Clarify Content-Type Handling When Using Component.Render in Documentation

[hu3bi]

Description:

Currently, when calling Render on a templ component, the Content-Type header is not set automatically. This requires users to manually set the appropriate header (e.g., "text/html; charset=utf-8") when serving components via HTTP.

This behavior can be surprising, especially since templ.Handler (or ComponentHandler) does handle the Content-Type when used with templ.WithContentType.

Proposal:

• Add clear documentation stating that users must set the Content-Type header themselves when using Component.Render.
• Optionally, consider whether Component.Render could accept options (similar to templ.Handler) to set the header automatically, or state in the documentation that this isn’t the default.

Benefits:

• Prevents confusion and subtle bugs when serving HTML responses.
• Improves the developer experience by making expectations explicit.

---

As a side note, extend the go comment for the component handler, to make it clear that it should only be used to render static sites.

[a-h]

I don't mind updating the docs, if people are getting confused by it, then sure. Is this something you've seen in production use?

The Component interface's Render method takes an io.Writer, not a http.ResponseWriter, so it shouldn't be too surprising that it doesn't set the Content-Type header, since it has no way to do so without inspecting the type of the interface and encoding specific behaviour.

Go's HTTP ResponseWriter uses a heuristic to set the Content-Type - see https://pkg.go.dev/net/http#ResponseWriter, based on the first 512 bytes of the response, so users aren't strictly required to set the header. In addition, most browsers will attempt to sniff.

Once headers are written, IIRC, you can't go back and change the status, which is why ComponentHandler has some behaviour around rendering to a buffer, then writing the correct status code (because it could be a 500 error) before rendering the body.

Do you have an idea of what the docs should say?

[hu3bi]

The Component interface's Render method takes an io.Writer, not a http.ResponseWriter, so it shouldn't be too surprising that it doesn't set the Content-Type header, since it has no way to do so without inspecting the type of the interface and encoding specific behaviour.

That makes sense. Somehow I didn't notice, that it didn't take a ResponseWriter. I would still prefer to add a nice go doc comment to render, that makes this behavior clear. When passing in a ResponseWriter the status code will be set!

Go's HTTP ResponseWriter uses a heuristic to set the Content-Type - see https://pkg.go.dev/net/http#ResponseWriter, based on the first 512 bytes of the response, so users aren't strictly required to set the header. In addition, most browsers will attempt to sniff.

Some reverse proxies set the header X-Content-Type-Options: nosniff. I prefer explicit settings instead of relying on a fallback mechanism in case the developer forgot something.

@a-h I would love to extend go doc with this.
HTTP Response handling could be omitted by just referencing http.ResponseWriter's documentation.

// Component is the interface that all templates implement.
type Component interface {
	// Render executes the template and writes the rendered output to w.
	//
	// Output is buffered internally and only written to w upon successful completion.
	// This ensures w receives either complete output or nothing at all.
	//
	// HTTP Response Handling:
	// When w implements [http.ResponseWriter] and neither status code nor Content-Type
	// header have been set, [Component.Render] automatically:
	//   - Sets Content-Type utilizing [http.DetectContentType]
	//   - Writes HTTP 200 OK status
	//
	// Cancellation:
	// The ctx parameter enables cancellation and timeout control. If ctx is canceled
	// or its deadline exceeded during rendering, Render immediately returns ctx.Err()
	// without writing any output to w.
	//
	// Error Handling in order:
	// Returns an error if:
	//    1. Context cancellation occurs
	//    2. Template execution fails (syntax errors, missing data, etc.)
	//    3. Writing to w fails (I/O errors, closed connections, etc.)
	Render(ctx context.Context, w io.Writer) error
}

// Handler creates a http.Handler that renders the template.
// As outlined in the [templ docu], [Handler] should only be used to display static data.
// To display dynamic data use [Component.Render].
//
// [templ docu]: https://templ.guide/server-side-rendering/creating-an-http-server-with-templ
func Handler(c Component, options ...func(*ComponentHandler)) *ComponentHandler {
	ch := &ComponentHandler{
		Component:   c,
		ContentType: "text/html; charset=utf-8",
	}
	for _, o := range options {
		o(ch)
	}
	return ch
}