Added all math By snippets

snippets/maxBy.md

@@ -0,0 +1,13 @@
+### maxBy
+
+Returns the maximum value of an array, after mapping each element to a value using the provided function.
+
+Use `Array.map()` to map each element to the value returned by `fn`, `Math.max()` to get the maximum value.
+
+```js
+const maxBy = (arr, fn) => Math.max(...arr.map(fn));
+```
+
+```js
+maxBy([{ 'n': 4 }, { 'n': 2 }, { 'n': 8 }, { 'n': 6 }], o => o.n);  // 8
+```