Convert number from float64 to Int64 is incorrect [closed] - json

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I try to write a function, the code:
package main
import (
"encoding/json"
"fmt"
)
type Test struct {
Data map[string]interface{} `json:"data"`
}
func main() {
jsonStr := "{\"data\": {\"id\": 999804707614896129}}"
t := &Test{}
json.Unmarshal([]byte(jsonStr), t)
fmt.Println(int64(t.Data["id"].(float64)))
var x float64 = 999804707614896129
fmt.Println(int64(x))
}
The result is below:
999804707614896128
999804707614896128
Why the results are 999804707614896128, not 999804707614896129.

Answer 1

Because 999804707614896129 cannot be represented exactly with a value of type float64. float64 uses the IEEE 754 standard for representing floating point numbers. It's a format with limited precision (roughly 16 decimal digits).
If you need to "transfer" the exact number, use string and not JSON number. If the number "fits" into an int64, you will be able to parse it "exactly" (else just work with it as a string or use big.Int):
jsonStr := `{"data": {"id": "999804707614896129"}}`
t := &Test{}
if err := json.Unmarshal([]byte(jsonStr), t); err != nil {
panic(err)
}
s := t.Data["id"].(string)
fmt.Println(s)
var x int64
x, err := strconv.ParseInt(s, 10, 64)
fmt.Println(x, err)
This will output (try it on the Go Playground):
999804707614896129
999804707614896129 <nil>