24 lines
572 B
Markdown
24 lines
572 B
Markdown
---
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title: Index of all matches
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tags: array
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expertise: intermediate
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cover: blog_images/jars-on-shelf-2.jpg
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firstSeen: 2018-01-06T12:07:56+02:00
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lastUpdated: 2020-10-22T20:23:47+03:00
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---
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Finds all indexes of `val` in an array.
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If `val` never occurs, returns an empty array.
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- Use `Array.prototype.reduce()` to loop over elements and store indexes for matching elements.
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```js
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const indexOfAll = (arr, val) =>
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arr.reduce((acc, el, i) => (el === val ? [...acc, i] : acc), []);
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```
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```js
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indexOfAll([1, 2, 3, 1, 2, 3], 1); // [0, 3]
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indexOfAll([1, 2, 3], 4); // []
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```
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