Creating readJSON method to manage errors that can happen once.

cmd/api/helpers.go

@@ -3,7 +3,9 @@ package main
 import (
 	"encoding/json"
 	"errors"
+	"fmt"
 	"github.com/julienschmidt/httprouter"
+	"io"
 	"net/http"
 	"strconv"
 )
@@ -82,3 +84,45 @@ func (app *application) writeJSON(w http.ResponseWriter, status int, data envelo
 
 	return nil
 }
+
+func (app *application) readJSON(w http.ResponseWriter, r *http.Request, dst any) error {
+	// Decode the request body into the target destination.
+	err := json.NewDecoder(r.Body).Decode(dst)
+	if err != nil {
+		// If there is an error during decoding, start the triage...
+		var syntaxError *json.SyntaxError
+		var unmarshalTypeError *json.UnmarshalTypeError
+		var invalidUnmarshalError *json.InvalidUnmarshalError
+
+		switch {
+		// Use the errors.As() function to check whether the error has the type *json.SyntaxError. If it does, then return a plain-english error message which includes the location problem.
+		case errors.As(err, &syntaxError):
+			return fmt.Errorf("body contains badly-formed JSON (at character %d)", syntaxError.Offset)
+
+		// In some circumstances Decode() may also return an io.ErrUnexpectedEOF error for syntax errors in the JSON. So we check for this using errors.Is() and return a generic error message.
+		case errors.Is(err, io.ErrUnexpectedEOF):
+			return errors.New("body contains badly-formed JSON")
+
+		// Likewise, catch any *json.UnmarshalTypeError errors. These occur when the JSON value is the wrong type for the target destination. If the error relates to a specific field, then we include that in our error message to make it easier for the client to debug.
+		case errors.As(err, &unmarshalTypeError):
+			if unmarshalTypeError.Field != "" {
+				return fmt.Errorf("body contains incorrect JSON type for field %q", unmarshalTypeError.Field)
+			}
+			return fmt.Errorf("body contains incorrect JSON type (at character %d)", unmarshalTypeError.Offset)
+
+		// An io.EOF error will be returned by Decode() if the request body is empty. We check for this with errors.Is() and return a plain-english error message instead.
+		case errors.Is(err, io.EOF):
+			return errors.New("body must not be empty")
+
+		// A json.InvalidUnmarshalError error will be returned if we pass something that is not a non-nil pointer to Decode(). We catch this and panic, rather than returning an error to our handler.
+		case errors.As(err, &invalidUnmarshalError):
+			panic(err)
+
+		// For anything else, return the error mesasge as-is.
+		default:
+			return err
+		}
+	}
+
+	return nil
+}