Add solution #2631

Author: JoshCrozier

README.md

@@ -387,6 +387,7 @@
 2627|[Debounce](./2627-debounce.js)|Medium|
 2629|[Function Composition](./2629-function-composition.js)|Easy|
 2630|[Memoize II](./2630-memoize-ii.js)|Hard|
+2631|[Group By](./2631-group-by.js)|Medium|
 2634|[Filter Elements from Array](./2634-filter-elements-from-array.js)|Easy|
 2635|[Apply Transform Over Each Element in Array](./2635-apply-transform-over-each-element-in-array.js)|Easy|
 2637|[Promise Time Limit](./2637-promise-time-limit.js)|Medium|

solutions/2631-group-by.js

@@ -0,0 +1,35 @@
+/**
+ * 2631. Group By
+ * https://leetcode.com/problems/group-by/
+ * Difficulty: Medium
+ *
+ * Write code that enhances all arrays such that you can call the array.groupBy(fn) method on any
+ * array and it will return a grouped version of the array.
+ *
+ * A grouped array is an object where each key is the output of fn(arr[i]) and each value is an
+ * array containing all items in the original array which generate that key.
+ *
+ * The provided callback fn will accept an item in the array and return a string key.
+ *
+ * The order of each value list should be the order the items appear in the array. Any order of
+ * keys is acceptable.
+ *
+ * Please solve it without lodash's _.groupBy function.
+ */
+
+/**
+ * @param {Function} fn
+ * @return {Object}
+ */
+Array.prototype.groupBy = function(fn) {
+  return this.reduce((grouped, item) => {
+    const key = fn(item);
+    grouped[key] = grouped[key] || [];
+    grouped[key].push(item);
+    return grouped;
+  }, {});
+};
+
+/**
+ * [1,2,3].groupBy(String) // {"1":[1],"2":[2],"3":[3]}
+ */