Node.js global variable and TypeScript

Question

I need to have some strongly-typed global variables.

As mentioned here: Extending TypeScript Global object in node.js, in order to add fields to the global variable I need to add a .d.ts file that extends the Global interface that's specified in node.d.ts.

Also, as Basarat mentioned:

Your file needs to be clean of any root level import or exports. That would turn the file into a module and disconnect it from the global type declaration namespace.

Now, I need to have fields on the Global interface whose types are custom interfaces that I created:

declare namespace NodeJS{
    interface Global {
        foo: Foo
        bar: Bar
    }
}

I'm extremely not willing to use the any type.

I can move/copy all the interface declarations to this declaration file, but it's a bad solution for me, since both Foo and Bar in turn, aggregate many fields of other interfaces, including third party interfaces like Moment etc.

I need a solution for this paradox

Answer 1

Here's an approach. I don't know if this is the 'correct' way of doing things, but it works for me with TypeScript 3.7.4.

1. Assuming your source files live in a folder src, create a new folder src/types and create a file global.d.ts in this folder.
2. Author your declarations using one of the following strategies:
   • If you need to import external types into your declaration file, use the following syntax:

import { Express } from 'express';

declare global {
  namespace NodeJS {
    interface Global {
      __EXPRESS_APP__: Express;
    }
  }
}

• If your declaration file does not contain any imports, the above will not work, and you'll need to use this syntax instead:

declare namespace NodeJS {
  interface Global {
    __CONNECTION_COUNT__: number;
  }
}

4. Make sure your global.d.ts file (and any other files you might add to src/types) is picked up by the TypeScript compiler, by adding the following to your tsconfig.json file:

{
  "paths": {
    "*": ["node_modules/*", "src/types/*"]
  }
}

5. Use the global variable as normal inside your code.

// Below, `app` will have the correct typings
const app = global.__EXPRESS_APP__;