25 lines
594 B
Markdown
25 lines
594 B
Markdown
---
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title: unionWith
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tags: array,intermediate
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---
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Returns every element that exists in any of the two arrays at least once, using a provided comparator function.
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- Create a `new Set()` with all values of `a` and values in `b` for which the comparator finds no matches in `a`, using `Array.prototype.findIndex()`.
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```js
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const unionWith = (a, b, comp) =>
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Array.from(
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new Set([...a, ...b.filter(x => a.findIndex(y => comp(x, y)) === -1)])
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);
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```
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```js
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unionWith(
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[1, 1.2, 1.5, 3, 0],
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[1.9, 3, 0, 3.9],
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(a, b) => Math.round(a) === Math.round(b)
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);
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// [1, 1.2, 1.5, 3, 0, 3.9]
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```
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