[Easy] Implement findIndex function

Question Prompt

Imagine you're tasked with developing a function that efficiently locates specific elements within arrays. This function, named findIndex, should accept three key parameters:

1. array: The array to be searched.
2. predicate: A testing function that determines whether an element matches the desired criteria.
3. fromIndex (optional): A starting index for the search, defaulting to 0 if not specified.

Your goal is to implement findIndex so that it returns the index of the first element within array that satisfies the predicate function. If no matching element is found, it should return -1.

findIndex(array, predicate, [(fromIndex = 0)]);

findIndex([1, 13, 7, 54], (num: number) => num > 10)) // 1
findIndex([1, 13, 7, 54], (num: number) => num > 200)) // -1

Solutions

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To find the index, we can do the following

• First determine the accurate starting index for the search. If fromIndex is negative, adjust it by adding array.length. This is because the last item in an array is at index -1 and by that we can infer -1 is the same as array.length - 1. Thus, to adjust the negative index, we can do it by adding array.length.
• Then loop through the array, starting from the startIndex.
• For each element, call the predicate function, which acts as a "tester" to see if the element meets the specified criteria.
• If the predicate function returns true for an element, meaning a match is found, the function immediately returns that element's index within the array.
• If the loop completes without finding any elements that satisfy the predicate, the function returns -1 to indicate that no match was found.

function findIndex(array, predicate, fromIndex = 0) {
  const startIndex = fromIndex < 0 ? fromIndex + array.length : fromIndex;

  for (let i = startIndex; i < array.length; i++) {
    if (predicate(array[i])) {
      return i;
    }
  }

  return -1;
}