How to Write JSON File in Go: Simple Guide with Example

To write a JSON file in Go, use the encoding/json package to convert data into JSON format and the os package to create and write to a file. Use json.MarshalIndent for readable JSON and os.WriteFile or os.Create with file.Write to save the JSON data to a file.

Syntax

Here is the basic syntax to write JSON data to a file in Go:

• json.MarshalIndent(data, "", " "): Converts Go data to formatted JSON bytes.
• os.WriteFile(filename, jsonBytes, 0644): Writes JSON bytes to a file with permissions.

go

jsonBytes, err := json.MarshalIndent(data, "", "  ")
if err != nil {
    // handle error
}

err = os.WriteFile("file.json", jsonBytes, 0644)
if err != nil {
    // handle error
}

Example

This example shows how to create a struct, convert it to JSON, and write it to a file named data.json. It uses json.MarshalIndent for pretty JSON formatting and os.WriteFile to save the file.

go

package main

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

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

func main() {
    person := Person{Name: "Alice", Age: 30}

    jsonBytes, err := json.MarshalIndent(person, "", "  ")
    if err != nil {
        fmt.Println("Error encoding JSON:", err)
        return
    }

    err = os.WriteFile("data.json", jsonBytes, 0644)
    if err != nil {
        fmt.Println("Error writing file:", err)
        return
    }

    fmt.Println("JSON file written successfully")
}

Output

JSON file written successfully

Common Pitfalls

Common mistakes when writing JSON files in Go include:

• Not checking errors from json.MarshalIndent or file operations, which can cause silent failures.
• Using json.Marshal without indentation, making JSON hard to read.
• Forgetting to set proper file permissions, which can cause permission errors.
• Trying to write to a file without creating or opening it properly.

go

package main

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

func main() {
    data := map[string]string{"hello": "world"}

    // Wrong: ignoring error from Marshal
    jsonBytes, _ := json.Marshal(data)

    // Wrong: writing to file without checking error
    _ = os.WriteFile("output.json", jsonBytes, 0000) // wrong permissions

    // Right way:
    jsonBytes, err := json.MarshalIndent(data, "", "  ")
    if err != nil {
        fmt.Println("Error encoding JSON:", err)
        return
    }

    err = os.WriteFile("output.json", jsonBytes, 0644)
    if err != nil {
        fmt.Println("Error writing file:", err)
        return
    }

    fmt.Println("File written correctly")
}

Output

File written correctly

Quick Reference

Remember these key points when writing JSON files in Go:

• Use json.MarshalIndent for readable JSON output.
• Use os.WriteFile to write bytes to a file simply.
• Always check errors from JSON encoding and file writing.
• Set file permissions to 0644 for normal read/write access.

Key Takeaways

Use encoding/json's MarshalIndent to convert Go data to pretty JSON bytes.

Write JSON bytes to a file with os.WriteFile and proper permissions (0644).

Always check errors when encoding JSON and writing files to avoid silent failures.

Set file permissions correctly to ensure the file is accessible.

Avoid ignoring errors and use indentation for better JSON readability.