How to use imports when using export default new ClassName();

Question

For example:

File 1:

const env = process.env.VARIABLE;
export const config: any = config[env.toLowerCase()];

File 2:

import { config } from '../file1';
class SomeClass {
  constructor() {
    console.log(config.someProperty);
  }
}
export default new SomeClass();

This doesn't seem to work. config.someProperty is undefined. Any reason why?

Edit per request:

import { config } from '../file1';
export class SomeClass {
  private myConfig = config.someProperty;
  constructor() {
    console.log(myConfig);
  }
}

Because `config` is a string. Not sure what properties you expect it to have? — Bergi, Nov 02 '20 at 16:23
While your problem is actually unrelated to the `export default new ClassName()`, notice [one should not use that](https://stackoverflow.com/q/39076190/1048572). — Bergi, Nov 02 '20 at 16:25
Wait, no, `env` is a string (weird name choice), and `config` is coming from a circular definition that throws an exception when evaluating file1. — Bergi, Nov 02 '20 at 16:29
So circular reference, ok. I am using the default export new pattern because I want it to be a singleton. I was told that using the pattern of having a private instance, and a static method getInstance() would not guarantee a singleton. — cyberguy, Nov 02 '20 at 16:39
Yes, `const config = config[env.toLowerCase()];` can't work, on its own already, regardless of any exports/imports. It's in the temporal dead zone. What did you attempt to do there, did you mean to refer to something else? — Bergi, Nov 02 '20 at 16:45
Neither does `export default new ClassName` guarantee a single instance of `ClassName`. — Bergi, Nov 02 '20 at 16:46
`const config = config[...]` does work though (I am able to import and use it in other files). Just not in any classes that are using `export default new`. Do you happen to have any resources other than the stack overflow answer above that describes how to implement a singleton pattern in node? — cyberguy, Nov 02 '20 at 17:02
No, it certainly doesn't. Not in any standard-compliant implementation of JavaScript. What do you think the `config` on the right hand side refers to? What does your actual code look like that you say works? — Bergi, Nov 02 '20 at 17:11
Regarding your singleton, just don't use a `class` and you'll be good. See the linked answer. Btw, the problem with the `config` is not the export, it's the immediate usage. Try a `console.log(config.someProperty)` in the top-level scope and it won't work either. — Bergi, Nov 02 '20 at 17:14
I updated the question with an example of how it is working in my app. I'm using webpack if that makes any difference, but it certainly does work. — cyberguy, Nov 02 '20 at 17:29
I meant the actual code that declares and initialises `config`. (Yes, webpack might suppress the TDZ error, but it should still throw an exception when accessing `config[env.toLowerCase()]` on `undefined`) — Bergi, Nov 02 '20 at 17:33

score 0 · Accepted Answer · answered Nov 02 '20 at 19:11

The answer is to ensure there are no circular dependencies if your environment isn't equipped to handle them. For me, I had a circular dependency between the two classes (not shown in the question), which was resolved by moving the config setup to its own module.

How to use imports when using export default new ClassName();

1 Answers1