29 lines
849 B
Markdown
29 lines
849 B
Markdown
---
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title: Count grouped elements
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type: snippet
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language: javascript
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tags: [array,object]
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cover: tools
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dateModified: 2020-11-03T22:11:18+02:00
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---
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Groups the elements of an array based on the given function and returns the count of elements in each group.
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- Use `Array.prototype.map()` to map the values of an array to a function or property name.
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- Use `Array.prototype.reduce()` to create an object, where the keys are produced from the mapped results.
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```js
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const countBy = (arr, fn) =>
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arr.map(typeof fn === 'function' ? fn : val => val[fn]).reduce((acc, val) => {
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acc[val] = (acc[val] || 0) + 1;
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return acc;
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}, {});
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```
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```js
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countBy([6.1, 4.2, 6.3], Math.floor); // {4: 1, 6: 2}
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countBy(['one', 'two', 'three'], 'length'); // {3: 2, 5: 1}
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countBy([{ count: 5 }, { count: 10 }, { count: 5 }], x => x.count)
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// {5: 2, 10: 1}
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```
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