Type-safe custom globals

fn.ts

function maybeGlobal<T extends object>(): Partial<T> | undefined {
    return (typeof window !== "undefined" ? window : undefined) as any;
}

maybeGlobal<{
    MY_GLOBAL_VARIABLE: string;
}>()?.MY_GLOBAL_VARIABLE?.toUpperCase();

helper-type.ts

interface MyGlobals {
    MY_GLOBAL_VARIABLE?: string;
}

type MyWindow = (Window & MyGlobals) | undefined;

const myWindow: MyWindow = typeof window !== "undefined" ? window : undefined;

console.log(myWindow?.MY_GLOBAL_VARIABLE?.toUpperCase() + "!");