Dynamic Code Evaluation prevents Flagsmith to run on Next.js middlewares

[cagrispan]

Hi,

I'm trying to run flagsmith on a Next.js middleware to load different routes depending on flags as below:

import flagsmith from "flagsmith/isomorphic";
import { NextRequest, NextResponse } from "next/server";

flagsmith.init({
  environmentID: {{ENVIRONMENT_KEY}},
});

export default async function middleware(request: NextRequest) {
  const url = request.nextUrl.clone();

  if (flagsmith.hasFeature(url.pathname)) {
    const UUID = crypto.randomUUID();

    await flagsmith.identify(UUID);
    url.pathname = flagsmith.getValue(url.pathname) as string;

    const response = NextResponse.rewrite(url);

    return response;
  }
}

It runs locally, but fails on build with the error:

./node_modules/flagsmith/index.js
Dynamic Code Evaluation (e. g. 'eval', 'new Function') not allowed in Middleware pages/_middleware

Import trace for requested module:
./pages/_middleware.ts

I would like to know if there is a plan to support Next.js Middlewares in the near future.

For reference:

Edge Runtime Unsupported APIs

---

UPDATE

The Function calls are added when AsyncStorage is used. If you don't pass it as a parameter and build the project, it will not include any dynamic code evaluation on the index files.

// index.ts - line 6
const flagsmith = core({ fetch });

This is not a solution, just some progress on the investigation.

[alizain]

@dabeeeenster all of the flagsmith code is obfuscated and compiled. you don't provide unobfuscated source. you've added the help-wanted label, but what's the best way for us to help?

[dabeeeenster]

Hi @alizain - i don't follow - the source is in this repo?

[alizain]

my bad, i couldn't find the correct file :)

[kyle-ssg]

Hey @cagrispan, this wasn't on my radar but I'd like to help with this - am a massive fan of Next.js. It might actually be worth trying with the latest version >3, calls to async storage now check for typeof window, e.g https://github.com/Flagsmith/flagsmith-js-client/blob/main/flagsmith-core.ts#L270

[kyle-ssg]

This is now available in flagsmith 3.3.2, there's a working example here

[cagrispan]

Nice @kyle-ssg! Thank you!

[kyle-ssg]

No problem! I'd love to hear about what you do with it, had a play around with it today, It looks like the main use cases would be redirecting for abn tests maybe

[cagrispan]

Precisely. Since it was not supported then, we are calling Flagsmith API to retrieve the flags and route based on that. And you are right, it's for ab tests.