21 lines
588 B
Markdown
21 lines
588 B
Markdown
---
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title: differenceBy
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tags: array,intermediate
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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`, then use `Array.prototype.map()` to apply `fn` to each element in `a`, then `Array.prototype.filter()`
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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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