JSON Encoding and Decoding

The encoding/json package converts between Go values and JSON text used in files and web APIs.

json.Marshal serializes a Go value into bytes. json.Unmarshal decodes bytes into a pointer to a destination. Exported struct fields are visible to encoding/json; tags such as json:"name" customize JSON field names.

Check encoding errors and decide whether unknown or missing fields matter to your application. Unmarshaling into a struct leaves fields at zero values if absent unless you add separate validation.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import (
	"encoding/json"
	"fmt"
)

type Student struct {
	Name  string `json:"name"`
	Score int    `json:"score"`
}

func main() {
	original := Student{Name: "Ava", Score: 92}
	data, err := json.Marshal(original)
	if err != nil {
		panic(err)
	}
	fmt.Println(string(data))
	var parsed Student
	if err := json.Unmarshal(data, &parsed); err != nil {
		panic(err)
	}
	fmt.Println(parsed.Name, parsed.Score)
}

Output

{"name":"Ava","score":92}
Ava 92

Marshal produces compact JSON with lowercase name and score keys. Unmarshal fills an addressable Student value, which prints Ava 92.

• Unexported struct fields are not serialized by encoding/json.
• Decoding and validating business requirements are separate steps.

Add a boolean Active field with a json tag; serialize it and verify it decodes correctly.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced JSON Encoding and Decoding in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.