for swizec

navigator.connection.d.ts

// stick this somewhere in your local @types/** files

interface NetworkInformation extends EventTarget {
	readonly downlink: number;
	readonly downlinkMax?: number;
	readonly effectiveType: 'slow-2g' | '2g' | '3g' | '4g';
	readonly rtt: number;
	readonly saveData: boolean;
	readonly type: 'bluetooth' | 'cellular' | 'ethernet' | 'none' | 'wifi' | 'wimax' | 'other' | 'unknown';
	onchange: EventListener;
}

interface Navigator {
	connection: NetworkInformation;
}

It's not included in DOM types by default because it's technically still experimental. Even Chrome 80, for example, doesn't have the downlinkMax property, despite MDN (and other places) reporting that it does.