How to Preserve Int64 Values When Parsing JSON in Go?

Preserve Int64 Values when Parsing JSON in Go

When processing JSON POST requests containing arrays of objects with int64 values, it's common to encounter conversion issues. JSON.Unmarshal automatically converts these values to float64, which can be inconvenient. This article explores solutions to preserve the original int64 values.

Solution 1: Using Number Type

The json.Decoder type includes a UseNumber method that allows you to decode numbers into a custom type. By defining a Number type that represents a JSON number literal, you can convert it back to int64 easily:

func main() {
    body := []byte("{\"tags\":[{\"id\":4418489049307132905},{\"id\":4418489049307132906}]}")
    var dat map[string]interface{}
    d := json.NewDecoder(bytes.NewBuffer(body))
    d.UseNumber()
    if err := d.Decode(&dat); err != nil {
        panic(err)
    }
    // ...
    n := tags[0].(map[string]interface{})["id"].(json.Number)
    i64, _ := strconv.ParseUint(string(n), 10, 64)
}

Solution 2: Decoding into Custom Structure

You can also decode directly into a specific structure tailored to your data:

type A struct {
    Tags []map[string]uint64
}
func main() {
    body := []byte("{\"tags\":[{\"id\":4418489049307132905},{\"id\":4418489049307132906}]}")
    var a A
    if err := json.Unmarshal(body, &a); err != nil {
        panic(err)
    }
    // ...
    println(a.Tags[0]["id"])
}

Caution:

Note that if your application interacts with JavaScript, which only supports IEEE754 double precision floats, you may encounter parsing issues with int64 values.

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