Array.findLastIndex()

The Array.findLastIndex() method in JavaScript returns the index of the last element in the array that satisfies the provided testing function. If no elements satisfy the testing function, it returns -1.

Syntax

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findLastIndex(callbackFn)
findLastIndex(callbackFn, thisArg)

Parameters

ParameterDescription
callbackFnA function that is called for every element in the array, in reverse order, until it returns a truthy value. It takes three arguments: element (current array element), index (index of the element), and array (the array itself).
thisArgOptional. A value to use as this when executing callbackFn.

Return Value

The findLastIndex() method returns the index of the last element in the array that satisfies the callbackFn. If no such element is found, it returns -1.

Examples

1. Finding the Last Even Number

This example finds the last even number in an array and returns its index.

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const numbers = [5, 12, 8, 130, 44];

const lastEvenIndex = numbers.findLastIndex(num => num % 2 === 0);
console.log(lastEvenIndex);

Output

4
  1. The last even number is 44, located at index 4.

2. Finding Based on Object Property

This example finds the index of the last person over 30 in an array of objects.

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const people = [
    { name: 'Akash', age: 25 },
    { name: 'Bhairav', age: 35 },
    { name: 'Charlie', age: 40 }
];

const lastOver30Index = people.findLastIndex(person => person.age > 30);
console.log(lastOver30Index);

Output

2
  1. The last person over 30 is Charlie, located at index 2.

3. Using thisArg

The thisArg parameter is used to provide a custom context for the callback function.

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

const context = { threshold: 3 };

const lastAboveThreshold = numbers.findLastIndex(function(num) {
    return num > this.threshold;
}, context);

console.log(lastAboveThreshold);

Output

4
  1. The custom context sets a threshold of 3, and the last number greater than 3 is 5, located at index 4.

4. No Matching Element

When no elements satisfy the callbackFn, -1 is returned.

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const numbers = [1, 2, 3];

const index = numbers.findLastIndex(num => num > 5);
console.log(index);

Output

-1

Since no numbers are greater than 5, findLastIndex() returns -1.