Create types for the configuration in next.config.js (and the NextConfig type)

[eweilow]

Feature request

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

Since I transpile a next.config.ts to next.config.js, it would be good to have typings for the configuration object. This is what is currently typed as any in https://github.com/zeit/next.js/blob/fce7e4eaee45e89557ceaa81fbd42057f8596e22/packages/next-server/server/next-server.ts#L37

This is also a problem when using the programmatic API, since the conf property on the options given to next is typed as any:

import next from "next";
const app = next({
  conf // any
});

Describe the solution you'd like

The previous types provided by @types/next and @types/next-server can be seen here:
https://github.com/DefinitelyTyped/DefinitelyTyped/blob/master/types/next-server/next-config.d.ts

Perhaps the easiest solution is to type NextConfig with something similar to the version that @types/next-server is based on:
https://github.com/zeit/next.js/blob/7.0.0/server/config.js#L9-L20

Describe alternatives you've considered

The option that I see is to maintain types for the configuration object in parallel to this repository, or simply use the configuration untyped.

[timneutkens]

We're currently not planning on allowing next.config.ts, and I wouldn't recommend adding an intermediate build step for just one config file.

[evgenymarkov]

We're currently not planning on allowing next.config.ts, and I wouldn't recommend adding an intermediate build step for just one config file.

Types can be used via JSDoc, it'll help developers to annotate their next.config.js files.

[zenVentzi]

Any movement on this?

[tylerbrostrom]

Types can be used via JSDoc, it'll help developers to annotate their next.config.js files.

Snowpack’s official TypeScript templates use @type annotations in this way. Makes for great DX (and could be made better if they were to add annotations).

Implementing a NextConfig type seems like it might be a pain in the butt, especially if it’s to be integrated into the codebase (as opposed to a third-party Definitely Typed package). Might be more of a maintenance burden than it’s worth?

@timneutkens would you be open to a PR integrating a NextConfig type? I figured you might have a better idea of how feasible/worthwhile this might be 🙂 Thanks!

[timneutkens]

@timneutkens would you be open to a PR integrating a NextConfig type? I figured you might have a better idea of how feasible/worthwhile this might be 🙂 Thanks!

Sure there's already multiple PRs to add such a type but they have not been merged yet given they were incomplete/did not have tests. If you open a new PR we'd be happy to take a look.

[balazsorban44]

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