https://apoorv.blog/json-key-not-set-null-golang/

main.go

package main

import (
	"encoding/json"
	"fmt"
)

// OptionalInt is for json unmarshal
type OptionalInt struct {
	Value int
	isValid bool
	isSet bool
}

// User is user props
type User struct {
	Views OptionalInt `json:"views"`
}

func main()  {

	body1 := `{"views": 5}`

	body2 := `{"views": null}`
	
	body3 := `{"age": 5}`

	var currentUser1 User

	if err := json.Unmarshal([]byte(body1), &currentUser1); err != nil {

		fmt.Println(err)
	}

	fmt.Printf("isSet: %v, isValid: %v, value: %v\n", currentUser1.Views.isSet, currentUser1.Views.isValid, currentUser1.Views.Value)
	
	var currentUser2 User
	
	if err := json.Unmarshal([]byte(body2), &currentUser2); err != nil {
		
		fmt.Println(err)
	}
	
	fmt.Printf("isSet: %v, isValid: %v, value: %v\n", currentUser2.Views.isSet, currentUser2.Views.isValid, currentUser2.Views.Value)
	
	var currentUser3 User
	
	if err := json.Unmarshal([]byte(body3), &currentUser3); err != nil {
		
		fmt.Println(err)
	}
	
	fmt.Printf("isSet: %v, isValid: %v, value: %v\n", currentUser3.Views.isSet, currentUser3.Views.isValid, currentUser3.Views.Value)
}

// UnmarshalJSON is custom unmarshal
func (i *OptionalInt) UnmarshalJSON(bytes []byte) error  {
	
	i.isSet = true

	if string(bytes) == "null" {

		i.isValid = false

		return nil
	}

	var value int

	if err := json.Unmarshal(bytes, &value); err != nil {

		return err
	}
	
	i.isValid = true

	i.Value = value

	return nil
}