Serialize deserialize from Go int64/uint64 from/to Javascript string

Question

Is it possible to modify json serialization and deserialization so that a struct like this:

type Foo struct {
   A int64
   B uint64
   // and other stuff with int64 and uint64 and there's a lot of struct that are like this
}

x := Foo{A: 1234567890987654, B: 987654321012345678}
byt, err := json.Marshal(x)
fmt.Println(err)
fmt.Println(string(byt))

//                                 9223372036854775808      9223372036854775808
err = json.Unmarshal([]byte(`{"A":"12345678901234567", "B":"98765432101234567"}`), &x)
// ^ must be quoted since javascript can't represent those values properly (2^53)
// ^ json: cannot unmarshal string into Go struct field Foo.A of type int64
fmt.Println(err)
fmt.Printf("%#v\n", x)
// main.Foo{A:1234567890987654, B:0xdb4da5f44d20b4e}

https://play.golang.org/p/dHN-FcJ7p-N

So that It could receive json string but parsed as int64/uint64, and can deserialize int64/uint64 as json string without have to modify the struct or struct tag at all

Answer 1

Use the string option in the JSON struct tag. It's designed exactly for this use case:

The "string" option signals that a field is stored as JSON inside a JSON-encoded string. It applies only to fields of string, floating point, integer, or boolean types. This extra level of encoding is sometimes used when communicating with JavaScript programs

type Foo struct {
    A int64  `json:"A,string"`
    B uint64 `json:"B,string"`
}

func main() {
    x := &Foo{}
    _ = json.Unmarshal([]byte(`{"A":"12345678901234567", "B":"98765432101234567"}`), x)

    fmt.Println(x) // &{12345678901234567 98765432101234567}

    b, _ := json.Marshal(x)

    fmt.Println(string(b)) // {"A":"12345678901234567","B":"98765432101234567"}
}

Playground: https://play.golang.org/p/IfpcYOlcKMo

If you can't modify existing struct tags (but your example has none), then you have to reinvent the implementation of this string tag option in a custom UnmarshalJSON and MarshalJSON methods.