[ReadonlyArray] .at() fix undefined issue

[DeepDoge]

This will return undefined regardless if strictNullChecks is true or false.

But this is already the behaviour of native ReadonlyArray .at().

As seen here:

interface Array<T> {
    /**
     * Returns the item located at the specified index.
     * @param index The zero-based index of the desired code unit. A negative index will count back from the last item.
     */
    at(index: number): T | undefined;
}

interface ReadonlyArray<T> {
    /**
     * Returns the item located at the specified index.
     * @param index The zero-based index of the desired code unit. A negative index will count back from the last item.
     */
    at(index: number): T | undefined;
}

Also fixed an issue with index union types, and added tests for it.

Issue was being caused by how Subtract util type works with unions types:

type Foo = Subtract<10, 1 | 2> 
// Foo should give `9 | 8` but gives `9` only

Also handled the cases where index number is a "float" and not an "int" correctly, based on how .at() handles them

[Sheraff]

Nice job, that's clean. Not sure if Matt will want this change, when mine was (slightly) smaller and already too much overhead! But at least yours is complete! And it looks neat too.

[Sheraff]

Wouldn't this be simpler with a forced type like this?

Suggested change

	function index() {
	return Math.random() > 0.5 ? 0 : 1;
	}
	const a = arr.at(index());
	const index = 1 as 1 | 2;
	const a = arr.at(index);

Or is it done like this for "expressivity" of the tests (like saying "sometimes we don't know the type in advance" but with code)?

[Sheraff]

Same comment as above.

[DeepDoge]

sure