := m["diller"].([]any)
fmt.Println(int(yas), len(diller))
}Expected output
36 2
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 What is the Go type of the number <code>36</code> decoded into a <code>map[string]any</code>?
- int
- int64
- string
- float64 Correct answer
Explanation: JSON has no integer/decimal distinction; with an any target the json package makes every number float64. For large integers use Decoder.UseNumber.
2 Postpone decoding
type Mesaj struct {
Tur string `json:"tur"`
Govde json.____ `json:"govde"`
}type Mesaj struct {
Tur string `json:"tur"`
Govde json.RawMessage `json:"govde"`
}Explanation: RawMessage stores the field's raw JSON bytes without decoding them.
3 JSON → any
| Term | Meaning |
|---|---|
{...} |
map[string]any |
[...] |
[]any |
36 |
float64 |
"metin" |
string |