Why convert ES modules to CommonJS?

Question

JavaScript

There is a plugin for babel that converts ES modules to CommonJS. For now, it's gone under the hood @babel/preset-env to the modules option.

But I don't quite understand its purpose. Tell me, please, why in the final bundle - code already compiled by babel - change imports / exports to CommonJS?

From
export default 42;

To

Object.defineProperty(exports, "__esModule", {
  value: true,
});

exports.default = 42;

Answer 1

Well, because old browsers don't know how to use esmodules, and babel's original idea was to support old browsers - everything else piled up after.
Also, esm cannot be bundled into a single file, unlike CommonJS.
In any case, these are just defaults that you can always change.