Marshaling JSON []byte as strings in Go

Question

I have a struct containing strings as []byte fields which I'd like to encode into JSON. However, the generated JSON contains a non expected string representation of the slice contents. Here is an example of what I refer:

package main

import (
    "fmt"
    "encoding/json"
    )

type Msg struct {
    Content []byte
}

func main() {
    helloStr := "Hello"
    helloSlc := []byte(helloStr)
    fmt.Println(helloStr, helloSlc)

    obj := Msg{helloSlc}
    json, _ := json.Marshal(obj)
    fmt.Println(string(json))
}

This produces the following output:

Hello [72 101 108 108 111]
{"Content":"SGVsbG8="}

What kind of conversion is the json.Marshal() method performing to the []byte encoded string. How can I generate a JSON with the original content of my string {"Content":"Hello"}?

Answer 1

A []byte is marshalled as base64 encoded string. From the documentation:

Array and slice values encode as JSON arrays, except that []byte encodes as a base64-encoded string, and a nil slice encodes as the null JSON object.

These values are decoded properly when unmarshalled.

The reason why this is done is that JSON does not have a native representation for raw bytes. See this question for a detailed explanation.