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I maintain a library also, and the shift to ESM was incredibly painful, because you still have to ship CJS, only now you have work out how to write the code in a way that can be bundled either way, can be tested, etc etc.

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It was a pain, but rollup can export both if you write the source in esm. The part I find most annoying is exporting the typescript types. There's no tree-shaking for that!

For simple projects you needed now to add rollup or other build system that didn't have or need it before. For complex systems (with non-trivial exports), now you have a mess since it wouldn't work straight away.

Now with ESM if you write plain JS it works again. If you use Bun, it also works with TS straight away.

This is where I actually appreciated Deno's start with a clean break from npm, and later in pushing jsr. I'm mixed on how much of Node has come into Deno, however.

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