IIFE transformed modules are incompatible es module babel plugins

[gabemeola]

IIFE transformed modules are incompatible with babel plugins which transform es modules.

Sort of a mouthful but I'll try to explain what's happening below.

Consider this CJS module

this.name = 'true';

Running this through transform-commonjs wraps it in a IIFE to preserve the this module context.

var module = {
  exports: {}
};
var exports = module.exports;
(function () {
  this.name = 'true';
}).call(module.exports);
export default module.exports;

If we use another ESM babel transform (such as transform-modules-systemjs) it causes nested duplication of code

System.register([], function (_export2, _context) {
  "use strict";

  return {
    setters: [],
    execute: function () {
      System.register([], function (_export, _context) {
        "use strict";

        var module, exports;
        return {
          setters: [],
          execute: function () {
            module = {
              exports: {}
            };
            exports = module.exports;
            (function () {
              this.name = 'true';
            }).call(module.exports);

            _export("default", module.exports);
          }
        };
      });
    }
  };
});

BABEL REPL

---

The issue seems to be coming from this line in the ThisExpression branch: https://github.com/tbranyen/babel-plugin-transform-commonjs/blob/6b3ac9bf0870a995b67d3d93f60735ce798f3950/lib/index.ts#L62

I'm not too familiar with babel's helper functions. What's your thoughts on this? Is there another method which will prevent duplication of the program?

[tbranyen]

Is the nested duplication breaking anything, or does it just look inefficient?

[gabemeola]

For SystemJS in this case, it breaks dependency loading since the System.register call is duplicated.

[shrinktofit]

I use multi-pass to solve this problem. babel.transformFile + babel.transformAst.

[gabemeola]

Replacing top level this exports with the exports keyword worked for me. https://github.com/tbranyen/babel-plugin-transform-commonjs/pull/16/files#diff-13b5b151431c7e7a17f273559ed212d5R316

Although this implementation may not cover all use cases and doesn't handle early returns