Lodash _.sortedLastIndexBy() method

Beginner
⏱️ 7 min read
📚 Updated: May 2026
🎯 3 Code examples
🚀 3 Try-it labs
Lodash

What you’ll learn

  • How _.sortedLastIndexBy(array, value, iteratee) finds the upper insertion boundary on projected keys.
  • Why pairing it with _.sortedIndexBy() counts duplicate projections in O(log n) time.
  • When function iteratees diverge from simple property paths.
  • Try each example in the editor (?tryit=1, 2, 3) with Lodash from a CDN.

Prerequisites

Comfort with _.sortedLastIndex() (bisect right without iteratees) and _.sortedIndexBy() (bisect left with iteratees) smooths the mental model.

  • Your dataset stays sorted ascending by whatever the iteratee measures.
  • You can run snippets in Node or open the Try-it labs in a browser.

Overview

_.sortedLastIndexBy merges iteratee ergonomics with bisect-right semantics: project rows and needles uniformly, then ask Lodash for the trailing edge of the band where that projection could live.

Upper fence

Duplicate streaks end where this index begins—ideal for histogram buckets or quota bands.

Iteratee parity

Strings, paths, or unary functions—Lodash applies the same rule to stored rows and the probe payload.

Count shortcut

Subtract paired left/right indexes to tally tied projections without extra scans.

Syntax

javascript
_.sortedLastIndexBy(array, value, iteratee)
  • array: ascending sorted by iteratee(element).
  • value: candidate compared through iteratee(value).
  • iteratee: Lodash shorthand as in other *By helpers.
  • Returns: greatest insertion index in [0, array.length] preserving ascending projected order.
1

Bracketing duplicate scores

Subtract sortedIndexBy from sortedLastIndexBy to learn how many rows share the same projection.

javascript
import sortedIndexBy from "lodash/sortedIndexBy";
import sortedLastIndexBy from "lodash/sortedLastIndexBy";

const rows = [
  { id: "a", score: 10 },
  { id: "b", score: 20 },
  { id: "c", score: 20 },
  { id: "d", score: 20 },
  { id: "e", score: 30 }
];
const needle = { id: "x", score: 20 };
const lo = sortedIndexBy(rows, needle, "score");
const hi = sortedLastIndexBy(rows, needle, "score");
const width = hi - lo;
// lo → 1, hi → 4, width → 3
Try it Yourself
2

Function iteratee

Even when each projection occurs once, bisect right lands after the equal neighbor—one slot past bisect left.

javascript
import sortedIndexBy from "lodash/sortedIndexBy";
import sortedLastIndexBy from "lodash/sortedLastIndexBy";

const words = ["a", "bb", "ccc"];
const len = (w) => w.length;
sortedIndexBy(words, "xx", len);      // → 1
sortedLastIndexBy(words, "xx", len); // → 2
Try it Yourself
3

Property path gap

Between distinct years both helpers agree—the interval collapses to a single insertion point.

javascript
import sortedLastIndexBy from "lodash/sortedLastIndexBy";

const releases = [
  { name: "v1", year: 2000 },
  { name: "v2", year: 2010 },
  { name: "v3", year: 2020 }
];
sortedLastIndexBy(releases, { name: "patch", year: 2015 }, "year");
// → 2
Try it Yourself

📋 _.sortedLastIndexBy vs _.sortedIndexBy vs _.sortedLastIndex

APIEdge
_.sortedIndexBy(...)Bisect left on projections.
_.sortedLastIndexBy(...)Bisect right on projections.
_.sortedLastIndex(array, value)Bisect right without an iteratee.

Pitfalls to avoid

Sort

Projection mismatch

Sorting by one field but indexing another breaks monotonicity—keep iteratee aligned with the comparator that produced the array.

Eq

sortedLastIndexOf confusion

Membership helpers return -1 on misses; insertion helpers always pick a fence inside [0, length].

Cost

Heavy iteratees

Each probe evaluates the iteratee—prefer cheap getters when arrays grow large.

❓ FAQ

No. Like every sortedIndex* helper, it returns an integer boundary only.
Projections must already be sorted ascending; the iteratee must match the field or derivation used when the array was ordered.
sortedIndexBy performs bisect left on iteratee outputs; sortedLastIndexBy performs bisect right—widening the insertion interval when duplicates share the same projection.
No. sortedLastIndexOf searches for the last equal element (or returns -1). sortedLastIndexBy always reports an insertion index in [0, length], even when the needle is absent.
Use import sortedLastIndexBy from "lodash/sortedLastIndexBy" or require("lodash/sortedLastIndexBy") for tree-shaking friendly bundles.

Summary

Did you know?

_.sortedIndexBy() pins the lower edge of a projection band; sortedLastIndexBy closes the upper edge—subtract the two to count rows sharing the same projected key without scanning.

About the author

Mari Selvan M P
Mari Selvan M P 🔗

Developer, cloud engineer, and technical writer

  • Experience 12 years building web and cloud systems
  • Focus Full Stack Development, AWS, and Developer Education

I write practical tutorials so students and working developers can learn by doing—from databases and APIs to deployment on AWS.

AWS Certified Developer – Associate (Credly)

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Lodash _.sortedLastIndex() Lodash _.sortedLastIndexOf()