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+---
+title: indexBy
+tags: array,object,intermediate
+firstSeen: 2021-06-20T05:00:00-04:00
+---
+
+Creates an object from an array, using a function to map each value to a key.
+
+- Use `Array.prototype.reduce()` to create an object from `arr`.
+- Apply `fn` to each value of `arr` to produce a key and add the key-value pair to the object.
+
+```js
+const indexBy = (arr, fn) =>
+  arr.reduce((obj, v, i) => {
+    obj[fn(v, i, arr)] = v;
+    return obj;
+  }, {});
+```
+
+```js
+indexBy([
+  { id: 10, name: 'apple' },
+  { id: 20, name: 'orange' }
+], x => x.id);
+// { '10': { id: 10, name: 'apple' }, '20': { id: 20, name: 'orange' } }
+```