27 lines
918 B
Markdown
27 lines
918 B
Markdown
---
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title: sortedLastIndexBy
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tags: array,intermediate
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---
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Finds the highest index at which a value should be inserted into an array in order to maintain its sort order, based on a provided iterator function.
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- Loosely check if the array is sorted in descending order.
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- Use `Array.prototype.map()` to apply the iterator function to all elements of the array.
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- Use `Array.prototype.reverse()` and `Array.prototype.findIndex()` to find the appropriate last index where the element should be inserted, based on the provided iterator function.
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```js
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const sortedLastIndexBy = (arr, n, fn) => {
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const isDescending = fn(arr[0]) > fn(arr[arr.length - 1]);
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const val = fn(n);
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const index = arr
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.map(fn)
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.reverse()
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.findIndex(el => (isDescending ? val <= el : val >= el));
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return index === -1 ? 0 : arr.length - index;
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};
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```
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```js
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sortedLastIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 1
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```
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