findLastIndex (Lodash Compatibility)

Use Array.prototype.findLastIndex

This findLastIndex function operates slowly due to additional features such as handling null or undefined, partial object matching, and property name matching.

Use the faster and more modern Array.prototype.findLastIndex instead.

Finds the index of the last element in an array that satisfies a condition.

const lastIndex = findLastIndex(array, predicate, fromIndex);

Reference

findLastIndex(array, predicate, fromIndex)

Use findLastIndex when you want to find the index of the first element that matches a given condition starting from the end of the array. Returns -1 if no element satisfies the condition.

This function can specify the condition in various ways. When you pass a function, it executes the function for each element. When you pass a partial object, it checks if the element has those properties. When you pass an array-formatted key-value pair, it checks if a specific property matches the given value. When you pass a string, it checks if that property evaluates to a truthy value.

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

const users = [
  { user: 'barney', active: true },
  { user: 'fred', active: false },
  { user: 'pebbles', active: false },
];

// Specify the condition using a function
findLastIndex(users, o => o.user === 'pebbles');
// Returns: 2

// Find a matching element using a partial object
findLastIndex(users, { user: 'barney', active: true });
// Returns: 0

// Find a matching element using a property-value pair
findLastIndex(users, ['active', false]);
// Returns: 2

// Find an element with a truthy value using a property name
findLastIndex(users, 'active');
// Returns: 0

You can also specify a starting position for the search. If fromIndex is negative, it's calculated from the end of the array.

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

const numbers = [1, 2, 3, 4, 5];

// Search in reverse from index 3
findLastIndex(numbers, n => n < 4, 2);
// Returns: 2

// If you use a negative index, it's calculated from the end
findLastIndex(numbers, n => n > 2, -2);
// Returns: 3

null or undefined are treated as empty arrays.

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

findLastIndex(null, n => n > 0); // -1
findLastIndex(undefined, n => n > 0); // -1

Parameters

• array (ArrayLike<T> | null | undefined): The array to search.
• predicate (((item: T, index: number, arr: any) => unknown) | Partial<T> | [keyof T, unknown] | PropertyKey, optional): The condition to test each element. Can be a function, partial object, property-value pair, or property name. Default is the identity function.
• fromIndex (number, optional): The index to start the search from. If negative, it's calculated from the end of the array. Default is array.length - 1.

Returns

(number): Returns the index of the last element that satisfies the condition. Returns -1 if no element satisfies the condition.