JSON unmarshaling with long numbers gives floating point number

Question

I was marshaling and unmarshaling JSONs using golang and when I want to do it with number fields golang transforms it in floating point numbers instead of use long numbers,

Answer 1

There are times when you cannot define a struct in advance but still require numbers to pass through the marshal-unmarshal process unchanged.

In that case you can use the UseNumber method on json.Decoder, which causes all numbers to unmarshal as json.Number (which is just the original string representation of the number). This can also useful for storing very big integers in JSON.

For example:

package main

import (
    "strings"
    "encoding/json"
    "fmt"
    "log"
)

var data = `{
    "id": 12423434, 
    "Name": "Fernando"
}`

func main() {
    d := json.NewDecoder(strings.NewReader(data))
    d.UseNumber()
    var x interface{}
    if err := d.Decode(&x); err != nil {
        log.Fatal(err)
    }
    fmt.Printf("decoded to %#v\n", x)
    result, err := json.Marshal(x)
    if err != nil {
        log.Fatal(err)
    }
    fmt.Printf("encoded to %s\n", result)
}

Result:

decoded to map[string]interface {}{"id":"12423434", "Name":"Fernando"}
encoded to {"Name":"Fernando","id":12423434}