main.go

import (
	...
	rc "firebase-authentication-with-react-and-go/backend/router/context"
	m "firebase-authentication-with-react-and-go/backend/router/middleware"
)

func main() {
    	// ...

	e.Use(middleware.CORSWithConfig(middleware.CORSConfig{
		AllowOrigins: []string{"*"},
		AllowMethods: []string{http.MethodGet, http.MethodPost, http.MethodOptions},
		AllowHeaders: []string{echo.HeaderOrigin, echo.HeaderXRequestedWith, echo.HeaderContentType, echo.HeaderAccept, echo.HeaderAuthorization},
	}))
	
	//--- restricted api ---//
	g := e.Group("api", m.Auth())

	g.POST("/users",  wrapCustomContext(func(c *rc.Context) error {
		var params model.User
		user := model.User{}

		if err := c.Bind(&params); !errors.Is(err, nil) {
			return c.JSON(http.StatusBadRequest, err.Error())
		}

		// Check to see if the user already registered
		err = db.Where(&model.User{UUID: params.UUID}).First(&user).Error
		if !errors.Is(err, nil) && !gorm.IsRecordNotFoundError(err) {
			return c.JSON(http.StatusBadRequest, err.Error())
		}
		if user.UUID != "" {
			return c.JSON(http.StatusBadRequest, "The user already registered")
		}

		// Create a new user when not registered
		user = params
		err = db.Create(&user).Error
		if !errors.Is(err, nil) {
			return err
		}
		return c.JSON(http.StatusCreated, user)
	}))

    	// ...
}

func wrapCustomContext(fn func(c *rc.Context) error) echo.HandlerFunc {
	return func(ctx echo.Context) error {
		return fn(ctx.(*rc.Context))
	}
}