'Cannot redeclare block-scoped variable' in unrelated files

Question

There is a simple TS package that is used as CommonJS modules and has no exports. TS files are compiled to JS files with the same name and used as require('package/option-foo').

tsconfig.json:

{
  "compilerOptions": {
    "target": "es5"
  }
}

option-foo.ts:

declare const GlobalVar: any;

function baz() {}

if (GlobalVar.foo) GlobalVar.baz = baz;

option-bar.ts:

declare const GlobalVar: any;

function baz() {}

if (GlobalVar.bar) GlobalVar.baz = baz;

The important part here is that option-foo and option-bar are never used together. There are other complimentary TS files in the project, but they don't affect anything, just needed to be transpiled to JS in one tsc run.

When tsc runs, it throws

Cannot redeclare block-scoped variable 'GlobalVar'.

Duplicate function implementation.

Cannot redeclare block-scoped variable 'GlobalVar'.

Duplicate function implementation.

for GlobalVar and baz in both files.

How can this be treated without complicating build process or the output from these two TS files?

Answer 1

TL;DR Just write export {} in the outermost scope of your files.

---

At some point there needs to be a semantic disambiguation for whether a file should be treated as a module (and have its own scope) or a script (and share the global scope with other scripts).

In the browser, this is easy - you should be able to use a <script type="module"> tag and you'll be able to use modules.

But what about any other place that utilizes JavaScript? Unfortunately there isn't a standard way at this point to make that distinction.

The way that TypeScript decided to tackle the problem was to simply state that a module is any file which contains an import or export.

So if your file doesn't have any sort of top-level import or export statements, then you'll occasionally see issues with global declarations interfering with each other.

To get around this, you can simple have an export statement that exports nothing. In other words, just write

export {};

somewhere at the top-level of your file.

Answer 2

GlobalVar and the functions are exposed to the global namespace, and TypeScript is warning you that the variable and the method name are re-declared. Because the two function are declared in the global namespace, you only need to declare them one time.

If you want to do this, use namespaces.

namespace foo {    
    declare const GlobalVar: any;
    function baz() {}
}

namespace bar {
    declare const GlobalVar: any;
    function baz() {}
}

You can call the functions in the same way as you call them in C#, by using the namespace name: bar.baz or foo.baz.