How to Parse JSON in Go: Simple Guide with Examples

In Go, you parse JSON using the encoding/json package by calling json.Unmarshal to convert JSON data into Go structs or maps. You provide the JSON bytes and a pointer to the variable where you want the data stored.

Syntax

The main function to parse JSON in Go is json.Unmarshal. It takes two arguments: the JSON data as a byte slice and a pointer to the variable where the parsed data will be stored.

• jsonData []byte: The raw JSON data.
• v interface{}: A pointer to a Go variable (usually a struct or map) to hold the parsed data.

The function returns an error if parsing fails.

go

err := json.Unmarshal(jsonData, &v)
if err != nil {
    // handle error
}

Example

This example shows how to parse a JSON string representing a person into a Go struct and print the fields.

go

package main

import (
    "encoding/json"
    "fmt"
)

type Person struct {
    Name string `json:"name"`
    Age  int    `json:"age"`
}

func main() {
    jsonData := []byte(`{"name": "Alice", "age": 30}`)

    var p Person
    err := json.Unmarshal(jsonData, &p)
    if err != nil {
        fmt.Println("Error parsing JSON:", err)
        return
    }

    fmt.Println("Name:", p.Name)
    fmt.Println("Age:", p.Age)
}

Output

Name: Alice
Age: 30

Common Pitfalls

Common mistakes when parsing JSON in Go include:

• Not using a pointer when calling json.Unmarshal, which prevents the data from being stored.
• Struct fields not being exported (starting with lowercase letters), so JSON cannot fill them.
• Missing or incorrect JSON tags on struct fields, causing mismatches.
• Ignoring errors returned by json.Unmarshal.

go

package main

import (
    "encoding/json"
    "fmt"
)

type Person struct {
    name string `json:"name"` // lowercase field: unexported
    Age  int    `json:"age"`
}

func main() {
    jsonData := []byte(`{"name": "Bob", "age": 25}`)

    var p Person
    // Wrong: passing value instead of pointer
    err := json.Unmarshal(jsonData, p)
    if err != nil {
        fmt.Println("Error:", err)
        return
    }

    fmt.Println("Name:", p.name) // empty because unexported
    fmt.Println("Age:", p.Age)

    // Correct way:
    var p2 Person
    err = json.Unmarshal(jsonData, &p2) // pointer used
    if err != nil {
        fmt.Println("Error:", err)
        return
    }
    fmt.Println("Correct Name:", p2.name) // still empty because unexported
    fmt.Println("Correct Age:", p2.Age)
}

Output

Name: 
Age: 25
Correct Name: 
Correct Age: 25

Quick Reference

Concept	Description	Example
json.Unmarshal	Parse JSON bytes into Go variable	json.Unmarshal(data, &v)
Pointer	Pass pointer to store parsed data	&v
Struct Tags	Map JSON keys to struct fields	`json:"key"`
Exported Fields	Struct fields must start with uppercase	Name string
Error Handling	Always check error from Unmarshal	if err != nil {...}

Key Takeaways

Use json.Unmarshal with a pointer to parse JSON into Go structs or maps.

Struct fields must be exported (start with uppercase) to be filled by JSON.

Add json tags to struct fields to match JSON keys exactly.

Always check the error returned by json.Unmarshal to catch parsing issues.

Passing a non-pointer or unexported fields will cause parsing to fail silently.