Edge Runtime Incompatibility

[LilaRest]

Hello @ljharb!
First, thanks for all the hard work on your libraries.

Our NextJS builds are failing with the following error since our last dependencies upgrade:

Dynamic Code Evaluation (e. g. 'eval', 'new Function', 'WebAssembly.compile') not allowed in Edge Runtime 
Learn More: https://nextjs.org/docs/messages/edge-dynamic-code-evaluation
The error was caused by importing 'traverse/index.js' in 'REDACTED'

After hours of investigation, it led us to this repository and is-async-function. Those two are using Function('return async function () {}')(); which is considered dynamic code by the compiler.

We are not using these two libraries directly, but instead use traverse which indirectly relies on those two libraries. And it's likely that in the most recent version of traverse (or of one of its dependencies), is-async-function and is-generator-function have been introduced as dependencies, so breaking our builds.

Is there an easy way for you to remove these "dynamic code" parts to unlock edge runtime support? Or should we explore other solutions?

[ljharb]

If Edge Runtime doesn't support a standard JS feature like new Function, then it's definitely not a reasonable expectation that you'll be able to run JS code in it.

If you can configure custom rewrites in your bundler, you could replace Function('return async function () {}')() with async function () {}, and similarly for this package.

It's also possible that I could add an ESM entry point to these two packages that uses the syntax directly, and then perhaps next.js would automatically pick that up instead?

[LilaRest]

@ljharb That's a known limitation in NextJS middleware (which is compiled for edge runtime by default).

You can learn more about it here: https://nextjs.org/docs/messages/edge-dynamic-code-evaluation

[ljharb]

Seems like unstable_allowDynamic will allow it to bundle, but it'd still not work at runtime.

[LilaRest]

@ljharb Indeed, I tried that already, it silents the error and seems to work at runtime (for now).
So this issue is of low severity as we are not blocked by it anymore.

I'm trying the bundler approach (Webpack in our case), I'll let you know!

Thanks for the support here.

[ljharb]

One thing you could try is, duplicate index.js in this package to index.mjs, replace return Function('return async function () {}')(); with return async function () {};, change module.exports = to export default and add this JSON above the main field in package.json:

"exports": {
  ".": {
    "import": "./index.mjs",
    "default": "./index.js"
  },
  "./package.json": "./package.json"
},

and see if it automatically picks up the Function-less file?

[LilaRest]

So I managed to solve the issue with the following Webpack config in my next.config.mjs file.

  // ...
  webpack: (config) => {
    // 'traverse' depends on 'is-async-function' and 'is-generator-function,'
    // both of which use edge-runtime anti-patterns. The code below replaces them.
    config.module.rules.push({
      test: /is-async-function\/index\.js$/,
      loader: "string-replace-loader",
      options: {
        search: "Function('return async function () {}')()",
        replace: "async function () {}",
      },
    });
    config.module.rules.push({
      test: /is-generator-function\/index\.js$/,
      loader: "string-replace-loader",
      options: {
        search: "Function('return function*() {}')()",
        replace: "function*() {}",
      },
    });
    return config;
  },
  // ...

We're satisfied with this fix, bun I'm worried that many NextJS projects will struggle or stop using traverse (or other libs of your ecosystem) because of that issue. Considering that it was quite challenging to find the root source of it, because of the very imprecise NextJS error messages and the deep/wide hierarchy of the packages you're maintaining.

I don't have time right now to push a PR (already more than a day off on my weekly roadmap), but happy to at least help reproducing if you wanted to invest time fixing that for other users @ljharb.

Thanks again!