A Complete Guide - GoLang Web Programming Working with JSON and CSV

1. How do you encode a Go struct to JSON in a web server response?

Answer: In Go, the encoding/json package is used to handle JSON encoding. To send a JSON response, you need to encode a Go struct to JSON format and write it to the http.ResponseWriter. Here's an example:

package main import ( "encoding/json" "net/http"
) type Message struct { Text string `json:"text"`
} func messageHandler(w http.ResponseWriter, r *http.Request) { msg := Message{Text: "Hello, world!"} w.Header().Set("Content-Type", "application/json") json.NewEncoder(w).Encode(msg)
} func main() { http.HandleFunc("/message", messageHandler) http.ListenAndServe(":8080", nil)
}

2. How do you decode a JSON request body in a Go web server?

Answer: To decode a JSON request body, you can use json.NewDecoder on the http.Request body and unmarshal it into a Go struct. Here's an example:

package main import ( "encoding/json" "io/ioutil" "net/http" "log"
) type Person struct { Name string `json:"name"` Age int `json:"age"`
} func personHandler(w http.ResponseWriter, r *http.Request) { var person Person reqBody, err := ioutil.ReadAll(r.Body) if err != nil { http.Error(w, err.Error(), http.StatusBadRequest) return } err = json.Unmarshal(reqBody, &person) if err != nil { http.Error(w, err.Error(), http.StatusBadRequest) return } log.Printf("Received person: %+v", person)
} func main() { http.HandleFunc("/person", personHandler) http.ListenAndServe(":8080", nil)
}

3. How do you read a CSV file in Go?

Answer: The encoding/csv package allows you to read CSV files. Here's an example of reading a CSV file:

package main import ( "encoding/csv" "fmt" "log" "os"
) func main() { file, err := os.Open("data.csv") if err != nil { log.Fatal(err) } defer file.Close() reader := csv.NewReader(file) records, err := reader.ReadAll() if err != nil { log.Fatal(err) } for _, record := range records { fmt.Println(record) }
}

4. How do you write to a CSV file in Go?

Answer: To write to a CSV file, you can use csv.NewWriter and call Write for each record you wish to add. Here's how:

package main import ( "encoding/csv" "log" "os"
) func main() { file, err := os.Create("data.csv") if err != nil { log.Fatal(err) } defer file.Close() writer := csv.NewWriter(file) defer writer.Flush() records := [][]string{ {"Name", "Age"}, {"Alice", "30"}, {"Bob", "25"}, } for _, record := range records { if err := writer.Write(record); err != nil { log.Fatalf("error writing record to csv: %v", err) } }
}

5. How do you encode a slice of structs to JSON for a web response?

Answer: Encoding a slice of structs takes a similar approach to encoding a single struct, but uses a slice type. Here's an example:

package main import ( "encoding/json" "net/http"
) type User struct { ID int `json:"id"` Name string `json:"name"`
} func usersHandler(w http.ResponseWriter, r *http.Request) { users := []User{ {ID: 1, Name: "Alice"}, {ID: 2, Name: "Bob"}, } w.Header().Set("Content-Type", "application/json") json.NewEncoder(w).Encode(users)
} func main() { http.HandleFunc("/users", usersHandler) http.ListenAndServe(":8080", nil)
}

6. How do you handle JSON decoding errors in Go web requests?

Answer: Handling JSON decoding errors is crucial for robust web services. You should check the errors after attempting to decode JSON. Here's a comprehensive example:

func personHandler(w http.ResponseWriter, r *http.Request) { var person Person decoder := json.NewDecoder(r.Body) err := decoder.Decode(&person) if err != nil { w.WriteHeader(http.StatusBadRequest) json.NewEncoder(w).Encode(map[string]string{"error": "invalid JSON format"}) return } log.Printf("Received person: %+v", person)
}

7. How do you ensure a JSON struct always returns a specific field name?

Answer: You can control the JSON field names using struct tags in your Go code. The json tag specifies the JSON field name. For instance:

type User struct { UserName string `json:"username"` UserID int `json:"user_id"`
}

8. How can you filter fields when encoding a JSON struct in Go?

Answer: Go's JSON encoding ignores struct fields with empty json tags or fields that are unexported (non-public). For more control, use struct tags or embed another struct with selected fields:

type FullUser struct { UserID int `json:"user_id"` UserName string `json:"username"` Password string `json:"-"`
} type PublicUser struct { UserID int `json:"user_id"` UserName string `json:"username"`
} func fullUserHandler(w http.ResponseWriter, r *http.Request) { user := FullUser{UserID: 1, UserName: "Alice", Password: "secret"} publicUser := PublicUser(user) json.NewEncoder(w).Encode(publicUser)
}

9. How do you handle large CSV files in Go?

Answer: Handling large CSV files efficiently in Go involves reading them line by line. This approach minimizes memory usage:

func processLargeCSV(file *os.File) { reader := csv.NewReader(file) for { record, err := reader.Read() if err == io.EOF { break } if err != nil { log.Fatal(err) } fmt.Println(record) }
}

10. How do you handle CSV files with headers in Go?

Answer: To handle CSV files with headers, you can use csv.NewReader and read the first row as headers. Here's an example: