extract interface{} from json

main.go

package main

import (
	"encoding/json"
	"fmt"
	"github.com/ismdeep/parser"
)

var contentA = `{"a": "123"}`
var contentB = `{"a": 123}`

type Content struct {
	A interface{} `json:"a"`
}

func main() {
	var c Content
	if err := json.Unmarshal([]byte(contentB), &c); err != nil {
		panic(err)
	}

	var size uint64

	switch c.A.(type) {
	case float64:
		size = uint64(c.A.(float64))
	case string:
		var err error
		size, err = parser.ToUint64(c.A.(string))
		if err != nil {
			panic(err)
		}
	default:
		panic("test")
	}

	fmt.Println(size)
}