=== tests/cases/compiler/extension.d.ts ===
declare global {
>global : typeof global

    namespace globalThis  {
>globalThis : typeof globalThis

        var test: string;
>test : string
    }
}

export {}

=== tests/cases/compiler/index.ts ===
import "./extention";

globalThis.tests = "a-b";
>globalThis.tests = "a-b" : "a-b"
>globalThis.tests : any
>globalThis : typeof globalThis
>tests : any
>"a-b" : "a-b"

console.log(globalThis.test.split("-"));
>console.log(globalThis.test.split("-")) : void
>console.log : (...data: any[]) => void
>console : Console
>log : (...data: any[]) => void
>globalThis.test.split("-") : string[]
>globalThis.test.split : (separator: string | RegExp, limit?: number) => string[]
>globalThis.test : string
>globalThis : typeof globalThis
>test : string
>split : (separator: string | RegExp, limit?: number) => string[]
>"-" : "-"