Fixing circular case

[guybedford]

Here is a code example of what is breaking with cycles:

a.js

exports.run = function () {
  console.log('parent');
}
require('./b.js');

b.js

require('./a.js').run();

The issue is that this gets converted into an ES module tree looking like:

a.js

import './b.js';
import 'commonjs-proxy:./b.js';

var fn = function () {
  console.log('parent');
}

var a = {
        fn: fn
};

export default a;
export { a as __moduleExports };
export { fn };

b.js

import './a.js';
import require$$0 from 'commonjs-proxy:./a.js';

require$$0.fn();

var b = {

};

export default b;
export { b as __moduleExports };

And so "fn" isn't defined as b.js executes before a.js entirely, no exports are permitted before the require statement.

Now one naive fix for this particular case would be to use a function binding in the output, as these hoist through circular references in ES modules. That would only fix exported functions though, so executed binding values would still throw. This may possibly be adequate though as most test cases are functions as classes, as it seems from most of the bug reports. So it could be an interesting first step to try, although would be invalidated by a single exports.x = 'evaluated value'; require('./circular') bug.