Refactor code-style

Author: wooorm

lib/index.js

@@ -1,7 +1,9 @@
 /**
  * @typedef {import('unist').Node} Node
  * @typedef {import('unist-util-visit').Test} Test
- *
+ */
+
+/**
  * @callback KeyFunction
  *   Function called with every added node (`Node`) to calculate the key to
  *   index on.
@@ -23,23 +25,19 @@ export class Index {
    * If `tree` is given, the index is initialized with all nodes, optionally
    * filtered by `test`.
    *
-   * @param {string | KeyFunction} prop
+   * @param {KeyFunction | string} prop
    *   Field (`string`) to look up in each node to find keys or function called
    *   with each node to calculate keys.
    * @param {Node | null | undefined} [tree]
-   *   Tree to index.
+   *   Tree to index (optional).
    * @param {Test | null | undefined} [test]
-   *   `is`-compatible test.
+   *   `is`-compatible test (optional).
    */
   constructor(prop, tree, test) {
     /** @type {Map<unknown, Array<Node>>} */
     this.index = new Map()
     /** @type {KeyFunction} */
-    this.key =
-      typeof prop === 'string'
-        ? // @ts-expect-error: Looks indexable.
-          /** @type {KeyFunction} */ ((node) => node[prop])
-        : prop
+    this.key = typeof prop === 'string' ? createKeyFunction(prop) : prop
 
     if (tree) {
       visit(tree, test, (node) => {
@@ -108,3 +106,18 @@ export class Index {
     return this
   }
 }
+
+/**
+ * @param {string} field
+ * @returns {KeyFunction}
+ */
+function createKeyFunction(field) {
+  return keyFunction
+
+  /** @type {KeyFunction} */
+  function keyFunction(node) {
+    // @ts-expect-error: all nodes are plain objects and so indexable.
+    const result = /** @type {unknown} */ (node[field])
+    return result
+  }
+}

readme.md

@@ -77,12 +77,20 @@ const tree = fromMarkdown(await fs.readFile('readme.md'))
 // Index on heading depth:
 const indexOnDepth = new Index('depth', tree, 'heading')
 
-console.log(indexOnDepth.get(2).map((d) => toString(d)))
+console.log(
+  indexOnDepth.get(2).map(function (d) {
+    return toString(d)
+  })
+)
 
 // Index on definition identifier:
 const indexOnIdentifier = new Index('identifier', tree, 'definition')
 
-console.log(indexOnIdentifier.get('unist').map((node) => node.url))
+console.log(
+  indexOnIdentifier.get('unist').map(function (node) {
+    return node.url
+  })
+)
 ```
 
 Yields: