Decode any JSON value to string in Golang

Question

Is there a way in golang to decode any json value to a string. Similar to json.Number why isn't there a json.String?

For example, the following could be decoded as indicated

{"number": 123}      => "123"
{"string": "123"}    => "123"
{"float" : 123.45}   => "123.45"
{"bool"  : true}     => "true"
{"empty" : ""}       => ""
{"null"  : null}     => ""

Not entirely sure what you mean because json.Number is a type (basically just a string) which stands for a numeral literal inside the loaded JSON structure, used because you can't map the JSON number type 1:1. As I understand, it's not a conversion function the way you think. You just need to access the field you want and convert it to a string if you want a string... — CherryDT
– CherryDT, Commented Jul 12, 2016 at 11:29
(JSON string fields don't need their own special type, they are just stored as Go string in the structure.) — CherryDT
– CherryDT, Commented Jul 12, 2016 at 11:31
Plus, the json.Number type is only used if you use Decoder.UseNumber(), I think. Otherwise it is just stored as float64 and the problem is that huge numbers can't properly stored this way (that's why the json.Number type exists). See attilaolah.eu/2013/11/29/json-decoding-in-go/… — CherryDT
– CherryDT, Commented Jul 12, 2016 at 11:32
@CherryDT A use case for me would be where I want to read a url query param from json. I don't care what the type of the json value is, it must always be decoded as a string — mozey
– mozey, Commented Jul 12, 2016 at 11:46

mozey · Accepted Answer · 2016-07-12 12:00:30Z

Inspired by this post I created a JsonString type. It will decode any string, number, boolean or null values to a string.

https://play.golang.org/p/ucAxwriL2K

type JsonString string
type jsonString JsonString
func (st *JsonString) UnmarshalJSON(bArr []byte) (err error) {
    j, n, f, b := jsonString(""), uint64(0), float64(0), bool(false)
    if err = json.Unmarshal(bArr, &j); err == nil {
        *st = JsonString(j)
        return
    }
    if err = json.Unmarshal(bArr, &n); err == nil {
        *st = JsonString(string(bArr[:]))
        return
    }
    if err = json.Unmarshal(bArr, &f); err == nil {
        *st = JsonString(string(bArr[:]))
        return
    }
    if err = json.Unmarshal(bArr, &b); err == nil {
        *st = JsonString(string(bArr[:]))
        return
    }
    return
}

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