The problem

I'm trying to use mathjs inside node, using ES modules, but without a package bundler (e.g. Webpack, Rollup).

When I do:

import { evaluate } from 'mathjs';

console.log(evaluate('1+2'));

It throws:

import { evaluate } from 'mathjs';
         ^^^^^^^^
SyntaxError: The requested module 'mathjs' is expected to be of type CommonJS, which does not support named exports. CommonJS modules can be imported by importing the default export.
For example:
import pkg from 'mathjs';
const { evaluate } = pkg;

From what I can see, the module entry in package.json is only used for package bundlers. Without bundlers, node will look at the main entry, so it tries to load the CommonJS / UMD script.

Node does support mixing ES and CommonJS modules, but expects a single, default export.
Changing the import to this works, because it correctly loads the CommonJS module:

import mathjs from 'mathjs';

console.log(mathjs.evaluate('1+2'));

So that's a kind of workaround, but it does mean that it's not using ES modules in an ES module project.

But the bigger issue is that it's now incompatible if I choose to use to use a bundler because the bundler will use the ES module version, which has no default import.
In particular, this causes my Jest tests to fail. In my test I have something like:

TypeError: Cannot read property 'evaluate' of undefined

I think that conditional exports may be a solution. It seems to allow you to tell node which files to use for CommonJS require and which to use for ES import.

Potential solution

I've not had a proper play with it, this seemed to work for me:

Change the package.json to this, to tell node to use the esm files for import statements, and es5 files for require:

{
  "main": "./main/es5",
  "module": "./main/esm",
  "exports": {
    "import": "./main/esm/index.js",
    "require": "./main/es5/index.js"
  },
  // this is required, otherwise the `exports` doesn't work
  "type": "module"
}

But because the type is now module, node will now complain when trying to use CommonJS.
To sort that, you can either rename all of the CommonJS files to have the .cjs extension, or to add a package.json in the root directories containing the CommonJS files (e.g. main/es5/, lib/), with just the following:

{
  "type": "commonjs"
}

I'm not sure if doing this has any adverse affects on other parts of the library though, but I'm happy to get a PR in for those changes.

I thought it seemed similar to #1766. Not quite the same issue, but possibly the same resolution, perhaps used with Subpath Exports.