How to solve this basic ES6-module circular dependency problem?

[/#!/JoePea]

> Every time you access `foo` in the second module, you are literally
reaching into the first module to grab the current value of `foo`.

Exactly! So, that means that in theory, I should be able to remove the `C`
parameters from my `setUpA` and `setUpB` functions in my [last example](
https://esdiscuss.org/topic/how-to-solve-this-basic-es6-module-circular-dependency-problem#content-23),
so it becomes the following (but it doesn't work in some Babel environments
(Meteor, Webpack) or Rollup).

```js
// --- Entrypoint
import A from './A'
console.log('Entrypoint', new A)
```

```js
// --- Module A

import C from './C'
import {setUpB} from './B'

let A

export
function setUpA() {

    if (!A) {
        A = class A extends C {
            // ...
        }
    }

}

if (setUpA && C) setUpA()
if (setUpB && C) setUpB()

export {A as default}
```

```js
// --- Module B

import C from './C'
import {setUpA} from './A'

let B

export
function setUpB() {

    if (!B) {
        B = class B extends C {
            // ...
        }
    }

}

if (setUpA && C) setUpA()
if (setUpB && C) setUpB()

export {B as default}
```

```js
// --- Module C

import A, {setUpA} from './A'
import B, {setUpB} from './B'

class C {
    constructor() {
        // this may run later, after all three modules are evaluated, or
        // possibly never.
        console.log(A)
        console.log(B)
    }
}

if (setUpA && C) setUpA()
if (setUpB && C) setUpB()

export {C as default}
```

I found that with Ben Newman's deferred imports idea, the problem literally
goes away:

```js
// --- Entrypoint
import A from './A'
console.log('Entrypoint', new A)
```

```js
// --- Module A

import C from './C'

class A extends C {
    // ...
}

export {A as default}
```

```js
// --- Module B

import C from './C'

class B extends C {
    // ...
}

export {B as default}
```

```js
// --- Module C

class C {
    constructor() {
        // this may run later, after all three modules are evaluated, or
        // possibly never.

        // reach into the A and B modules
        import A from './A'
        import B from './B'

        console.log(A)
        console.log(B)
    }
}

export {C as default}
```

*/#!/*JoePea
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