"strict": true collides with mdast types

[205g0]

This is related to the prior discussion. Please check the example repo https://github.com/205g0/mdast-example and there index.ts:

You should see in your editor following contextual error in index.ts at line 21 over (node: Heading) which says:

[tsserver 2345] [E] Argument of type '(node: Heading) => void' is not assignable to parameter of type '(value: Node, index: number, array: Node[]) => void'. Types of parameters 'node' and 'value' are incompatible. Type 'Node' is missing the following properties from type 'Heading': depth, children

If you remove "strict": true, this error goes away. Be aware that I already broke down the strict compiler option in all its single strict options at the top of tsconfig.json in order to isolate the causing option, which is:

"strictFunctionTypes": true

Remove this and the error goes away. Not sure if this is intended and I misunderstood something.

[wooorm]

But selectAll('root > *', ast) could result in other nodes as well, though, right? Not just headings?

[205g0]

Yes, you are right, I just realized once I hit submit. Then how do I properly type this node?

[205g0]

Now, I refactored again, kicked-out any yaml, frontmatter, frontmatter processing and now I get the same error as above on .depth at:

switch (true) {
  case is<Heading>(node, 'heading'):
    if (node.depth == 1) {
      ...

The error again: [tsserver 2339] [E] Property 'depth' does not exist on type 'Root | Paragraph | Heading | ThematicBreak | Blockquote | List | Table | HTML | Code | YAML | ... 16 more ... | FootnoteReference'. Property 'depth' does not exist on type 'Root'.

🤯

Edit: I use visit to walk the AST

[ChristianMurphy]

switch (true) {
  case is<Heading>(node, 'heading'):
    if (node.depth == 1) {
      ...

TypeScript doesn't support narrowing types in case the way you show.
See this playground for an example, note the value of something in the different blocks https://www.typescriptlang.org/play?#code/PTAEBcAsEsGdQA4CcCmATaBjAhuFocA7UQ7JJAewHdRtiBXQga0OuPAE8F9wLbRY4JNEIBzAFAAzRpnDQKxOAGUhI0QAoAbtgA29FAC5QjFmwCUR7XvxwBqsaADeoVOHpJ2XFBUmgr+0ABeYNAAckFhMVCAX3FxEFAqfCo6cAg+UnJqCBhYcUwFQQEKAFsUKDUjE1YqYkCSeh0dOOhfdWV7DVhS8pgxMzMncVAR0ASK+FtMyiS0YdHusoqxcVjxWCpocExIUHUhfUHHeZGcWBtYFUiunuXRCyHRp7GwCdApinAT58XetW-YrEgA

You need to use an if to get a predicate to narrow the value in scope.

I use visit to walk the AST

A good idea, visit can handle narrowing types itself, which makes using specific node types easier

[wooorm]

@205g0 your javascript is wrong, switch doesn't work like that. https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/switch.

[205g0]

@wooorm right,switch/ case are the way around and weird anyway. @ChristianMurphy thanks Christian, I refactored again and I think the golden rule is just to use ifs and visit and call it a day. While you can do anything with visit I like the stuff selectAll offers but I could create some clean code just with visit as well, btw how do you get syntax highlighting in your code fields?

[wooorm]

This example shows both how switch in JavaScript works, and how to attach a code language to code:

```js
// Assuming `node` is a known mdast node such as `mdast.Content`
switch (node.type) {
  case 'heading':
    // TS should know this is a heading.
    break
  case 'yaml':
    // TS should know this is YAML.
    break
}
```