Based on TypeScript 4.5

Extending globalThis, global, window

Declaration:

declare var var_name: string

Use it:

globalThis.var_name
global.var_name
window.var_name
var_name

Extending process and process.env

Declaration:

declare namespace NodeJS {
  interface Process {
    some_key: string
  }
  interface ProcessEnv {
    DATABASE_URL: string
  }
}

Use it:

process.some_key
process.env.DATABASE_URL

Extending import.meta

Declaration:

declare interface ImportMeta {
  env: {
    DEV: boolean
  }
}

Use it:

import.meta.env.DEV