JSON encoding

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Goal

Turn a Go value into a JSON byte stream. After this page you will know how encoding/json reads the struct tags from the Structs page and emits the right keys.

Prerequisites

• Structs
• Slices

What encoding/json does

The standard library ships encoding/json — a package that walks any value and emits JSON. The walk follows three rules:

1. Only exported fields (uppercase first letter) are written.
2. The output key comes from the json:"name" field tag if present; otherwise it's the field name verbatim.
3. The encoder writes bytes — you pass it a Writer (a file, an HTTP response, a buffer) and it writes into that.

For the full set of options (omitempty, custom marshalers, indentation), see the encoding/json package docs. For now we use one function: json.NewEncoder(w).Encode(v).

Code

package main

import (
	"encoding/json"
	"log"
	"os"
)

type Item struct {
	ID    int     `json:"id"`
	Name  string  `json:"name"`
	Price float64 `json:"price"`
}

func main() {
	items := []Item{
		{ID: 1, Name: "Notebook", Price: 4.0},
		{ID: 2, Name: "Pen", Price: 2.5},
	}

	// os.Stdout is a Writer — anything that satisfies io.Writer works here.
	if err := json.NewEncoder(os.Stdout).Encode(items); err != nil {
		log.Fatal(err)
	}
}

Run it:

go run main.go

Output:

[{"id":1,"name":"Notebook","price":4},{"id":2,"name":"Pen","price":2.5}]

Notice the keys are lowercase — the encoder followed the json:"id" tags. Drop the tags and the keys come back as ID, Name, Price.

The same json.NewEncoder works against an HTTP ResponseWriter, because ResponseWriter is also a Writer. That's the bridge we use on the next page.

Next → Serving JSON