23 lines
757 B
Markdown
23 lines
757 B
Markdown
---
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title: sortedIndexBy
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tags: array,math,function,intermediate
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---
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Returns the lowest index at which value should be inserted into array in order to maintain its sort order, based on a provided iterator function.
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- Check if the array is sorted in descending order (loosely).
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- Use `Array.prototype.findIndex()` to find the appropriate index where the element should be inserted, based on the iterator function `fn`.
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```js
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const sortedIndexBy = (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.findIndex(el => (isDescending ? val >= fn(el) : val <= fn(el)));
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return index === -1 ? arr.length : index;
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};
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```
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```js
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sortedIndexBy([{ x: 4 }, { x: 5 }], { x: 4 }, o => o.x); // 0
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```
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