Copy polyfills source maps

Feature request

I would like to request a feature to copy source maps from polyfills to next build. Currently, the source maps are skipped but js file still contains a reference to it.

This request is very related to https://github.com/zeit/next.js/issues/7579 but I decided to report it separately since it probably requires a different bunch of work. Feel free to close if you find it duplicate.

Is your feature request related to a problem? Please describe.

We use source map loader in our app which allows us to include third-party source maps in our build. It is extremely helpful especially in conjunction with Sentry. It helps us track down bugs easily and eventually if it is not a bug on our side then report it to correct package maintainers.

I prepared MWE for you when you run the app and open chrome console you will see something like:

Can't resolve './unfetch.js.map' in '/mnt/c/Users/joozty/Documents/next-source-maps/node_modules/next/dist/build/polyfills'

it happens because unfetch.js file copied here contains a reference to a source map which is not copied. (It’s probably the same for other polyfills too.)

Describe the solution you’d like

I would like to have these source maps included. It’s not a big deal and this warning can be ignored but as I mentioned earlier it’s a nice feature for having bug free JS ecosystem. 😃

[Joozty]

For anybody affected by this issue. You may disable source map preloader for these specific modules/files like this:

    config.module.rules.push(
      {
        test: /.*\.js$/,
        // disable these modules because they generate a lot of warnings about
        // non existing source maps
        // we cannot filter these warnings via config.stats.warningsFilter
        // because Next.js doesn't allow it
        // https://github.com/vercel/next.js/pull/7550#issuecomment-512861158
        // https://github.com/vercel/next.js/issues/12861
        exclude: [
          /@next\/react-dev-overlay/,
          /@next\/react-refresh-utils\/runtime.js$/,
          /@next\/react-refresh-utils\/internal\/helpers.js$/,
          /next\/dist\/build\/polyfills\/unfetch.js$/,
          /next\/dist\/build\/polyfills\/unfetch.js$/,
        ],
        use: ['source-map-loader'],
        enforce: 'pre',
      }
    )

[maxigimenez]

@timneutkens I would love to take this one, one approach could be copying the .map file onto the build, what do you think?