Converting Go struct to JSON

I am trying to convert a Go struct to JSON using the json package but all I get is {}. I am certain it is something totally obvious but I don't see it.

package main

import (
    "fmt"
    "encoding/json"
)

type User struct {
    name string
}

func main() {
    user := &User{name:"Frank"}
    b, err := json.Marshal(user)
    if err != nil {
        fmt.Printf("Error: %s", err)
        return;
    }
    fmt.Println(string(b))
}

Then when I try to run it I get this:

$ 6g test.go && 6l -o test test.6 && ./test 
{}

You need to export the User.name field so that the json package can see it. Rename the name field to Name.

package main

import (
	"fmt"
	"encoding/json"
)

type User struct {
	Name string
}

func main() {
	user := &User{Name: "Frank"}
	b, err := json.Marshal(user)
	if err != nil {
		fmt.Println(err)
		return
	}
	fmt.Println(string(b))
}

Output:

{"Name":"Frank"}

Related issue:

I was having trouble converting struct to JSON, sending it as response from Golang, then, later catch the same in JavaScript via Ajax.

Wasted a lot of time, so posting solution here.

In Go:

// web server

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

foo_marshalled, err := json.Marshal(Foo{Number: 1, Title: "test"})
fmt.Fprint(w, string(foo_marshalled)) // write response to ResponseWriter (w)

In JavaScript:

// web call & receive in "data", thru Ajax/ other

var Foo = JSON.parse(data);
console.log("number: " + Foo.number);
console.log("title: " + Foo.title);

Struct values encode as JSON objects. Each exported struct field becomes a member of the object unless:

• the field's tag is "-", or
• the field is empty and its tag specifies the "omitempty" option.

The empty values are false, 0, any nil pointer or interface value, and any array, slice, map, or string of length zero. The object's default key string is the struct field name but can be specified in the struct field's tag value. The "json" key in the struct field's tag value is the key name, followed by an optional comma and options.

You can define your own custom MarshalJSON and UnmarshalJSON methods and intentionally control what should be included, ex:

package main

import (
    "fmt"
    "encoding/json"
)

type User struct {
    name string
}

func (u *User) MarshalJSON() ([]byte, error) {
	return json.Marshal(&struct {
		Name     string `json:"name"`
	}{
		Name:     "customized" + u.name,
	})
}

func main() {
    user := &User{name: "Frank"}
    b, err := json.Marshal(user)
    if err != nil {
        fmt.Println(err)
        return
    }
    fmt.Println(string(b))
}

This is an interesting question, it is very easy using the new go versions. You should do this:

package main

import (
    "fmt"
    "encoding/json"
)

type User struct {
    Name string `json:"name"`
}

func main() {
    user := &User{name:"Frank"}
    b, err := json.Marshal(user)
    if err != nil {
        fmt.Printf("Error: %s", err)
        return;
    }
    fmt.Println(string(b))
}

Change this name to Name.