Sometimes, you want to add a property to the window interface. TypeScript doesn't really document how to do this properly, and this situation is more likely to arise in Astro since we support TypeScript directly in script tags.

The solution is to add to env.d.ts (or any other .d.ts that's in the project scope) a Window interface, like such:

interface Window {
  helloChris: string;
}

The property will then appear in completions and by type-checked with no issues:

Side note

A similar situation happens for globalThis, but the way to add property to it is completely different, as you need to do the following:

export {}
declare global {
  function myFunction(): boolean;
  var myVariable: number;
}