esm and cjs config file support

use prettier.config.cjs and prettier.config.mjs for config file.

[SMotaal]

Glad to see .cjs and .mjs out here.

This comes from Node.js finally unflagging ESM (ie --experimental-modules).

The Node.js Modules team took time to explore matters of source disambiguation where unlike browsers that only ever deal with text/javascript files based on clear intent of loading, tooling and Node.js itself cannot always assume the format of .js files.

Package authors can indicate that .js is either ESM of CommonJS using the package.type field — and so to close the gap for tools expecting CommonJS configuration in a package.type = "module" (ie ESM) the .cjs extension may be necessary when relying on require(configuration) in their implementation.

The reverse can be accomplished with .mjs.

Worth noting that the limitation only applies to require — where it cannot at least for now seamlessly load from ESM — but CommonJS and ESM code can always (await import('…/configuration.cjs')).default to return the module.exports for that file.

Hope this helps.

[akauppi]

Situation in Dec 2020 (prettier 2.2.1):

One can use a .cjs configuration file in package.type = "module" project. Plain .js does not work which is pretty common still these days.

Beware of this: (could deserve to be a separate issue, but no time, sorry…)

If there is a syntax error in the JavaScript configuration file (prettier.config.cjs), prettier seems to ignore the file and try looking for something else. My error message was:

$ npm run _format.js
...
> prettier --config ./prettier.config.cjs --write ./src/**/*.js ./app/**/*.js

[error] Invalid configuration file `src/Draggable/draggableX.js`: Unexpected identifier
npm ERR! code 2
npm ERR! path /Users/.../Git/svelte-with-SVG
npm ERR! command failed
npm ERR! command sh -c prettier --config ./prettier.config.cjs --write ./src/**/*.js ./app/**/*.js

npm ERR! A complete log of this run can be found in:
npm ERR!     /Users/.../.npm/_logs/2020-12-16T10_14_00_410Z-debug.log

I would expect to rather get an error stating that the config file prettier.config.cjs is bad.