Webpack config.externals is undefined in Next 8.1.0?

Question about Next.js

I’m not sure if this qualifies as a “bug” per se, as we’re engaging with some wild chicanery in our next.config.js, but there was a change in next@8.1.0 that broke our current configuration.

next.config.js (simplified)

module.exports = withTypescript({
    webpack: (config, options) => {
        const updatedConfig = withSass({
            // SASS options here
        }).webpack(config, options);

        updatedConfig.resolve.symlinks = false;
        updatedConfig.externals = config.externals.map(external => {
            // Some custom dependency resolution here
        });

        // Returns the customized Next.js configuration
        return updatedConfig;
    }
});

This causes no problems on next@8.0.4 - however, after updating to 8.1.0 I started seeing the following error:

(node:45880) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'map' of undefined
    at Object.webpack (/Users/aeksco/project/web/next.config.js:43:52)
    at Object.webpack (/Users/aeksco/project/web/node_modules/@zeit/next-typescript/index.js:57:27)
    at Object.getBaseWebpackConfig [as default] (/Users/aeksco/project/web/node_modules/next/dist/build/webpack-config.js:308:32)
    at HotReloader.getWebpackConfig (/Users/aeksco/project/web/node_modules/next/dist/server/hot-reloader.js:162:37)

It appears that the Webpack configuration object passed into the webpack function does not include an externals array, which causes our customized configuration to explode when we try to run config.externals.map(...)

I was curious if the missing config.externals array is a bug, or if we’re doing something that’s so completely non-standard that the problem is really with our approach.

Any help or insight is appreciated - thanks!

[timneutkens]

Note that you should actually make the value undefined if you’re not adding anything to it.

[aeksco]

@timneutkens Thanks for the reply and the context! I’ve added the following line to my next.config.js file which solves the problem:

config.externals = config.externals || [];

Glad to know this is actually a solution to the problem and not just a kludgey band-aid. Thanks again!