Use module as singleton

Question

In javascript we can use module as singleton.

//a.js
export default {str:'aaa'}

//b.js
import singleton from 'a.js'
singleton.str='bbb'

//c.js
import singleton from 'a.js'
singleton.str='ccc'

I want to use it to be the configure of my module. Is there any risk? It will always be a singleton in every module system?

Answer 1

I want to use it to be the configure of my module. Is there any risk? It will always be a singleton in every module system?

Yes, an ESM module (and a commonjs module) is guaranteed to return the same instance. **

While this is bad design (because it's mutable global state anyone can import and change and it's hard to keep track of) - it's entirely guaranteed to always work.

If you're interested in the details of how Node.js loads modules - you can see the reference of the resolver algorithm in our docs and the relevant bits for CommonJS here.

---

This is safe to rely on. I'll try to explain why. Imagine modules didn't work this way:

//a.js
export default class Foo { /* some code */ }

//b.js
import Foo from './a';

If whenever b and other files import the class Foo they might get a different instance - stuff like instanceof wouldn't work across modules and it would be impossible to rely on references.

This is generally why this is both guaranteed and supported.

As a fun side node - ES Modules are live references :]

_{**the caveat is that it's possible to "opt out" - for example by deleting the module from require.cache and setting require.parent in commonjs. However, this is never the default behavior.}