sortedLastIndex (Lodash compatibility)

::: warning Implement binary search directly

This sortedLastIndex function operates slowly due to complex binary search handling and type validation.

Instead, implement faster, more modern binary search directly.

:::

Finds the highest index at which a value should be inserted into a sorted array.

const index = sortedLastIndex(array, value);

Usage

sortedLastIndex(array, value)

Use sortedLastIndex to find the highest position to insert a value in a sorted array. When duplicate values exist, it returns the index after the last position.

import { sortedLastIndex } from 'es-toolkit/compat';

// Find last insertion position in array with duplicate values
sortedLastIndex([4, 5, 5, 5, 6], 5);
// Returns 4 (position after the last 5)

// Find insertion position for new value
sortedLastIndex([10, 20, 30], 25);
// Returns 2 (25 is positioned before 30)

// When value doesn't exist
sortedLastIndex([1, 2, 3], 0);
// Returns 0 (positioned at the front)

For null or undefined arrays, returns 0.

import { sortedLastIndex } from 'es-toolkit/compat';

sortedLastIndex(null, 1); // 0
sortedLastIndex(undefined, 1); // 0

Parameters

• array (ArrayLike<T> | null | undefined): The sorted array. Using an unsorted array can produce incorrect results.
• value (T): The value to insert.

Returns

(number): Returns the highest index to insert the value. If the array is null or undefined, returns 0.