How to Stream JSON with json.Decoder and json.Encoder

Use json.Decoder to read JSON values sequentially from an io.Reader and json.Encoder to write them sequentially to an io.Writer without loading the entire payload into memory.

package main

import (
	"encoding/json"
	"fmt"
	"io"
	"strings"
)

func main() {
	// Streaming Decode
	reader := strings.NewReader(`{"id":1,"name":"Alice"}{"id":2,"name":"Bob"}`)
	dec := json.NewDecoder(reader)
	for {
		var item map[string]interface{}
		if err := dec.Decode(&item); err == io.EOF {
			break
		}
		if err != nil {
			fmt.Println("Error:", err)
			break
		}
		fmt.Println("Decoded:", item)
	}

	// Streaming Encode
	var writer strings.Builder
	enc := json.NewEncoder(&writer)
	items := []map[string]interface{}{{"id": 1}, {"id": 2}}
	for _, item := range items {
		if err := enc.Encode(item); err != nil {
			fmt.Println("Error:", err)
			return
		}
	}
	fmt.Println("Encoded:", writer.String())
}

Think of streaming like reading a book page by page instead of photocopying the whole book at once. json.Decoder reads one JSON object at a time from a data stream, and json.Encoder writes them out one by one. This saves memory and lets you process huge files or live data feeds efficiently.