8ee50178f3
Correct: `Array.from()` (it’s a static method) Incorrect: `Array.join()` (doesn’t exist; it’s a prototype method) This patch uses the common `#` syntax to denote `.prototype.`.
20 lines
629 B
Markdown
20 lines
629 B
Markdown
### countBy
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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, i) => {
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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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```
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