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findLastIndex javascript Code Snippet in 2024

arrayintermediate

Last Updated: 19 May 2024

Finds the index of the last element for which the provided function returns a truthy value.

  • Use Array.prototype.map() to map each element to an array with its index and value.
  • Use Array.prototype.filter() to remove elements for which fn returns falsy values
  • Use Array.prototype.pop() to get the last element in the filtered array.
  • Return -1 if there are no matching elements.
const findLastIndex = (arr, fn) => (arr .map((val, i) => [i, val]) .filter(([i, val]) => fn(val, i, arr)) .pop() || [-1])[0];
findLastIndex([1, 2, 3, 4], n => n % 2 === 1); // 2 (index of the value 3) findLastIndex([1, 2, 3, 4], n => n === 5); // -1 (default value when not found)

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