JavaScript .findLastIndex()

The .findLastIndex() iterates through the array in reverse order and returns the index that passes the provided testing function. If no item passes, it returns -1.

Syntax

array.findLastIndex((element, index) => {...});

The function can be invoked with two arguments:

• element: The current element in the iteration.
• index (optional): The index of the array element.

Example

The following examples highlight the difference in how .findIndex() and .findLastIndex() iterate through an array:

let age = [13, 20, 15, 45, 1, 44, 80];

console.log('findIndex() method: ');

let findIndex = age.find((value, index) => {
  console.log('Checking age index: ', index);
  return value === 80;
});

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findIndex() method:
Checking age index: 0
Checking age index: 1
Checking age index: 2
Checking age index: 3
Checking age index: 4
Checking age index: 5
Checking age index: 6

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// Now .findLastIndex().

age = [13, 20, 15, 45, 1, 44, 80];

console.log('findLastIndex() method: ');

findIndex = age.findLastIndex((value, index) => {
  console.log('Checking age index: ', index);
  return value === 80;
});

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findLastIndex() method:
Checking age index: 6

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Note: If an element is known to be at the end of an array, using .findLastIndex can shorten the iteration path.