Why Does JSON Marshaling of a Go []byte Result in a Base64-Encoded String?

Marshalling []byte to JSON: Decoding the Mysteries

When attempting to marshal a byte slice ([]byte) to JSON format, it's common to encounter a strange, encoded string as the output. To delve into this peculiar behavior, let's examine the code sample provided in the question:

type ColorGroup struct {
    ByteSlice    []byte
    SingleByte   byte
    IntSlice     []int
}
...
b, err := json.Marshal(group)

The JSON marshaling process converts a Go data structure into its JSON representation. Understanding how []byte is treated during this conversion is crucial. According to the Go documentation for json.Marshal, "[]byte encodes as a base64-encoded string."

The Encoding Enigma

What this means is that when you attempt to marshal a []byte to JSON, the slice is not directly included as an array in the JSON output. Instead, it is converted to a base64-encoded string. This conversion is employed because JSON does not have a native byte slice type.

Decoding the Encoded String

To decode the encoded string back to its original byte slice, you can use the Go base64 package. Here's how it would look:

import (
    "encoding/base64"
    "log"
)
...
b64 := "AAAAAQID" // encoded byte slice from JSON
b, err := base64.StdEncoding.DecodeString(b64)
if err != nil {
    log.Fatal(err)
}
fmt.Printf("%v", b) // Outputs: [0 0 0 1 2 3]

Why the Strange String?

The encoded string that you encountered is the result of the base64 encoding process. Base64 encoding replaces binary data with a subset of ASCII characters to make it suitable for transmission or storage in non-binary formats like JSON. In the case of the byte slice, this encoding transforms the byte values into a string representation.

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