how can I declare a global variable in typescript

in global.ts

export namespace Global {
    export var test: string = 'Hello World!';
}

in your.ts

import { Global } from "./global";
console.log(Global.test)

I try to do that I get the error property ‘test’ does not exist on type ‘Global’.

Create a file globals.d.ts with

interface Global {
 test: string;
}

More

Declaration files : https://basarat.gitbook.io/typescript/docs/types/ambient/d.ts.html