String to float64 receiving format ".01"

If I receive from an API a string obeying the format of ".01", and I have a struct like this:

type Mystruct struct {
Val float64 json:"val,string"
}

In this case, I receive trying to unmarshal val into float64. Is there a way I can accomplish this?

Add a string field to capture the string value:

type Mystruct struct {
	Val  float64 `json:"-"`
	XVal string  `json:"val"`
}

Unmarshal the JSON document. Convert the string value to a float value:

var v Mystruct
err := json.Unmarshal([]byte(data), &v)
if err != nil {
	log.Fatal(err)
}
v.Val, err = strconv.ParseFloat(v.XVal, 64)
if err != nil {
	log.Fatal(err)
}

I recommand defining a type alias which you can use it anywhere.

package main

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

type MyFloat64 float64

func (f *MyFloat64) UnmarshalJSON(data []byte) error {
	raw := string(data)
	raw = strings.TrimPrefix(raw, "\"")
	raw = strings.TrimSuffix(raw, "\"")
	if parsedFloat, err := strconv.ParseFloat(raw, 64); err != nil {
		return err
	} else {
		*f = MyFloat64(parsedFloat)
		return nil
	}
}

type MyObj struct {
	Val1 MyFloat64
	Val2 string
}

func main() {
	j := `{"Val1":"0.01", "Val2":"0.01"}`
	o := MyObj{}
	err := json.Unmarshal([]byte(j), &o)
	if err != nil {
		fmt.Println(err)
	} else {
		b, _ := json.Marshal(o)
		fmt.Println("in:", j)
		fmt.Println("out:", string(b))
	}
}

output:

in: {"Val1":"0.01", "Val2":"0.01"}
out: {"Val1":0.01,"Val2":"0.01"}