27 lines
767 B
Markdown
27 lines
767 B
Markdown
---
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title: groupBy
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tags: array,object,intermediate
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firstSeen: 2017-12-17T17:55:51+02:00
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lastUpdated: 2020-10-22T20:23:47+03:00
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---
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Groups the elements of an array based on the given function.
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- Use `Array.prototype.map()` to map the values of the 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 groupBy = (arr, fn) =>
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arr
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.map(typeof fn === 'function' ? fn : val => val[fn])
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.reduce((acc, val, i) => {
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acc[val] = (acc[val] || []).concat(arr[i]);
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return acc;
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}, {});
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```
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```js
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groupBy([6.1, 4.2, 6.3], Math.floor); // {4: [4.2], 6: [6.1, 6.3]}
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groupBy(['one', 'two', 'three'], 'length'); // {3: ['one', 'two'], 5: ['three']}
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```
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