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How can I find the insertion index of an element in a sorted JavaScript array?

Submitted by: @import:30-seconds-of-code·Mar 2, 2026·

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Problem

A sorted array is useful as long as it can maintain its sorting order. Unfortunately, inserting a new element into a sorted array is not as simple as pushing it to the end of the array. Instead, it requires finding the correct index to insert it at.
> [!NOTE]
>
> The code snippets presented in this article loosely check if the array is sorted in ascending or descending order. If you want to be more strict or have checks for this in place, you should alter the code to reflect that.
To find the lowest insertion index of an element in a sorted array, you can use Array.prototype.findIndex() to find the appropriate index where the element should be inserted.

Solution

const insertionIndex = (arr, n) => {
  const isDescending = arr[0] > arr[arr.length - 1];
  const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
  return index === -1 ? arr.length : index;
};

insertionIndex([5, 3, 2, 1], 4); // 1
insertionIndex([30, 50], 40); // 1

>
> The code snippets presented in this article loosely check if the array is sorted in ascending or descending order. If you want to be more strict or have checks for this in place, you should alter the code to reflect that.
To find the lowest insertion index of an element in a sorted array, you can use Array.prototype.findIndex() to find the appropriate index where the element should be inserted.
Conversely, to find the highest insertion index of an element in a sorted array, you can use Array.prototype.findLastIndex() to find the appropriate index where the element should be inserted.
For more complex data, you might need a comparator function to determine the insertion index. For example, if you have an array of objects, you might want to find the insertion index based on a specific property of each object.
The technique is the same as above, except for applying the given comparator function to each element of the array before comparing it to the given value.

Code Snippets

const insertionIndex = (arr, n) => {
  const isDescending = arr[0] > arr[arr.length - 1];
  const index = arr.findIndex(el => (isDescending ? n >= el : n <= el));
  return index === -1 ? arr.length : index;
};

insertionIndex([5, 3, 2, 1], 4); // 1
insertionIndex([30, 50], 40); // 1

const lastInsertionIndex = (arr, n) => {
  const isDescending = arr[0] > arr[arr.length - 1];
  const index = arr.findLastIndex(el => (isDescending ? n <= el : n >= el));
  return index === -1 ? arr.length : index;
};

lastInsertionIndex([10, 20, 30, 30, 40], 30); // 3

const insertionIndexBy = (arr, n, comparatorFn) => {
  const index = arr.findIndex(el => comparatorFn(n, el) < 0);
  return index === -1 ? arr.length : index;
};

insertionIndexBy([{ x: 4 }, { x: 6 }], { x: 5 }, (a, b) => a.x - b.x); // 1
insertionIndexBy([{ x: 6 }, { x: 4 }], { x: 5 }, (a, b) => b.x - a.x); // 1

Context

From 30-seconds-of-code: insertion-index-in-sorted-array

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