Refactor code-style

*   Add more docs to JSDoc
*   Add support for `null` in input of API types

Author: wooorm

lib/index.js

@@ -9,6 +9,7 @@
  *   Node to calculate a key for.
  * @returns {unknown}
  *   Key to index on.
+ *
  *   Can be anything that can be used as a key in a `Map`.
  */
 
@@ -22,20 +23,23 @@ export class Index {
    * If `tree` is given, the index is initialized with all nodes, optionally
    * filtered by `test`.
    *
-   * @param {string|KeyFunction} prop
+   * @param {string | KeyFunction} prop
    *   Field (`string`) to look up in each node to find keys or function called
    *   with each node to calculate keys.
-   * @param {Node} [tree]
+   * @param {Node | null | undefined} [tree]
    *   Tree to index.
-   * @param {Test} [test]
+   * @param {Test | null | undefined} [test]
    *   `is`-compatible test.
    */
   constructor(prop, tree, test) {
     /** @type {Map<unknown, Array<Node>>} */
     this.index = new Map()
     /** @type {KeyFunction} */
-    // @ts-expect-error: Looks indexable.
-    this.key = typeof prop === 'string' ? (node) => node[prop] : prop
+    this.key =
+      typeof prop === 'string'
+        ? // @ts-expect-error: Looks indexable.
+          /** @type {KeyFunction} */ ((node) => node[prop])
+        : prop
 
     if (tree) {
       visit(tree, test, (node) => {
@@ -49,6 +53,7 @@ export class Index {
    *
    * @param {unknown} key
    *   Key to retrieve.
+   *
    *   Can be anything that can be used as a key in a `Map`.
    * @returns {Array<Node>}
    *   List of zero or more nodes.

test.js

@@ -21,7 +21,7 @@ test('Index', (t) => {
     'new Index(prop)'
   )
 
-  // @ts-expect-error: fine.
+  // @ts-expect-error: Look for nodes with an `id`.
   instance = new Index((/** @type {IdNode} */ node) => node.id)
   instance.add(node)
 
@@ -166,7 +166,6 @@ test('index.get', (t) => {
   })
 
   t.test('empty index', (st) => {
-    // @ts-expect-error: runtime.
     st.deepEqual(new Index('foo', null).get('bar'), [])
     st.deepEqual(new Index('foo').get('bar'), [])
     st.end()