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+// @flow strict
+import type { ObjMap } from './ObjMap';
+
+/**
+ * Creates a keyed JS object from an array, given a function to produce the keys
+ * for each value in the array.
+ *
+ * This provides a convenient lookup for the array items if the key function
+ * produces unique results.
+ *
+ *     const phoneBook = [
+ *       { name: 'Jon', num: '555-1234' },
+ *       { name: 'Jenny', num: '867-5309' }
+ *     ]
+ *
+ *     // { Jon: { name: 'Jon', num: '555-1234' },
+ *     //   Jenny: { name: 'Jenny', num: '867-5309' } }
+ *     const entriesByName = keyMap(
+ *       phoneBook,
+ *       entry => entry.name
+ *     )
+ *
+ *     // { name: 'Jenny', num: '857-6309' }
+ *     const jennyEntry = entriesByName['Jenny']
+ *
+ */
+export default function keyMap<T>(
+  list: $ReadOnlyArray<T>,
+  keyFn: (item: T) => string,
+): ObjMap<T> {
+  return list.reduce((map, item) => {
+    map[keyFn(item)] = item;
+    return map;
+  }, Object.create(null));
+}