The encoding/json package

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.

Exercise 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).

Show example 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))
}

Test yourself

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
Show answer

Because the field name starts with a lower-case letter — The json package works through reflection and can't access unexported fields; zero values are written unless omitempty is set.

Encode to JSON

b, err := json.____(urun)

Show answer

b, err := json.Marshal(urun)

Go and JSON

[]int{1, 2}

[1,2]
map[string]int{"a": 1}

{"a":1}
nil pointer

null
[]byte("hi")

"aGk="

Next lesson: Struct tags

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