Chapter 11: JSON (The Universal Language)

Goal: Learn how to trade data with the outside world. Concept: Your Go program sends structs. The Browser wants JSON (JavaScript Object Notation). We need a translator.

1. The Sticker (Struct Tags)

A struct groups named fields. Here type Book struct { ... } defines the type, Book{...} creates a value, and book.Title selects a field. Exported fields start with uppercase letters. See Middle chapter 3 for methods and copying. But how does Go know that BookTitle in Go code should be title in JSON? Tags. Think of them as sticky notes attached to the fields for the translator to read.

type Book struct {
    Title  string  `json:"title"`       // lowercase "title" in JSON
    PriceMinor int64 `json:"price_minor"`
    Currency string `json:"currency"`
    Hidden string  `json:"-"`           // Ignore this field!
}

2. Marshaling (Packing)

Marshal = Converting Go Struct -> JSON String. (Ideally, a byte slice, but think of it as text).

myBook := Book{Title: "Go Guide", PriceMinor: 1999, Currency: "USD"}
jsonData, err := json.Marshal(myBook)
if err != nil {
    log.Fatal(err)
}
fmt.Println(string(jsonData))
// Output: {"title":"Go Guide","price_minor":1999,"currency":"USD"}

3. Unmarshaling (Unpacking)

Unmarshal = JSON String -> Go Struct.

jsonStr := `{"title":"Go Guide","price_minor":1999,"currency":"USD"}`
var newBook Book
if err := json.Unmarshal([]byte(jsonStr), &newBook); err != nil {
    log.Fatal(err) // CLI example; HTTP handlers should return an error response.
}

4. Visual Signal: The Translator 🗣️

Concept: Serialization/Deserialization. Signal: A Universal Translator. The Go Gopher speaks "Struct", the Browser speaks "JSON". The Translator converts between them.

Why do we care?

When you build your API in the next level, JSON endpoints may involve these operations (HTML pages, downloads, and requests without a body use different paths):

1. Unmarshaling user Input (JSON -> Struct).
2. Marshaling your Response (Struct -> JSON).