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findLastIndex()

findLastIndex<T>(array, predicate): number

Returns the index of the last element that satisfies the predicate, or -1 if none found.

note

Prefer this over native Array.findLastIndex() for ES2020 compatibility.

Type Parameters​

T: T​

The type of elements in the array.

Parameters​

array: readonly T[]​

The source array to inspect.

predicate: (value, index, array) => boolean​

The function invoked per iteration.

Returns: number​

The index of the found element, else -1.

See Also​

Array.findLastIndex()

Since​

2.0.0

Performance​

O(n) time, O(1) space, reverse iteration with early return on match.

Also known as​

findLastIndex (Lodash, es-toolkit, Remeda, Ramda, Effect) · ❌ (Radashi, Modern Dash, Antfu)

Example​

findLastIndex([1, 2, 3, 4, 3, 2, 1], (v) => v > 2);
// => 4

findLastIndex(
  [{ name: 'Bob', age: 30 }, { name: 'Alice', age: 30 }],
  (u) => u.age === 30
);
// => 1

How it works?​

Returns the index of the last element that matches the predicate. Iterates from right to left.

Use Cases​

Find the last occurrence index of a matching element 📌​

Locate the index of the last element that satisfies a condition. Perfect for finding the most recent match position in logs, history, or ordered data.

const activities = [
  { type: "login", time: "08:00" },
  { type: "purchase", time: "09:30" },
  { type: "login", time: "12:00" },
  { type: "logout", time: "18:00" },
];

const lastLoginIndex = findLastIndex(activities, (a) => a.type === "login");
// => 2

Find the last valid entry index before errors​

Identify the position of the last successful or valid record in a sequence. Perfect for error recovery, checkpointing, or rollback scenarios.

const operations = [
  { id: 1, status: "success" },
  { id: 2, status: "success" },
  { id: 3, status: "failed" },
  { id: 4, status: "failed" },
];

const lastSuccessIndex = findLastIndex(
  operations,
  (op) => op.status === "success"
);
// => 1

Locate the last element index meeting a numeric threshold​

Find the index of the last value that passes a numeric condition. Perfect for analyzing trends, finding peaks, or locating threshold crossings.

const temperatures = [18, 22, 25, 30, 28, 24, 19, 17];

const lastHotDayIndex = findLastIndex(temperatures, (temp) => temp >= 25);
// => 4 (value: 28)