Go: Converting JSON string to map[string]interface{}

Question

I'm trying to create a JSON representation within Go using a map[string]interface{} type. I'm dealing with JSON strings and I'm having a hard time figuring out how to avoid the JSON unmarshaler to automatically deal with numbers as float64s. As a result the following error occurs.

Ex. "{ 'a' : 9223372036854775807}" should be map[string]interface{} = [a 9223372036854775807 but in reality it is map[string]interface{} = [a 9.2233720368547758088E18]

I searched how structs can be used to avoid this by using json.Number but I'd really prefer using the map type designated above.

Answer 1

The go json.Unmarshal(...) function automatically uses float64 for JSON numbers. If you want to unmarshal numbers into a different type then you'll have to use a custom type with a custom unmarshaler. There is no way to force the unmarshaler to deserialize custom values into a generic map.

For example, here's how you could parse values of the "a" property as a big.Int .

package main

import (
  "encoding/json"
  "fmt"
  "math/big"
)

type MyDoc struct {
  A BigA `json:"a"`
}

type BigA struct{ *big.Int }

func (a BigA) UnmarshalJSON(bs []byte) error {
  _, ok := a.SetString(string(bs), 10)
  if !ok {
    return fmt.Errorf("invalid integer %s", bs)
  }
  return nil
}

func main() {
  jsonstr := `{"a":9223372036854775807}`
  mydoc := MyDoc{A: BigA{new(big.Int)}}

  err := json.Unmarshal([]byte(jsonstr), &mydoc)
  if err != nil {
    panic(err)
  }

  fmt.Printf("OK: mydoc=%#v\n", mydoc)
  // OK: mydoc=main.MyDoc{A:9223372036854775807}
}

Answer 2

func jsonToMap(jsonStr string) map[string]interface{} {
    result := make(map[string]interface{})
    json.Unmarshal([]byte(jsonStr), &result)
    return result
}

• Example - https://goplay.space/#ra7Gv8A5Heh
• Related questions - create a JSON data as map[string]interface with the given data