Add improved docs

Author: wooorm

readme.md

@@ -8,30 +8,69 @@
 [![Backers][backers-badge]][collective]
 [![Chat][chat-badge]][chat]
 
-[**unist**][unist] utility to create a mutable index mapping property values or
-computed keys back to nodes.
+[unist][] utility to create an index from certain nodes.
 
-## Install
+## Contents
+
+*   [What is this?](#what-is-this)
+*   [When should I use this?](#when-should-i-use-this)
+*   [Install](#install)
+*   [Use](#use)
+*   [API](#api)
+    *   [`Index(prop|keyFunction[, tree[, test]])`](#indexpropkeyfunction-tree-test)
+*   [Types](#types)
+*   [Compatibility](#compatibility)
+*   [Related](#related)
+*   [Contribute](#contribute)
+*   [License](#license)
+
+## What is this?
+
+This utility creates a mutable index data structure, that maps property values
+or computed keys, to nodes.
+For example, you can use this to index all (footnote) definitions in a tree,
+or all headings of a certain rank, to later retrieve them without having to walk
+the tree each time.
+
+## When should I use this?
 
-This package is [ESM only](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c):
-Node 12+ is needed to use it and it must be `import`ed instead of `require`d.
+This is a utility that helps you deal with indexing the tree.
+It’s pretty small, and you can definitely do it yourself, but this little
+wrapper makes it all a bit easier.
 
-[npm][]:
+## Install
+
+This package is [ESM only][esm].
+In Node.js (version 12.20+, 14.14+, 16.0+, 18.0+), install with [npm][]:
 
 ```sh
 npm install unist-util-index
 ```
 
+In Deno with [`esm.sh`][esmsh]:
+
+```js
+import {Index} from "https://esm.sh/unist-util-index@3"
+```
+
+In browsers with [`esm.sh`][esmsh]:
+
+```html
+<script type="module">
+  import {Index} from "https://esm.sh/unist-util-index@3?bundle"
+</script>
+```
+
 ## Use
 
 ```js
-import fs from 'node:fs'
-import {remark} from 'remark'
+import fs from 'node:fs/promises'
+import {fromMarkdown} from 'mdast-util-from-markdown'
 import {toString} from 'mdast-util-to-string'
 import {Index} from 'unist-util-index'
 
 // Parse and read this repo’s readme:
-const tree = remark.parse(fs.readFileSync('readme.md'))
+const tree = fromMarkdown(await fs.readFile('readme.md'))
 
 // Index on heading depth:
 const indexOnDepth = new Index('depth', tree, 'heading')
@@ -47,70 +86,109 @@ console.log(indexOnIdentifier.get('unist').map(node => node.url))
 Yields:
 
 ```js
-[ 'Install', 'Use', 'API', 'Related', 'Contribute', 'License' ]
+[
+  'Contents',
+  'What is this?',
+  'When should I use this?',
+  'Install',
+  'Use',
+  'API',
+  'Types',
+  'Compatibility',
+  'Related',
+  'Contribute',
+  'License'
+]
 [ 'https://github.com/syntax-tree/unist' ]
 ```
 
 ## API
 
-This package exports the following identifiers: `Index`.
+This package exports the identifier `Index`.
 There is no default export.
 
-### `class Index(prop|keyFn[, tree[, test]])`
+### `Index(prop|keyFunction[, tree[, test]])`
 
-Create an index data structure that maps keys (calculated by `keyFn` function or
-the values at `prop` in each node) to a list of nodes.
+Create a mutable index data structure, that maps property values or computed
+keys, to nodes.
 
 If `tree` is given, the index is initialized with all nodes, optionally filtered
 by `test`.
 
 ###### Parameters
 
-*   `prop` (`string`) — Property to look up in each node to find keys
-*   `keyFn` ([`Function`][keyfn]) — Function called with each node to calculate
-    keys
-*   `tree` ([`Node?`][node]) — [Tree][] to index
-*   `test` ([`Test`][is], optional) — [`is`][is]-compatible test (such as a
-    [type][])
+*   `prop` (`string`)
+    — property to look up in each node to find keys
+*   `keyFunction` ([`KeyFunction`][key-function])
+    — function called with each node to calculate keys
+*   `tree` ([`Node`][node], optional)
+    — tree to index
+*   `test` ([`Test`][is], optional)
+    — [`is`][is]-compatible test
 
 ###### Returns
 
-`Index` — an index instance.
+Instance (`Index`).
 
-#### `function keyFn(node)`
+#### `function keyFunction(node)`
 
-Function called with every added [node][] to return the key to index on.
+Function called with every added node ([`Node`][node]) to calculate the key to
+index on.
+
+###### Returns
+
+Key to index on (`unknown`).
+Can be anything that can be used as a key in a [`Map`][map].
 
 #### `Index#get(key)`
 
-Get nodes by `key` (`*`).
-Returns a list of zero or more nodes ([`Array<Node>`][node]).
+Get nodes by `key` (`unknown`).
+
+###### Returns
+
+List of zero or more nodes ([`Array<Node>`][node]).
 
 #### `Index#add(node)`
 
-Add [`node`][node] to the index (if not already present).
+Add `node` ([`Node`][node]) to the index (if not already present).
+
+###### Returns
+
+Nothing (`void`).
 
 #### `Index#remove(node)`
 
-Remove [`node`][node] from the index (if present).
+Remove `node` ([`Node`][node]) from the index (if present).
+
+###### Returns
+
+Nothing (`void`).
+
+## Types
+
+This package is fully typed with [TypeScript][].
+It exports the additional types `KeyFunction` and `Test`.
+
+## Compatibility
+
+Projects maintained by the unified collective are compatible with all maintained
+versions of Node.js.
+As of now, that is Node.js 12.20+, 14.14+, 16.0+, and 18.0+.
+Our projects sometimes work with older versions, but this is not guaranteed.
 
 ## Related
 
 *   [`unist-util-is`](https://github.com/syntax-tree/unist-util-is)
-    — Utility to check if a node passes a test
+    — utility to check if a node passes a test
 *   [`unist-util-visit`](https://github.com/syntax-tree/unist-util-visit)
-    — Utility to recursively walk over nodes
-*   [`unist-util-map`](https://github.com/syntax-tree/unist-util-map)
-    — Create a new tree by mapping by the provided function
-*   [`unist-util-flatmap`](https://gitlab.com/staltz/unist-util-flatmap)
-    — Create a new tree by mapping and then flattening
+    — utility to recursively walk over nodes
 *   [`unist-util-select`](https://github.com/syntax-tree/unist-util-select)
-    — Select nodes with CSS-like selectors
+    — select nodes with CSS-like selectors
 
 ## Contribute
 
-See [`contributing.md` in `syntax-tree/.github`][contributing] for ways to get
-started.
+See [`contributing.md`][contributing] in [`syntax-tree/.github`][health] for
+ways to get started.
 See [`support.md`][support] for ways to get help.
 
 This project has a [Code of Conduct][coc].
@@ -151,22 +229,28 @@ abide by its terms.
 
 [npm]: https://docs.npmjs.com/cli/install
 
+[esm]: https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c
+
+[esmsh]: https://esm.sh
+
+[typescript]: https://www.typescriptlang.org
+
 [license]: license
 
-[contributing]: https://github.com/syntax-tree/.github/blob/HEAD/contributing.md
+[health]: https://github.com/syntax-tree/.github
 
-[support]: https://github.com/syntax-tree/.github/blob/HEAD/support.md
+[contributing]: https://github.com/syntax-tree/.github/blob/main/contributing.md
 
-[coc]: https://github.com/syntax-tree/.github/blob/HEAD/code-of-conduct.md
+[support]: https://github.com/syntax-tree/.github/blob/main/support.md
+
+[coc]: https://github.com/syntax-tree/.github/blob/main/code-of-conduct.md
 
 [unist]: https://github.com/syntax-tree/unist
 
 [node]: https://github.com/syntax-tree/unist#node
 
-[tree]: https://github.com/syntax-tree/unist#tree
-
-[type]: https://github.com/syntax-tree/unist#type
-
 [is]: https://github.com/syntax-tree/unist-util-is
 
-[keyfn]: #function-keyfnnode
+[map]: https://developer.mozilla.org/docs/Web/JavaScript/Reference/Global_Objects/Map
+
+[key-function]: #function-keyfunctionnode