How Lodash sortedLastIndexBy Works in JavaScript

This article provides a comprehensive overview of the _.sortedLastIndexBy method from the Lodash JavaScript utility library. You will learn what the function does, understand its syntax and parameters, see practical code examples, and discover how it differs from similar search methods like _.sortedIndexBy.

What is _.sortedLastIndexBy?

The _.sortedLastIndexBy method uses a binary search algorithm to determine the highest index at which a given value should be inserted into an already sorted array in order to maintain its sort order.

Unlike standard index-finding methods that look for exact value matches, _.sortedLastIndexBy computes a criteria for each element using an iteratee function. If the value being checked matches one or more existing elements based on this iteratee, the method returns the index after the last matching element.

Syntax

_.sortedLastIndexBy(array, value, [iteratee=_.identity])

Parameters

  1. array (Array): The sorted array to inspect.
  2. value (*): The value to evaluate for insertion.
  3. iteratee (Function|string): The iteratee invoked per element to compute the sort criteria. Defaults to _.identity.

Returns

Code Examples

Example 1: Using a Property Name Shorthand

When dealing with arrays of objects, you can pass a string representing the object property by which the array is sorted.

const _ = require('lodash');

const users = [
  { 'name': 'Alice', 'age': 25 },
  { 'name': 'Bob',   'age': 30 },
  { 'name': 'Charlie', 'age': 30 },
  { 'name': 'Diana', 'age': 35 }
];

// Determine where to insert a user with age 30
const index = _.sortedLastIndexBy(users, { 'name': 'Eve', 'age': 30 }, 'age');

console.log(index);
// Output: 3

In this example, two users already have an age of 30 (indices 1 and 2). _.sortedLastIndexBy returns index 3, which places the new entry immediately after the last existing match.

Example 2: Using a Custom Iteratee Function

You can pass a custom function if the sorting logic depends on a computed value.

const _ = require('lodash');

const words = ['apple', 'banana', 'cherry', 'date'];

// The array is sorted by word length: 5, 6, 6, 4?
// Let's sort properly by length first:
const sortedByLength = ['date', 'apple', 'banana', 'cherry'];
// lengths: 4, 5, 6, 6

const newWord = 'orange'; // length 6

const index = _.sortedLastIndexBy(sortedByLength, newWord, function(word) {
  return word.length;
});

console.log(index);
// Output: 4

Because 'banana' and 'cherry' both have a length of 6, 'orange' is placed at index 4 (at the end of the matching group).

_.sortedIndexBy vs. _.sortedLastIndexBy

The primary distinction between _.sortedIndexBy and _.sortedLastIndexBy lies in how duplicates are handled:

const numbers = [{ x: 10 }, { x: 20 }, { x: 20 }, { x: 30 }];
const target = { x: 20 };

_.sortedIndexBy(numbers, target, 'x');     // Returns 1 (before existing 20s)
_.sortedLastIndexBy(numbers, target, 'x'); // Returns 3 (after existing 20s)

Important Considerations