A Day in a Pile of Work

My personal Web development blog

Content Negotiation in Valum

I recently finished and merged support for content negotiation.

The implementation is really simple: one provide a header, a string describing expecations and a callback invoked with the negotiated representation. If no expectation is met, a 406 Not Acceptable is raised.

app.get ("/", negotiate ("Accept", "text/xml; application/json",
                         (req, res, next, ctx, content_type) => {
    // produce according to 'content_type'

Content negotiation is a nice feature of the HTTP protocol allowing a client and a server to negotiate the representation (eg. content type, language, encoding) of a resource.

One very nice part allows the user agent to state a preference and the server to express quality for a given representation. This is done by specifying the q parameter and the negotiation process attempt to maximize the product of both values.

The following example express that the XML version is poor quality, which is typically the case when it’s not the source document. JSON would be favoured – implicitly q=1 – if the client does not state any particular preference.

accept ("text/xml; q=0.1, application/json", () => {


Mounted as a top-level middleware, it provide a nice way of setting a Content-Type: text/html; charset=UTF-8 header and filter out non-compliant clients.

using Tmpl;
using Valum;

var app = new Router ();

app.use (accept ("text/html", () => {
    return next ();

app.use (accept_charset ("UTF-8", () => {
    return next ();

var home = new Template.from_path ("templates/home.html");

app.get ("/", (req, res) => {
    home.expand (res.body, null);

This is another step forward a 0.3 release!

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