The encoding/json package
- Go course
- Intermediate
- Lesson 59/80
- 4 min
Learn the basics of the encoding/json package.
The encoding/json package
encoding/json turns Go values into JSON (Marshal) and JSON into Go values (Unmarshal). Only a struct's exported fields (starting with an upper-case letter) are encoded:
type Urun struct {
Ad string
Fiyat float64
stok int // lower case: not in the JSON
}
b, _ := json.Marshal(Urun{"Kalem", 12.5, 3})
fmt.Println(string(b)) // {"Ad":"Kalem","Fiyat":12.5}Go and JSON types
Mappings: bool ↔ true/false, numbers ↔ number, string ↔ string, slice/array ↔ array, struct and map[string]T ↔ object, nil ↔ null. []byte becomes base64 text and time.Time RFC 3339 text. Channels, functions and complex numbers can't be encoded; Marshal returns an error.
Practice First Marshal
Define type Urun struct { Ad string; Fiyat float64 }. Encode Urun{"Kalem", 12.5} with json.Marshal; print the error if there is one, otherwise print string(b).
Starter code
package main
import (
"encoding/json"
"fmt"
)
func main() {
// buraya yaz
}Sample solution
package main
import (
"encoding/json"
"fmt"
)
type Urun struct {
Ad string
Fiyat float64
}
func main() {
b, err := json.Marshal(Urun{"Kalem", 12.5})
if err != nil {
fmt.Println(err)
return
}
fmt.Println(string(b))
}Expected output
{"Ad":"Kalem","Fiyat":12.5}
Questions and answers
Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.
1 Why might a struct field not appear in the JSON output?
- Because the field is an int
- Because JSON supports at most 3 fields
- Because its value is zero (always)
- Because the field name starts with a lower-case letter Correct answer
Explanation: The json package works through reflection and can't access unexported fields; zero values are written unless omitempty is set.
2 Encode to JSON
b, err := json.____(urun)b, err := json.Marshal(urun)Explanation: json.Marshal converts the value into JSON bytes.
3 Go and JSON
| Term | Meaning |
|---|---|
[]int{1, 2} |
[1,2] |
map[string]int{"a": 1} |
{"a":1} |
nil pointer |
null |
[]byte("hi") |
"aGk=" |