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javascript/snippets/count-by.md

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+---
+title: Count grouped elements
+type: snippet
+tags: [array,object]
+cover: tools
+dateModified: 2020-11-03T22:11:18+02:00
+---
+
+Groups the elements of an array based on the given function and returns the count of elements in each group.
+
+- Use `Array.prototype.map()` to map the values of an array to a function or property name.
+- Use `Array.prototype.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) => {
+    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}
+countBy([{ count: 5 }, { count: 10 }, { count: 5 }], x => x.count)
+// {5: 2, 10: 1}
+```