How to Preserve Original String Content When Marshaling a `[]byte` Field in a Go Struct as JSON?

Marshaling JSON []byte as Strings

Question:

In Go, when encoding a struct containing a []byte field as JSON, the resulting JSON includes a non-expected string representation of the slice contents. For example, the code:

type Msg struct {
    Content []byte
}

func main() {
    helloStr := "Hello"
    helloSlc := []byte(helloStr)
    json, _ := json.Marshal(Msg{helloSlc})
    fmt.Println(string(json))
}

Produces the JSON string:

{"Content":"SGVsbG8="}

What conversion is performed by json.Marshal on the slice contents, and how can the original string content be preserved?

Answer:

Base64 Encoding

By default, Go's json.Marshal function encodes []byte arrays as base64-encoded strings to represent raw bytes in JSON. According to the JSON specification, JSON doesn't have a native representation for raw bytes.

Custom Marshaling:

To preserve the original string content, custom marshaling can be implemented by defining a custom MarshalJSON method for the Msg struct:

import (
    "encoding/json"
    "fmt"
)

type Msg struct {
    Content []byte
}

func (m Msg) MarshalJSON() ([]byte, error) {
    return []byte(fmt.Sprintf(`{"Content": "%s"}`, m.Content)), nil
}

func main() {
    helloStr := "Hello"
    helloSlc := []byte(helloStr)
    json, _ := json.Marshal(Msg{helloSlc})
    fmt.Println(string(json))
}

This custom implementation encodes the Content field as a string within the JSON object, preserving its original content.

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