feat(countBy): Add module countBy

src/__tests__/countBy.js

@@ -0,0 +1,78 @@
+import countBy from '../countBy';
+
+describe('Core.countBy', () => {
+  const albums = [
+    { title: 'Art of the Fugue', artist: 'Glenn Gould', genre: 'Baroque' },
+    { title: 'A Farewell to Kings', artist: 'Rush', genre: 'Rock' },
+    { title: 'Timeout', artist: 'Dave Brubeck Quartet', genre: 'Jazz' },
+    { title: 'Fly By Night', artist: 'Rush', genre: 'Rock' },
+    {
+      title: 'Goldberg Variations',
+      artist: 'Daniel Barenboim',
+      genre: 'Baroque',
+    },
+    {
+      title: 'New World Symphony',
+      artist: 'Leonard Bernstein',
+      genre: 'Romantic',
+    },
+    { title: 'Romance with the Unseen', artist: 'Don Byron', genre: 'Jazz' },
+    { title: 'Somewhere In Time', artist: 'Iron Maiden', genre: 'Metal' },
+    { title: 'In Times of Desparation', artist: 'Danny Holt', genre: 'Modern' },
+    { title: 'Evita', artist: 'Various', genre: 'Broadway' },
+    { title: 'Five Leaves Left', artist: 'Nick Drake', genre: 'Folk' },
+    {
+      title: 'The Magic Flute',
+      artist: 'John Eliot Gardiner',
+      genre: 'Classical',
+    },
+  ];
+
+  const prop = p => obj => obj[p];
+  const derivedGenre = album => {
+    const remap = {
+      Baroque: 'Classical',
+      Modern: 'Classical',
+      Romantic: 'Classical',
+      Metal: 'Rock' /*  etc */,
+    };
+
+    const genre = prop('genre')(album);
+    return remap[genre] || genre;
+  };
+
+  test('counts by a simple property of the objects', () => {
+    expect(countBy(prop('genre'), albums)).toEqual({
+      Baroque: 2,
+      Rock: 2,
+      Jazz: 2,
+      Romantic: 1,
+      Metal: 1,
+      Modern: 1,
+      Broadway: 1,
+      Folk: 1,
+      Classical: 1,
+    });
+  });
+
+  test('counts by a more complex function on the objects', () => {
+    expect(countBy(derivedGenre, albums)).toEqual({
+      Classical: 5,
+      Rock: 3,
+      Jazz: 2,
+      Broadway: 1,
+      Folk: 1,
+    });
+  });
+
+  test('will force items into strings due to the nature of js objects', () => {
+    const list = [() => {}, 1, {}, []];
+    const id = x => x;
+    expect(countBy(id, list)).toEqual({
+      '() => {}': 1,
+      '1': 1,
+      '[object Object]': 1,
+      '': 1,
+    });
+  });
+});

src/countBy.js

@@ -0,0 +1,16 @@
+import reduce from './reduce';
+import prop from './prop';
+import assoc from './assoc';
+
+const countBy = (fn, list) =>
+  reduce(
+    (acc, item) => {
+      const key = fn(item);
+      const existingKey = prop(key, acc);
+      return assoc(key, existingKey ? existingKey + 1 : 1, acc);
+    },
+    {},
+    list,
+  );
+
+export { countBy as default };

src/index.js

@@ -9,6 +9,7 @@ import assocPath from './assocPath';
 import clamp from './clamp';
 import compose from './compose';
 import concat from './concat';
+import countBy from './countBy';
 import curry from './curry';
 import dec from './dec';
 import divide from './divide';
@@ -65,6 +66,7 @@ export {
   clamp,
   compose,
   concat,
+  countBy,
   curry,
   dec,
   divide,