javascriptarraysloopssearchcallback

How to loop through array with FindIndex from its end?

I need to get errorIdx = 3, but I get 0. How can I loop through array from its end?

  const scrollPosition = 5007
  const errorsHeight = [947, 2498, 3495, 4805, 5755]

  errorIdx = errorsHeight.findIndex((itemHeight: number) => itemHeight < scrollPosition)
  console.log(errorIdx) // 0
Solution

  • An alternative implementation of findLastIndex where the callback function is aware of a thisArg context/target and also will be invoked with its three parameters which are [element, index, array]; thus following the standard of findIndex ...

    function findLastIndex(arr, test, target) {
  if (!arr && ((arr ?? true) === true)) {
    throw new TypeError('findLastIndex called on null or undefined');
  };
  if (typeof test !== 'function') {
    throw new TypeError(`${ test } is not a function`);
  };
  if (!Array.isArray(arr)) {
    arr = Array.from(arr);
  }
  target = target ?? null;

  let isContinue = true;
  let idx = arr.length;

  // assures -1 as return value for nothing found.
  while ((idx >= 0) && isContinue) {

    // be aware of sparse array slots ... and ...
    // be a guard for the negative index default.
    if (arr.hasOwnProperty(--idx)) {

      isContinue = !test.call(target, arr[idx], idx, arr);
    }
  }
  return idx;
}

const errorsHeight = [947, 2498, 3495, 4805, 5755];
const scrollPosition = 5007;

console.log(
  findLastIndex(errorsHeight, (height/*, idx, arr*/) =>
    height < scrollPosition
  )
);
console.log(
  findLastIndex(errorsHeight, height => height < 5)
);
    .as-console-wrapper { min-height: 100%!important; top: 0; }