How to Pretty Print JSON in Go

Key Takeaways

1. Use json.MarshalIndent to format Go data structures into readable JSON.
2. json.Indent can be used to pretty print existing JSON strings.
3. json.Encoder provides a memory-efficient way to format JSON output.

In Go, "pretty printing" JSON refers to formatting JSON data with indentation and line breaks to enhance readability. This is particularly useful when displaying JSON in logs or console outputs. Go's standard library provides built-in support for this through the encoding/json package.

Using json.MarshalIndent to Pretty Print JSON

The encoding/json package offers the MarshalIndent function, which serializes Go data structures into JSON with indentation. Here's how to use it:

package main

import (
    "encoding/json"
    "fmt"
)

func main() {
    // Sample data structure
    data := map[string]interface{}{
        "Name":   "Alice",
        "Age":    30,
        "Active": true,
        "Skills": []string{"Go", "Docker", "Kubernetes"},
    }

    // Convert to pretty-printed JSON
    jsonData, err := json.MarshalIndent(data, "", "    ")
    if err != nil {
        fmt.Println("Error marshalling JSON:", err)
        return
    }

    // Print the formatted JSON
    fmt.Println(string(jsonData))
}

In this example:

• data is a map representing a sample data structure.
• json.MarshalIndent serializes data into a JSON-formatted byte slice with indentation.
• The second argument ("") specifies the prefix for each line (none in this case).
• The third argument (" ") specifies the indentation string (four spaces here).
• The resulting jsonData is converted to a string and printed.

The output will be:

{
    "Active": true,
    "Age": 30,
    "Name": "Alice",
    "Skills": [
        "Go",
        "Docker",
        "Kubernetes"
    ]
}

Pretty Printing Existing JSON Strings

If you have a JSON string and wish to pretty print it, you can use the json.Indent function:

package main

import (
    "bytes"
    "encoding/json"
    "fmt"
)

func main() {
    // Minified JSON string
    jsonString := `{"Name":"Alice","Age":30,"Active":true,"Skills":["Go","Docker","Kubernetes"]}`

    // Convert string to byte slice
    jsonData := []byte(jsonString)

    // Create a buffer to hold the formatted JSON
    var prettyJSON bytes.Buffer

    // Indent the JSON
    err := json.Indent(&prettyJSON, jsonData, "", "    ")
    if err != nil {
        fmt.Println("Error indenting JSON:", err)
        return
    }

    // Print the formatted JSON
    fmt.Println(prettyJSON.String())
}

In this code:

• jsonString contains the minified JSON data.
• json.Indent reads the minified JSON and writes a formatted version to prettyJSON.
• The formatted JSON is then printed as a string.

The output will be identical to the previous example.

Using json.Encoder with Indentation

Alternatively, you can use json.Encoder to pretty print JSON directly to an io.Writer, such as os.Stdout:

package main

import (
    "encoding/json"
    "os"
)

func main() {
    // Sample data structure
    data := map[string]interface{}{
        "Name":   "Alice",
        "Age":    30,
        "Active": true,
        "Skills": []string{"Go", "Docker", "Kubernetes"},
    }

    // Create a JSON encoder with indentation
    encoder := json.NewEncoder(os.Stdout)
    encoder.SetIndent("", "    ")

    // Encode and print the data
    err := encoder.Encode(data)
    if err != nil {
        fmt.Println("Error encoding JSON:", err)
    }
}

This approach is efficient when you want to write formatted JSON directly to an output stream without holding it in memory.

Conclusion

Go's encoding/json package provides multiple methods to pretty print JSON data, enhancing readability in logs and console outputs. Whether using json.MarshalIndent, json.Indent, or json.Encoder with indentation, you can choose the method that best fits your use case.

FAQs

What is the difference between json.MarshalIndent and json.Indent?

json.MarshalIndent formats Go data structures into JSON, while json.Indent reformats existing JSON strings.

When should I use json.Encoder instead of json.MarshalIndent?

Use json.Encoder when writing directly to an io.Writer to avoid unnecessary memory usage.

Can I change the indentation character in json.MarshalIndent?

Yes, the third argument in json.MarshalIndent specifies the indentation string (e.g., spaces or tabs).

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