nit: can you add this as jsdoc and not an inline comment?

Sure. I didn't because it's a private interface.

Do we need threshold and thresholds? If we need a convenience of passing a single threshold, can we not just type it number | number[]?

My preference would be to go with thresholds and make it always number[]

Are these all lets? Does root need a default?

If thresholds is always an array, the let doesn't matter and I can allow root to be undefined in the details object.

We wouldn't need this thresholds, is typed as number | number[].

Could just do:

thresholds = Array.isArray(thresholds) ? thresholds : [ thresholds ];

Am I reading this wrong? is this using the threshold as an index?

No, this seems wrong.

I should have added a test for this to ensure the same set of options doesn't create additional observers.

I think we can simplify this if condition some what....

In terms of formatting or?

The logic is pretty complicated, i'd either break these logical conditions up so each one is done individually, like:

if (options !== detail.options) {
    continue;
}

Or perhaps even better, use a different mechanism for the options caching (like a map, using the serialised options as the key)? It would look something like this:

	private _details: Map<string, IntersectionDetail> = new Map();

	private _getDetails(options: IntersectionGetOptions): IntersectionDetail {
		const cacheKey = JSON.stringify(options);
		let cached = this._details.get(cacheKey);

		if (!cached) {
			const entries = new WeakMap<Element, IntersectionObserverEntry>();
			cached = { ...options, entries };
			this._details.set(cacheKey, cached);
		}
		return cached;
	}

Shows as uncovered in the test coverage

Yeah, I'm not sure why it's not getting called.

Do we have to cast like this?

Until TypeScript is updated to include this property.

rootMargin?: string; over rootMargin: string | undefined?