20 lines
453 B
Markdown
20 lines
453 B
Markdown
---
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title: indexOfAll
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tags: array,intermediate
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---
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Returns all indices of `val` in an array.
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If `val` never occurs, returns `[]`.
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Use `Array.prototype.reduce()` to loop over elements and store indices for matching elements.
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Return the array of indices.
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```js
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const indexOfAll = (arr, val) => 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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