Go: get the value of the json tag from a `struct` instance

go.mod

module dummy/foo

go 1.14

require (
)

main.go

package main

import (
	"fmt"
)

func main() {
	doggo := Dog{
		Name: "Fluffy",
	}

	fieldName, e := getJsonTagValue(doggo, "Name")

	if e != nil {
		fmt.Println(e.Error())
		return
	}

	fmt.Println(fieldName)
}

util.go

// main logic sourced from https://stackoverflow.com/a/69061124
package main

import (
	"fmt"
	"reflect"
	"strings"
)

type Dog struct {
	Name string `json:"bar,omitempty"`
}

func getStructTag(f reflect.StructField, tagName string) string {
	return string(f.Tag.Get(tagName))
}

// based on the function used in the go standard library "encoding/json" source
// https://cs.opensource.google/go/go/+/refs/tags/go1.17.6:src/encoding/json/tags.go;l=17
func extractFieldName(tag string) string {
	if idx := strings.Index(tag, ","); idx != -1 {
		return tag[:idx]
	}
	return tag
}

func GetStructField(f Dog, fieldName string) (reflect.StructField, error) {
	field, ok := reflect.TypeOf(&f).Elem().FieldByName(fieldName)
	if !ok {
		emptyValue := reflect.StructField{}
		return emptyValue, fmt.Errorf(`the field with the name "%s" was not found`, fieldName)
	}
	return field, nil
}

// Returns the json tag for a given struct field
func getJsonTagValue(f Dog, fieldName string) (string, error) {
	field, err := GetStructField(f, fieldName)

	tag := getStructTag(field, "json")
	return extractFieldName(tag), err
}