lazy: true won't ever trigger rebuild without undocumented option "filename" set.

[grrowl]

I have it narrowed down to line 135 of middleware.js:

if(options.lazy && filename === pathJoin(compiler.outputPath, options.filename))

When the undocumented option filename isn't set, it will attempt to match /Users/tom/Documents/somewebsite/dist/undefined, fail, and the request will fall through to the next handler, regardless of the file.

Setting { lazy: true, filename: "client.js" } in the dev-middleware config allows it to handle the request, but this won't work for webpack configurations with multiple entry points.

[grrowl]

It's also worth mentioning in the readme that lazy compilation is incompatible with Hot Module Replacement (although in hindsight, it should have been moderately obvious)

[hswolff]

Ran into same issue from the dev server: webpack/webpack-dev-server#103

[SpaceK33z]

Closing this very old issue, since it appears to have been fixed a while ago. filename is now a documented option, and this middleware also works if you don't define it.