Stop json.Marshal() from stripping trailing zero from floating point number

Question

I got the following problem: My golang program converts some information into JSON. For example it results in the following json:

{
   \"value\":40,
   \"uni         


        

Answer 1

By default floating point numbers are rendered without a decimal point and fractions if its value is an integer value. The representation is shorter, and it means the same number.

If you want control over how a number appears in the JSON representation, use the json.Number type.

Example:

type Pt struct {
    Value json.Number
    Unit  string
}

func main() {
    data, err := json.Marshal(Pt{json.Number("40.0"), "some_string"})
    fmt.Println(string(data), err)
}

Output (try it on the Go Playground):

{"Value":40.0,"Unit":"some_string"} 

If you have a number as a float64 value, you may convert it to json.Number like this:

func toNumber(f float64) json.Number {
    var s string
    if f == float64(int64(f)) {
        s = fmt.Sprintf("%.1f", f) // 1 decimal if integer
    } else {
        s = fmt.Sprint(f)
    }
    return json.Number(s)
}

Testing it:

f := 40.0
data, err := json.Marshal(Pt{toNumber(f), "some_string"})
fmt.Println(string(data), err)

f = 40.123
data, err = json.Marshal(Pt{toNumber(f), "some_string"})
fmt.Println(string(data), err)

Output (try it on the Go Playground):

{"Value":40.0,"Unit":"some_string"} 
{"Value":40.123,"Unit":"some_string"} 

The other direction, if you want the float64 value of a json.Number, simply call its Number.Float64() method.