Using Must, Funcs, and ParseFiles with multiple files in Go templates

I wanted to add a few custom template functions in my first Go web application, which I’m currently building. Something like this

// Setup template
funcMap := template.FuncMap{
    "trim": strings.Trim,
}
customTemplate := template.Must(template.New("test").Funcs(funcMap).ParseFiles("views/base.html", "views/submit.html"))
.
.
// Call template
d := ....
customTemplate.Execute(w, d)

I found out that this was the only way to generate a page with custom functions in a performant way. After code compilation and execution, the browser screen was empty.

It turns out that the customTemplate object contains more than one inner template object, including both the ones created by New and ParseFiles. Calling Execute applies the template object created by New. Unfortunately, this wouldn’t include the HTML string returned by ParseFiles. This was why my browser screen was empty. To fix it, I used the ExecuteTemplate function like this:

customTemplate.ExecuteTemplate(w, "base.html", d)

The ExecuteTemplate function allows me to pick a specific template object, which, in this case, will be one returned by ParseFiles.

I could also set the parameter in the New function to what I expect ParseFiles template object name to be (in this case, base.html). This will allow me to just call the Execute function. Here’s how it will be

customTemplate := template.Must(template.New("base.html").Funcs(funcMap).ParseFiles("views/base.html", "views/submit.html"))
....
customTemplate.Execute(w, d)

Both the New and ParseFiles template objects have the same name, ParseFiles overwrites the template object allocated by New with its own. Execute now applies the ParseFiles template object which contains the HTML string I want.

This Stackoverflow answer has way more details on Go Template names.