NamedNodeMap/getNamedItem is not defined to be potentially "null"

[DorianGrey]

TypeScript Version: 2.4.2

I've recently faced in issue accessing the NamedNodeMap of a Node - a user managed to insert an img tag without a src attribute. My code accessed attributes.getNamedItem("src") on the node, and (accidentially) did not check if the resulting attr exists before accessing its .value, and the function crashed with a type error.
While not performing this check is my personal mistake, I've figured out that getNamedItem is not defined to possibly return null: https://github.com/Microsoft/TypeScript/blob/master/lib/lib.es6.d.ts#L14075

However, this function will return null in case the attributes does not exist:
https://developer.mozilla.org/en-US/docs/Web/API/NamedNodeMap/getNamedItem
This should hold true for getNamedItemNS as well.

Thus, as I've strict resp. strictNullChecks enabled in in my tsconfig.json, the compiler should raise in error when not checking the result for null (see code below).

Code

const node: Node = // ... an <img> node
const src: Attr = this.attributes.getNamedItem("src"); // should complain that src should be of type Attr | null
const valueLength = src.value.length; // should complain that src might be null.

Expected behavior:
The code mentioned above compiles without an error that src might be null, even with the config strict resp. strictNullChecks enabled. It does because the definition mentions that the result of getNamedItem is always an Attr.

Actual behavior:
The code mentioned above should raise a compiler error in case strict resp. strictNullChecks is enabled. The definition should mention that the result of getNamedItem is of type Attr | null.

[mhegazy]

PRs welcomed. You can find more information about contributing lib.d.ts fixes at https://github.com/Microsoft/TypeScript/blob/master/CONTRIBUTING.md#contributing-libdts-fixes.

[dsifford]

@mhegazy I went in to try and fix this issue now and I'm having trouble understanding where exactly you want these fixes to take place?

Reading the contributing guide, it appears that all the DOM-related types come from https://github.com/Microsoft/TSJS-lib-generator, however, when I went into that repo to see where it was I could make the fix for this, the only location where there is any mention of NamedNodeMap is also in auto-generated files.

Any help pointing me to the correct file to edit would be greatly appreciated.

Thanks in advance.

[mhegazy]

The change should go into https://github.com/Microsoft/TSJS-lib-generator/blob/master/inputfiles/overridingTypes.json, see https://github.com/Microsoft/TSJS-lib-generator#contribution-guidelines for more details.

[NN---]

Is it possible to tell TypeScript that null check is not required in this code ?

for (let i = 0; i < nodeMap.length; ++i) {
   const node = nodeMap.item(i);
   // node is never null since i is less than nodeMap.length , but check is required.

[RyanCavanaugh]

@NN--- const node = nodeMap.item(i)!;