27 lines
780 B
Markdown
27 lines
780 B
Markdown
---
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title: Mapped array difference
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type: snippet
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language: javascript
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tags: [array]
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cover: keyboard
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dateModified: 2020-10-19T18:52:00+03:00
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---
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Returns the difference between two arrays, after applying the provided function to each array element of both.
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- Create a `Set` by applying `fn` to each element in `b`.
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- Use `Array.prototype.map()` to apply `fn` to each element in `a`.
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- Use `Array.prototype.filter()` in combination with `fn` on `a` to only keep values not contained in `b`, using `Set.prototype.has()`.
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```js
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const differenceBy = (a, b, fn) => {
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const s = new Set(b.map(fn));
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return a.map(fn).filter(el => !s.has(el));
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};
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```
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```js
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differenceBy([2.1, 1.2], [2.3, 3.4], Math.floor); // [1]
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differenceBy([{ x: 2 }, { x: 1 }], [{ x: 1 }], v => v.x); // [2]
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```
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