parse JSON field which may be string or number in Go

main.go

package main

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

func main() {
	data := `[
		{
			"name" : "foo",
			"value" : 100
		},
		{
			"name" : "bar",
			"value" : 200
		},
		{
			"name" : "baz",
			"value" : "X300"
		}
	]`

	var nvs []NameValue
	if err := json.Unmarshal([]byte(data), &nvs); err != nil {
		panic(err)
	}

	for i := 0; i < len(nvs); i++ {
		fmt.Printf("result[%d]: %+v\n", i, nvs[i])
	}
}

type NameValue struct {
	Name  string
	Value StringOrNumber
}

type StringOrNumber string

func (s *StringOrNumber) UnmarshalJSON(data []byte) error {
	if bytes.HasPrefix(data, []byte{'"'}) {
		var str string
		err := json.Unmarshal(data, &str)
		if err != nil {
			return err
		}
		*s = StringOrNumber(str)
	} else {
		*s = StringOrNumber(string(data))
	}
	return nil
}

The result.

$ go run main.go
result[0]: {Name:foo Value:100}
result[1]: {Name:bar Value:200}
result[2]: {Name:baz Value:X300}

The original article: blog.ichiban: How to parse JSON w/ a field which can be string or number in Go