Closure-compiler: in.js:3: ERROR - Duplicate let / const / class declaration in the same scope is not allowed.

[danykey]

Hi,

I have an error while compile a js file with ES6 code:

java -jar closure-compiler-v20170409.jar --compilation_level ADVANCED_OPTIMIZATIONS --js_output_file=in.js out.js

```
in.js:3: ERROR - Duplicate let / const / class declaration in the same scope is not allowed.
class Storage {
^^^^^^^

1 error(s), 0 warning(s)

There is conflict name `Storage` with something as I guess.
If I rename, for example, to `_Storage` then compilation ends successfully.

```javascript
"use strict";

class Storage {

}

[ChadKillingsworth]

It's conflicting with the Storage interface. Either wrap your class in an IIFE or rename it.

[alcnsahin]

Hi, is there any solution?

[prateekbh]

I also just found out the same problem with class Plugin. IIUC these aren't reserved words by the language, why from CC then?

Also is there a list of what else is reserved?

[brad4d]

If I do console.log(Storage) in my chrome browser debug console I get
f Storage() { [native code] }

In other words there is already a global variable named Storage, so it isn't safe to define your own class with that name.

Look at the files under https://github.com/google/closure-compiler/tree/master/externs for other names that are already defined.

[brad4d]

I meant to say:
"It isn't safe to define your own class named Storage in the global scope."

[prateekbh]

The problem here is that while using
`import {Plugin} from ./module-x

there is a clear indication that

• a. This is not on global
• b. I totally want it mangled

So I there should be safe way to do this in modules "atleast"

[brad4d]

The original example doesn't have an import or export statement in it, so closure-compiler doesn't consider that to be a module.
If you get the same error in a file that does contain either an import or export, I suggest you file a new bug with that example.

[workflo]

Hi @brad4d,

The original example doesn't have an import or export statement in it, so closure-compiler doesn't consider that to be a module.

You pretty much saved my life here :)
But, is there a way to make CC handle all my files as if they were _modules_?

Thanks in advance,
Flo.

[mprobst]

You can put an empty export {}; into a file to mark it as a module.

[samthegitguy]

Or you could put "type": "module" in your package.json for marking all your files under that package.json as es6 modules I think. It usually works with node but i haven't tested it with closure compiler