Add fallthrough option

README.md

@@ -89,6 +89,25 @@ provided locals:
 
 Display mode. `tiles` and `details` are available. Defaults to `tiles`.
 
+##### fallthrough
+
+Set the middleware to have client errors fall-through as just unhandled
+requests, otherwise forward a client error. The difference is that client
+errors like a bad request or a request to a non-existent file will cause
+this middleware to simply `next()` to your next middleware when this value
+is `true`. When this value is `false`, these errors (even 404s), will invoke
+`next(err)`.
+
+Typically `true` is desired such that multiple physical directories can be
+mapped to the same web address or for routes to fill in non-existent files.
+
+The value `false` can be used if this middleware is mounted at a path that
+is designed to be strictly a single file system directory, which allows for
+short-circuiting 404s for less overhead. This middleware will also reply to
+all methods.
+
+The default value is `true`.
+
 ## Examples
 
 ### Serve directory indexes with vanilla node.js http server

index.js

@@ -97,9 +97,14 @@ function serveIndex(root, options) {
   var stylesheet = opts.stylesheet || defaultStylesheet;
   var template = opts.template || defaultTemplate;
   var view = opts.view || 'tiles';
+  var fallthrough = opts.fallthrough !== false;
 
   return function (req, res, next) {
     if (req.method !== 'GET' && req.method !== 'HEAD') {
+      if (fallthrough) {
+        return next();
+      }
+
       res.statusCode = 'OPTIONS' === req.method ? 200 : 405;
       res.setHeader('Allow', 'GET, HEAD, OPTIONS');
       res.setHeader('Content-Length', '0');