findLastIndex

INFO

This function is only available in es-toolkit/compat for compatibility reasons. It either has alternative native JavaScript APIs or isn’t fully optimized yet.

When imported from es-toolkit/compat, it behaves exactly like lodash and provides the same functionalities, as detailed here.

Similar to findIndex, findLastIndex returns the index of the first element that matches the condition, but it starts searching from the right.

You can specify the condition in several ways:

• Predicate function: If you provide a predicate function, the function will be applied to each item. The first item that makes the predicate function return true will be selected.
• Partial object: If you provide a partial object, the function will return the first item that matches the properties of the partial object.
• Property-value pair: If you provide a property-value pair, the function will return the first item that matches the property and value from the pair.
• Property name: If you provide a property name, the function will return the first item where the specified property has a truthy value.

Signature

function findLastIndex<T>(
  arr: T[],
  doesMatch: (item: T, index: number, arr: T[]) => unknown,
  fromIndex?: number
): number;
function findLastIndex<T>(arr: T[], doesMatch: Partial<T>, fromIndex?: number): number;
function findLastIndex<T>(arr: T[], doesMatch: [keyof T, unknown], fromIndex?: number): number;
function findLastIndex<T>(arr: T[], doesMatch: string, fromIndex?: number): number;

Parameters

• arr (T[]): The array to search through.

arr can be ArrayLike<T> or null or undefined

To ensure full compatibility with lodash, the findLastIndex function processes arr as follows:

• If arr is ArrayLike<T>, it converts it to an array using Array.from(...).
• If arr is null or undefined, it is treated as an empty array.

• doesMatch:

  • Predicate function ((item: T, index: number, arr: readonly T[]) => unknown): A function that takes an item, its index, and the array, and returns a truthy value if the item matches the criteria.
  • Partial object (Partial<T>): A partial object that specifies the properties to match.
  • Property-value pair ([keyof T, unknown]): An array where the first element is the property key and the second element is the value to match.
  • Property name (string): The name of the property to check for a truthy value.

• fromIndex (number): The index to start the search from, defaults to the last index of the array (arr.length - 1).

Returns

(number): The index of the first item that has the specified property value, or -1 if no match is found.

Examples

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

// Using a predicate function
const items = [1, 2, 3, 4, 5];
const result = findLastIndex(items, item => item > 3);
console.log(result); // 4

// Using a partial object
const items = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
];
const result = findLastIndex(items, { name: 'Bob' });
console.log(result); // 1

// Using a property-value pair
const items = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
];
const result = findLastIndex(items, ['name', 'Alice']);
console.log(result); // 0

// Using a property name
const items = [
  { id: 1, name: 'Alice' },
  { id: 2, name: 'Bob' },
];
const result = findLastIndex(items, 'name');
console.log(result); // 1