Golang templates: using FuncMap with html/template produces empty response

Question

Using html/template I am trying to populate a date field in an HTML form from a time.Time type of field using template.FuncMap but it is not working for me.

The following code is working,

type Task struct {
    ID          int
    ProjectID   int
    Description string
    StartDate   time.Time
    Duration    float32
}

type ProjectTaskData struct {
    ProjectID          int
    ProjectName        string
    TaskDetails        Task
    FormattedStartDate string   // this field is a hack/workaround for me
}

My trimmed down main function,

func main() {
    db := GetConnection()

    r := mux.NewRouter()

    // other handlers here are removed

    r.HandleFunc("/project/{project-id}/add-task", AddTask(db))
    r.PathPrefix("/static/").Handler(http.StripPrefix("/static/", http.FileServer(http.Dir("static/"))))

    http.ListenAndServe(":8080", r)
}

The AddTask function,

func AddTask(db *sql.DB) func(w http.ResponseWriter, r *http.Request) {
    //tmpl := template.Must(template.New("").Funcs(template.FuncMap{
    //  "startdate": func(t time.Time) string { return t.Format("2006-01-02") },
    //}).ParseFiles("static/add_task.html"))
    tmpl := template.Must(template.ParseFiles("static/add_task.html"))
    return func(w http.ResponseWriter, r *http.Request) {
        vars := mux.Vars(r)
        projectID, err := strconv.Atoi(vars["project-id"])
        if err != nil {
            log.Fatal(err)
        }

        var projectName string
        var projectStartDate time.Time
        err = db.QueryRow(`select name, start_date from projects where id = ?`, projectID).Scan(&projectName, &projectStartDate)
        switch {
        case err == sql.ErrNoRows:
            log.Printf("No project with id %d\n", projectID)
            return
        case err != nil:
            log.Fatalf("Query error: %v\n", err)
        default:
            log.Printf("Got project %v with id %d\n", projectName, projectID)
        }

        if r.Method != http.MethodPost {
            data := ProjectTaskData{
                ProjectID:   projectID,
                ProjectName: projectName,
                TaskDetails: Task{
                    ProjectID:   projectID,
                    Description: "",
                    StartDate:   projectStartDate,
                    Duration:    1,
                },
                FormattedStartDate: projectStartDate.Format(time.DateOnly),
            }
            tmpl.Execute(w, data)
            return
        }

        // rest of code handling the post action here
        
        http.Redirect(w, r, "/project/"+fmt.Sprint(projectID)+"/tasks", http.StatusFound)
    }
}

In the add_task.html, if I put the following place-holder, it is able to populate the start date when I hit http://localhost:8080/project/1/add-task,

<input type="date" id="start_date" name="start_date" value="{{.FormattedStartDate}}">

However, if I replace the below first line in AddTask(),

    tmpl := template.Must(template.ParseFiles("static/add_task.html"))

with the below one,

    tmpl := template.Must(template.New("").Funcs(template.FuncMap{
        "startdate": func(t time.Time) string { return t.Format("2006-01-02") },
    }).ParseFiles("static/add_task.html"))

and if I change the add_task.html as below,

<input type="date" id="start_date" name="start_date" value="{{.TaskDetails.StartDate | startdate}}">

I get an empty response when I hit http://localhost:8080/project/1/add-task (but I get 200 OK)

I also referred the below question but wasn't successful,

https://stackoverflow.com/a/35550730/8813473

Answer 1

As mentioned by @icza in the comment, receiving the error from Template.Execute() revealed the issue.

I got the error, template: "" is an incomplete or empty template

Referring to the answer at https://stackoverflow.com/a/49043639/8813473, I changed the template.New("") call to template.New("add_task.html") to resolve the issue.

The final code looks like below,

    tmpl := template.Must(template.New("add_task.html").Funcs(template.FuncMap{
        "startdate": func(t time.Time) string { return t.Format("2006-01-02") },
    }).ParseFiles("static/add_task.html"))