27 lines
843 B
Markdown
27 lines
843 B
Markdown
---
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title: findLastIndex
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tags: array,intermediate
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firstSeen: 2018-01-24T13:01:51+02:00
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lastUpdated: 2020-10-22T20:23:47+03:00
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---
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Finds the index of the last element for which the provided function returns a truthy value.
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- Use `Array.prototype.map()` to map each element to an array with its index and value.
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- Use `Array.prototype.filter()` to remove elements for which `fn` returns falsy values
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- Use `Array.prototype.pop()` to get the last element in the filtered array.
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- Return `-1` if there are no matching elements.
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```js
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const findLastIndex = (arr, fn) =>
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(arr
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.map((val, i) => [i, val])
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.filter(([i, val]) => fn(val, i, arr))
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.pop() || [-1])[0];
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```
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```js
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findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3)
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findLastIndex([1, 2, 3, 4], n => n === 5); // -1 (default value when not found)
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```
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