Breaking change in 2.1: Node.firstChild can now be null

[felixfbecker]

TypeScript Version: 2.1.4

This code snippet used to compile fine in 2.0 with strictNullChecks, but now errors in 2.1:

if (documentElement.hasChildNodes() && documentElement.firstChild.nodeName === 'error') {
}

I agree that firstChild can in fact be null (even though in this case I checked with hasChildNodes), but this is a breaking change introduced in a minor version.

Workaround: check with documentElement.firstChild && or add a ! in the cases where I know from the protocol that the element will have a child node.

Full file: https://github.com/felixfbecker/vscode-php-debug/blob/master/src/xdebugConnection.ts
Failing build: https://travis-ci.org/felixfbecker/vscode-php-debug/builds/182024998

[blakeembrey]

Not sure if this is a good workaround, but also:

declare interface Node {
    hasChildNodes(): this is this & { firstChild: Node, lastChild: Node }
}

[felixfbecker]

That would be an improvement, but it's still a breaking change, so please at least document it. I solved this already in xdebug/vscode-php-debug@0ccad83

[gcnew]

I dont see this as a breaking change - this is not the compiler at work, it is the DOM definitions. They get improved every day and sometimes new errors get caught because of the changes. Previous definitions were just incomplete and thats getting fixed :)

PS: If I had stumbled on firstChild's 2.0 type I'd have been rather baffled by its incompleteness

[mhegazy]

here is the original issue that this change fixed: #11113
The PR with the change: https://github.com/Microsoft/TSJS-lib-generator/pull/149/files

Now added to the breaking change section for this release: https://github.com/Microsoft/TypeScript/wiki/Breaking-Changes#changes-to-dom-apis-in-the-standard-library

[felixfbecker]

Thanks @mhegazy. I think it does not make sense to release a new major here because in a way it could be considered a bug in my code (even though as shown I am doing appropriate checks and am also working with parsing a well-defined protocol).