An example of reading a json array from a file and appending a new object to it

json-array-append.go

package main

import (
	"encoding/json"
	"io/ioutil"
	"os"
)

type User struct {
	Username string `json:"username"`
	Password string `json:"password"`
}

type Users []User

func main() {
	// open the json file with read / write abilities
	file, err := os.OpenFile("users.json", os.O_RDWR, 0644)
	if err != nil {
		panic(err)
	}

	// good practice to ensure closing the file
	defer file.Close()

	// read our opened json file into bytes
	bytes, err := ioutil.ReadAll(file)
	if err != nil {
		panic(err)
	}

	// initialize a Users slice type
	var users Users

	// unmarshal the bytes into our users type
	err = json.Unmarshal(bytes, &users)
	if err != nil {
		panic(err)
	}

	// add our new user to json users
	users = append(users, User{"adam", "test"})

	// create new indented JSON with the added user
	newBytes, err := json.MarshalIndent(users, "", "    ")
	if err != nil {
		panic(err)
	}

	// write the new indented json over original file
	_, err = file.WriteAt(newBytes, 0)
	if err != nil {
		panic(err)
	}
}