You can't extend Window with a type alias

I wanted to extend the Window type in TypeScript and read the new property in my React app. TypeScript kept saying it wasn't defined, even though I had declared it in global.d.ts.

Because of the @typescript-eslint/consistent-type-definitions rule, I was using a type alias:

type Window = {
	dataLayer: Record<string, unknown>;
};

A type alias doesn't support augmentation. It only works when you declare every property in one place, so a second declaration with the same name is an error, not a merge. An interface does support augmentation, which is what extending the Window interface needs:

interface Window {
	dataLayer: Record<string, unknown>;
}

One thing to watch: if global.d.ts has a top-level import or export, TypeScript treats it as a module and the interface no longer touches the global scope. Wrap the block in declare global in that case.

In general, I prefer type for app code since it avoids hidden extensions. But if you're extending the Window object or any other built-in or third-party type, interface is the way to go.

For the full rules, see declaration merging on the TypeScript website.