Type of parts of the AST in Typescript unclear

[lkj4]

After getting a chunk (an array of nodes, to be clear) of nodes with unist-util-find-all-between in an AST I try to convert that chunk back to Markdown with mdast-util-to-markdown but IDK how. I tried embedding that chunk into a...

{ 
  type: "root",
  children: chunk 
}

and got

[tsserver 2345] [E] Argument of type 'object' is not assignable to parameter of type 'Node'. 
Type '{}' is missing the following properties from type 'FootnoteReference': type, identifier

then I asserted that chunk as Node and got

[tsserver 2345] [E] Argument of type 'Node<Data>' is not assignable to parameter of type 'Node'. 
Property 'identifier' is missing in type 'Node<Data>' but required in type 'FootnoteReference'.

then I asserted it as Node<Data> with Data being...

interface Data {
  identifier: string
}

and got the same again...

[tsserver 2345] [E] Argument of type 'Node<Data>' is not assignable to parameter of type 'Node'.
Property 'identifier' is missing in type 'Node<Data>' but required in type 'FootnoteReference'.

So, how I can I deal with AST chunks with TS?

[lkj4]

Think I found the correct type for that chunk embedded into a Root type: Root from mdast haha. Nice and wondering if I need to embed that chunk at all...

[ChristianMurphy]

probably not,
that cast sounds off, chunk is Node[] right? A list of Node. Root is just a single object.
Forcing TS to type cast a list of object to an object may make the compile time error go away, but may cause a runtime error.

[lkj4]

Oh yes, right chunk is Node[]. Just tried that too without embedding the nodes into that fake root and also Node<Data>[] paired with the interface Data from above and got...

[tsserver 2345] [E] Argument of type 'Node<Data>[]' is not assignable to
parameter of type 'Node'.
  Type 'Node<Data>[]' is missing the following properties from type
'FootnoteReference': type, identifier

[ChristianMurphy]

Right, most plugins are designed to take a tree, starting with a single node.
Not a list of nodes.

TS is warning about this, in a somewhat round about way, it knows Array does not have the type or identifier properties which must exist on a Node.

[lkj4]

Hm, mdast is deprecated and I should use remark but latter does not export a type Root...

[ChristianMurphy]

mdast was a JavaScript package which has since been deprecated and replaced by remark.
@types/mdast is a TypeScript typings package which is used by much of the remark ecosystem.

[lkj4]

Ok, I saw in some of the docs that I should import @types/mdast but I actually don't need to. Even without importing the types, I am able to do an import type { Root } from 'mdast' which is a bit confusing. So what is actually happening here, where are the types actually coming from? I do not have mdast installed and use pnpm which does not give access to indirect modules.

Btw, and re List, TS does not complain about List as assertion but it creates an RT error, so toMarkdown requires Root.

[ChristianMurphy]

Even without importing the types, I am able to do an import type { Root } from 'mdast' which is a bit confusing. So what is actually happening here, where are the types actually coming from?

Probably a transitive dependency.
Many remark=* packages include @types/mdast as a dependency. For example
https://github.com/remarkjs/remark/blob/383871e6307fa0c09b542a78c05974267b759fe4/packages/remark-parse/package.json#L44
yarn and recent versions of npm flatten dependencies.
meaning ./node_modules/@types/mdast will be present even if you haven't directly run npm install @types/mdast or yarn add @types/mdast

[lkj4]

Yeah right, however pnpm does not flatten the packages, so it shouldn't allow access. I could imagine though that it might be different with types...

[ChristianMurphy]

@lkj4 this incompatibility is likely coming from

https://github.com/mrzmmr/unist-util-find-all-between/blob/adb9565a4424bcd9bcd3db4199b4d15ba91c6ecb/types/index.d.ts#L16-L21

remark-* plugins expect to get mdast nodes, this is returning a list more generic unist Node, which doesn't have the properties generally expected on mdast.

---

a few options:
1, open an issue (or even better a PR) with https://github.com/mrzmmr/unist-util-find-all-between to add generics and type narrowing so it can figure out the type it is passed.
2. share a sample of the code you are trying, so folks can offer more concrete suggestions on what to do

In the meantime, you may want to checkout https://unifiedjs.com/learn/guide/syntax-trees-typescript/ and https://unifiedjs.com/learn/recipe/narrow-node-typescript/ which introduce the unified syntax trees and some of the approaches to getting the specific types.

[lkj4]

Thanks for sharing the links, let get me a bit more familiar with ASTs and then I might submit issue/PR, right now the fake root hack works ok.