declare global
 let myGlobal: number
Now we’re able to set global.myGlobal in the application without type errors.
For older versions of TypeScript
Getting right into it with a contrived example, given a global.d.ts declaring
a global variable, named myGlobal:
declare const myGlobal: number
namespace NodeJS {
  interface Global {
    myGlobal: number
And a file that defines the global via global.myGlobal = 100, now any script
that uses myGlobal via ts-node will fail with the following error:
TSError: ⨯ Unable to compile TypeScript:
test/mainTest.ts:3:17 - error TS2552: Cannot find name 'myGlobal'.
This is happening because ts-node isn’t looking for global.d.ts file. After
doing some research the first time I ran into this, I found two common
suggestions:
  
Pass the --files flag to ts-node so ts-node will compile all TypeScript
files defined by your tsconfigs files key. The large downside here being
that each time you use ts-node all of those files will be compiled
resulting in slower startup time.
  
Define typeRoots. Again, this works, but you no longer have automatic type
declaration detection.
Because of the downsides each solution came with, I searched for a better
approach and thankfully, found one. Well, two.
The first option was to use a triple slash
directive to associate the file that declares the global value with the declaration file.
/// <reference types="./global" />
global.myGlobal = 100
Note: Omit the .d.ts file extension.
This effectively says ‘this file depends on these types, when importing me
please import the types too’. Running npx tsc completed successfully, as did
our ts-node powered mocha tests.
The next solution seemed almost obvious after I found it. The global.d.ts file
can be imported.
import "./global.d"
global.myGlobal = 100
Now running tsc and our ts-node script, both complete successfully.
It took a bit of trial, error, and google, but I’m pretty happy with how simple
the solution ended up being.