Rename js snippets

snippets/js/s/mapped-array-average.md

@@ -0,0 +1,26 @@
+---
+title: Mapped array average
+type: snippet
+language: javascript
+tags: [math]
+cover: red-lake
+dateModified: 2020-10-21T21:17:45+03:00
+---
+
+Calculates the average of an array, after mapping each element to a value using the provided function.
+
+- Use `Array.prototype.map()` to map each element to the value returned by `fn`.
+- Use `Array.prototype.reduce()` to add each value to an accumulator, initialized with a value of `0`.
+- Divide the resulting array by its length.
+
+```js
+const averageBy = (arr, fn) =>
+  arr
+    .map(typeof fn === 'function' ? fn : val => val[fn])
+    .reduce((acc, val) => acc + val, 0) / arr.length;
+```
+
+```js
+averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 5
+averageBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 5
+```