Skip to main content

Dynamic code evaluation is not available in Middleware

Why This Error Occurred​

eval(), new Function() or compiling WASM binaries dynamically is not allowed in Middleware. Specifically, the following APIs are not supported:

  • eval()
  • new Function()
  • WebAssembly.compile
  • WebAssembly.instantiate with a buffer parameter

Possible Ways to Fix It​

You can bundle your WASM binaries using import:

import { NextResponse } from 'next/server'
import squareWasm from './square.wasm?module'

export default async function middleware() {
  const m = await WebAssembly.instantiate(squareWasm)
  const answer = m.exports.square(9)

  const response = NextResponse.next()
  response.headers.set('x-square', answer.toString())
  return response
}

In rare cases, your code could contain (or import) some dynamic code evaluation statements which can not be reached at runtime and which can not be removed by tree-shaking. You can relax the check to allow specific files with your Middleware configuration:

export const config = {
  unstable_allowDynamic: [
    '/lib/utilities.js', // allows a single file
    '/node_modules/function-bind/**', // use a glob to allow anything in the function-bind 3rd party module
  ],
}

unstable_allowDynamic is a glob, or an array of globs, ignoring dynamic code evaluation for specific files. The globs are relative to your application root folder.

Be warned that if these statements are executed on the Edge, they will throw and cause a runtime error.