Golang: Generate an error when a JSON field is present but the corresponding struct field is missing - Disallow Unknown Fields

main.go

package main

import (
	"bytes"
	"encoding/json"
	"fmt"
)

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

func main() {

	jsonString := `{"name": "John Doe", "age": 30, "location": "New York"}`
	var user User

	dec := json.NewDecoder(bytes.NewReader([]byte(jsonString)))
	dec.DisallowUnknownFields()

	err := dec.Decode(&user)
	if err != nil {
		fmt.Println("err:", "Decode:", err)
	}

	fmt.Printf("%+v\n", user)
}