When you have an ESM module like this:

// greetings.js
export const hello = 'hello';
export const hi = 'hi';

and you try to import it using default export:

import greatings from './greetings.js';

You will get The requested module './greetings.js' does not provide an export named 'default' error, which is expected.

But for some packages, like yup, I noticed you can import it like this:

import yup from 'yup';

if you install Yup and check index.esm.js, you will notice, it also doesn’t have any default export. so how we are able to import it using default?

If you want to say, Yup is shipped with a CommonJS module (index.js), I have to say yes, but some other packages like rotating-file-stream are also shipped with a CommonJS module, but we get the does not provide an export named 'default' error when we try to import it like using default import.

I trid these:

import yup from 'yup';

console.log(yup);

and

import rfs from 'rotating-file-stream';

console.log(rfs);

first one works, but second one doesn’t, why?!