feat(next): next.config.ts

[devjiwonchoi]

Note: port of #57656

What?

This PR initiated support of next.config.ts.

Follow ups:

• update docs (x-ref)
• update cna ts templates (x-ref)

Why?

Next.js provides a TypeScript-first development experience for building your React application.

This PR can resolve the two main needs:

1. Import TS files to the config.
2. Conduct a type check directly by importing type instead of JSDoc.

x-ref: #5318 #35969 #44632 #50121 #50126

How?

Goals

• do not bundle
• minimum fs operations
• zero restrictions

Used require.extensions hooks to transpile the imported files on the fly.

To reduce I/O operation on disk, used custom module.exports to require directly from a transpiled code string.

Expected

Added tests for the cases below:

• export default
• export config as default
• config as an async function
• use Node.js modules (fs, path, etc.)
• HMR on config change
• throw when type error
• turbopack

Importing Files

• nested imports (config -> A -> B)
• import alias (tsconfig baseUrl and paths)
• import from node_modules
• import JSON

Extensions

• .ts
• .cts
• .mts
• .cjs
• .mjs
• .js (both ESM and CJS w/o extra change)

[ijjk]

Amazing work, thank you!

[wojtekmaj]

If .mjs is supported, then why not .mts too?

[devjiwonchoi]

You can handle native ESM in CJS project with next.config.ts, no need for next.config.mts, it should work.

Extensions

• .ts
• .cts
• .mts
• .cjs
• .mjs
• .js (both ESM and CJS w/o extra change)