cloneNode should return sub type, not Node

When I do someElement.cloneNode(), I want the returned element to be the same type as someElement, but right now it’s returning type Node.

[DanielRosenwasser]

I assume that you have something like the following code

var d: Document;
var x = d.cloneNode(); // 'x' is of type 'Node'

but you wanted the resulting type of d.cloneNode() to be Document as well.

This is something that cannot “automatically” be expressed by our type system (nor can it be in type systems such as Java, C#, but has been “solved” in certain other languages).

One thing we could do is make cloneNode generic so that you could do:

elem.cloneNode<Document>();

and have that return a Document - but you’re not gaining much over using a type assertion.

Something that allows you to get exactly what you want would be to define your own helper function so that you generally won’t have to supply the generic parameter since it can be inferred.

function cloneNode<T extends Node>(node: T) {
    return <T>node.cloneNode();
}

var d: Document;
var x = cloneNode(d); // 'x' is of type 'Document'

[sandersn]

Unfortunately, we had to revert microsoft/TSJS-lib-generator#811 because it makes type parameters that extend HTMLElement invariant where they were previously covariant. See https://github.com/microsoft/TSJS-lib-generator/pull/842

@types/tablesorter is a good test case for future attempts; clone microsoft/DefinitelyTyped and then run tsc in types/tablesorter.