[unist-util-filter] Typescript: Property 'type' does not exist on type 'Test<Node>'

[bearcanrun]

Attempting to type a utility that removes frontmatter from markdown. Functions works as just JS, I just can't figure out how to add 'type' property to node. Help, please! (disclaimer: I'm still learning TS)

import filter from 'unist-util-filter'
import visit from 'unist-util-visit'
import type { Node } from 'unist'
import type { Test } from 'unist-util-is'

interface YamlNode extends Node {
  type: string
}

export const removeFrontmatter = () => async (tree: Node, file) => {
  let getFrontMatter
  await visit(tree, 'yaml', (node) => {
    getFrontMatter = node.data.parsedValue
    return
  })
  file.data.frontmatter = getFrontMatter
  return filter(tree, (node: Test<YamlNode>): node is YamlNode => node.type !== 'yaml')
// TS error here: Property 'type' does not exist on type 'Test<Node>' ^^^^^^^^
}

[ChristianMurphy]

A few different questions in here: 1 what is the input to a Test accept? 2 why is type missing? 3 what is a way to resolve this?

1. what is the input to a Test accept?

   (node: Test<YamlNode>): node is YamlNode => node.type !== 'yaml'

   is known as a Test https://github.com/syntax-tree/unist-util-is#function-testnode-index-parent. A test may or may not receive a valid node, as such it doesn't know what it's receiving and node is type unknown.
   a Test can also be null which checks that a node is valid (but not it's type), string which check that a node is valid and that node type matches the string passed in, or an object which checks that it matches passed in attributes.
   https://github.com/syntax-tree/unist-util-is#use

2. why is type missing? Because node is an unknown type, to access a property on an unknown type, it first has to be narrowed to guarantee that the property exists. https://fettblog.eu/typescript-hasownproperty

3. what is a way to resolve this? Since you mainly want to filter nodes where type is yaml, using the string test type or object test type since they validate that unknown is a Node before doing the validation type/attribute check.

   return filter(tree, 'yaml')

[bearcanrun]

Wow, super gratitude for spelling it out for me. I had reduced it down to your answer at some point, getting rid of the error, but I was really trying to understand the typing side of it.

I had indeed followed the type definitions from filter paramfilter?: Test<T> → type Test<T extends Node> = TestType<T> | TestObject<T> | TestFunction<T> → type TestType<T extends Node> = T['type'], but generics still don't automatically translate for me and I hadn't comprehended the now obvious meaning of T['type']. Level up, thanks to you!