compute a field in json output

computefield.go

package main

import (
	"encoding/json"
	"os"
)

type Point struct {
	X int `json:"x"`
	Y int `json:"y"`
	Z int `json:"z"`
}

func (p Point) MarshalJSON() ([]byte, error) {
	type doc Point
	d := doc(p)
	d.Z = 3
	return json.Marshal(d)
}

func main() {
	p := &Point{X: 1, Y: 2}
	if err := json.NewEncoder(os.Stdout).Encode(p); err != nil {
		panic(err)
	}
}