28 lines
885 B
Markdown
28 lines
885 B
Markdown
---
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title: Mapped array union
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tags: array
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expertise: intermediate
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cover: blog_images/lake-church.jpg
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firstSeen: 2018-01-24T12:19:41+02:00
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lastUpdated: 2020-10-22T20:24:44+03:00
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---
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Returns every element that exists in any of the two arrays at least once, after applying the provided function to each array element of both.
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- Create a `Set` by applying all `fn` to all values of `a`.
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- Create a `Set` from `a` and all elements in `b` whose value, after applying `fn` does not match a value in the previously created set.
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- Return the last set converted to an array.
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```js
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const unionBy = (a, b, fn) => {
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const s = new Set(a.map(fn));
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return Array.from(new Set([...a, ...b.filter(x => !s.has(fn(x)))]));
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};
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```
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```js
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unionBy([2.1], [1.2, 2.3], Math.floor); // [2.1, 1.2]
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unionBy([{ id: 1 }, { id: 2 }], [{ id: 2 }, { id: 3 }], x => x.id)
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// [{ id: 1 }, { id: 2 }, { id: 3 }]
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```
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