Golang JSON array of different types reflection: float64 vs int64

Consider this simple example:

package main

import (
	"encoding/json"
	"fmt"
	"log"
	"reflect"
)

var args = `[1, 2.5, "aaa", true, false]`

func main() {
	var x []interface{}
	err := json.Unmarshal([]byte(args), &x)

	if err != nil {
		log.Fatalf("%s", err.Error())
		panic(fmt.Sprintf("%s", err.Error()))
	}

	for _, arg := range x {
		t := reflect.TypeOf(arg).Kind().String()
		v := reflect.ValueOf(arg)

		if t == "int64" {
			fmt.Printf("int64 %v\n", v.Int())
		}

		if t == "float64" {
			fmt.Printf("float64 %v\n", v.Float())
		}

		if t == "string" {
			fmt.Printf("string %v\n", v.String())
		}

		if t == "bool" {
			fmt.Printf("bool %v\n", v.Bool())
		}
	}
}

The program outputs:

float64 1
float64 2.5
string aaa
bool true
bool false

As you can see, my input is a valid JSON which represents an array with five items:

- integer
- floating point number
- string
- boolean
- boolean

When I unmarshal the valid JSON string into []interface{} and try to check the types with reflection, the integer value from JSON has a type of float64. Any idea why? Is this expected behaviour?

This is documented behavior of Unmarshal. All numbers are unmarshaled into float64.

> To unmarshal JSON into an interface value, Unmarshal stores one of these in the interface value:

> - bool, for JSON booleans

• float64, for JSON numbers
• string, for JSON strings
• []interface{}, for JSON arrays
• map[string]interface{}, for JSON objects
• nil for JSON null

This is because JSON does not have integers, every number in JSON is defined to be a 64 bit floating point.

This is the default behaviour of the JSON decoder. You can change it to output json.Number instead by using the UseNumber method.