Node.js ESM support

Unfortunately, Node.js didn’t add support for "module" field that is supported by bundlers such as webpack, and introduced their own resolve logic.

To add Node.js ESM support instead of /esm submodule we need to add .mjs version next to CommonJS files. That would be a breaking change so we should introduce it in v3.

Thank you Node.js, very nice.

See the issue: https://github.com/date-fns/date-fns/issues/1779

[karlhorky]

@kossnocorp maybe it would be an idea to move to pure ESM for v3, like Sindre Sorhus has been doing with his modules:

https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c

[marnusw]

@kossnocorp it is possible to support native Node.js ESM without breaking changes. I’ve just done so successfully in date-fns-tz. There are a few things to do:

1. Change all imports to use the complete file names including the extension, e.g. ./format to ./format/index.js
2. Add "type": "commonjs" to the main package.json
3. Add a package.json inside the ./esm folder containing {"type": "module"}
4. Change the build script so it adds "type": "module" to the package.json of each function in the esm folder
5. Add the exports field to the main package.json similar to what I’ve done in date-fns-tz#package.json.

The last will be a proper mess because there are a gazillion files being exported from date-fns, but it should work…