Add countBy, update other By snippets

Updated all By snippets to have matching behavior, consistent argument naming and added countBy as per #100.

snippets/averageBy.md

@@ -5,9 +5,11 @@ Returns the average of an array, after mapping each element to a value using the
 Use `Array.map()` to map each element to the value returned by `fn`, `Array.reduce()` to add each value to an accumulator, initialized with a value of `0`, divide by the `length` of the array.
 
 ```js
-const averageBy = (arr, fn) => arr.map(fn).reduce((acc, val) => acc + val, 0) / arr.length;
+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
 ```

snippets/countBy.md

@@ -0,0 +1,19 @@
+### countBy
+
+Groups the elements of an array based on the given function and returns the count of elements in each group.
+
+Use `Array.map()` to map the values of an array to a function or property name.
+Use `Array.reduce()` to create an object, where the keys are produced from the mapped results.
+
+```js
+const countBy = (arr, fn) =>
+  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
+    acc[val] = (acc[val] || 0) + 1;
+    return acc;
+  }, {});
+```
+
+```js
+countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
+countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}
+```

snippets/groupBy.md

@@ -6,8 +6,8 @@ Use `Array.map()` to map the values of an array to a function or property name.
 Use `Array.reduce()` to create an object, where the keys are produced from the mapped results.
 
 ```js
-const groupBy = (arr, func) =>
-  arr.map(typeof func === 'function' ? func : val => val[func]).reduce((acc, val, i) => {
+const groupBy = (arr, fn) =>
+  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val, i) => {
     acc[val] = (acc[val] || []).concat(arr[i]);
     return acc;
   }, {});

snippets/maxBy.md

@@ -5,9 +5,11 @@ Returns the maximum value of an array, after mapping each element to a value usi
 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));
+const maxBy = (arr, fn) =>
+  Math.max(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));
 ```
 
 ```js
 maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 8
+maxBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }],'n'); // 8
 ```

snippets/minBy.md

@@ -5,9 +5,11 @@ Returns the minimum value of an array, after mapping each element to a value usi
 Use `Array.map()` to map each element to the value returned by `fn`, `Math.min()` to get the maximum value.
 
 ```js
-const minBy = (arr, fn) => Math.min(...arr.map(fn));
+const minBy = (arr, fn) =>
+  Math.min(...arr.map(typeof fn === 'function' ? fn : val => val[fn]));
 ```
 
 ```js
 minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 8
+minBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 8
 ```

snippets/sumBy.md

@@ -5,9 +5,11 @@ Returns the sum of an array, after mapping each element to a value using the pro
 Use `Array.map()` to map each element to the value returned by `fn`, `Array.reduce()` to add each value to an accumulator, initialized with a value of `0`.
 
 ```js
-const sumBy = (arr, fn) => arr.map(fn).reduce((acc, val) => acc + val, 0);
+const sumBy = (arr, fn) =>
+  arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => acc + val, 0);
 ```
 
 ```js
 sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], o => o.n); // 20
+sumBy([{ n: 4 }, { n: 2 }, { n: 8 }, { n: 6 }], 'n'); // 20
 ```