Global types in typescript

Yes this is possible. You can find all information here: https://www.typescriptlang.org/docs/handbook/declaration-files/templates/global-modifying-module-d-ts.html

The important part is this:

declare global {
    /*~ Here, declare things that go in the global namespace, or augment
     *~ existing declarations in the global namespace
     */
    interface String {
        fancyFormat(opts: StringFormatOptions): string;
    }
}

I found the accepted answer is not working (maybe it is some configuration that needs to be done?). So with some tinkering, I got it to work for me (maybe I also have some weird configuration option? Let me know if it doesn't work and I will remove this answer).

1. Create a definition file in a suitable folder. I will be using types/global.d.ts
2. Check your tsconfig.json and include "*": ["types/*.d.ts"] under paths. (You should also be able to directly address the created file if you care to).
3. Put your global definitions directly into the root of the file NO declare global or similar.

Now you should be good to go to use the types declared in this file (tested with typescript 3.9.6 and 3.7.5).

Example file:

// global.d.ts
declare interface Foo {
    bar: string;
    fooBar: string;
}

What your tsconfig should look like:

[...]
"paths": {
    "*": ["types/*.d.ts"]
},
[...]