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Important Go Topics

Go JSON: Marshal and Unmarshal

Turn structs into JSON and JSON into structs, with struct tags to choose the field names.

  • Intermediate
  • 3 min read
  • 1 runnable example
What you'll learn
  • json.Marshal
  • json.Unmarshal
  • Struct tags and omitempty
Run code →
Round trip
  1. 1Product{...}a struct
  2. 2json.Marshal{"name":"Go Book",...}
  3. 3json.Unmarshalback to a struct

Structs to JSON and back

The encoding/json package converts Go values to JSON text and JSON text back to Go values. Struct tags choose the names used in the JSON.

Marshal turns the struct into JSON bytes, using the names from the json tags. Unmarshal fills a struct from JSON and needs a pointer so it can change the value. omitempty leaves a field out when it is empty, and a missing field keeps its zero value.

Key points

  1. Tags choose the names used in JSON.
  2. Unmarshal needs a pointer.
  3. omitempty leaves out empty fields.

Runnable Go example

Go
Run code →
main.go
package main

import (
	"encoding/json"
	"fmt"
)

type Product struct {
	Name  string   `json:"name"`
	Price float64  `json:"price"`
	Tags  []string `json:"tags,omitempty"`
}

func main() {
	book := Product{Name: "Go Book", Price: 12.5, Tags: []string{"go", "learning"}}
	data, err := json.Marshal(book)
	if err != nil {
		fmt.Println("error:", err)
		return
	}
	fmt.Println(string(data))

	var parsed Product
	input := `{"name":"Pen","price":1.25}`
	if err := json.Unmarshal([]byte(input), &parsed); err != nil {
		fmt.Println("error:", err)
		return
	}
	fmt.Printf("%s costs %.2f, tags: %d\n", parsed.Name, parsed.Price, len(parsed.Tags))
}
Get a Run button for this example Result of running the code
{"name":"Go Book","price":12.5,"tags":["go","learning"]}
Pen costs 1.25, tags: 0

Practice

Open the example in the online compiler, change a value or add a line, and predict the output before you run it. Then break the code on purpose and read the compiler's message.

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