First and Last Occurrence of Element in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Find first and last occurrence indexes

What is the output of the following JavaScript code that finds the first and last occurrence of the number 3 in the array?

DSA Javascript

const arr = [1, 3, 5, 3, 7, 3, 9];
const target = 3;

function firstLastOccurrence(array, x) {
  let first = -1;
  let last = -1;
  for (let i = 0; i < array.length; i++) {
    if (array[i] === x) {
      if (first === -1) first = i;
      last = i;
    }
  }
  return { first, last };
}

console.log(firstLastOccurrence(arr, target));

A {"first":1,"last":3}

B {"first":2,"last":5}

C {"first":0,"last":6}

D {"first":1,"last":5}

💡 Hint

Remember array indexes start at 0 and check all positions where the target appears.

The number 3 appears at indexes 1, 3, and 5. The first occurrence is at index 1 and the last at index 5.

🧠 Conceptual

intermediate

Understanding output when element is absent

What will be the output of the function when the target element is not present in the array?

DSA Javascript

const arr = [2, 4, 6, 8];
const target = 5;

function firstLastOccurrence(array, x) {
  let first = -1;
  let last = -1;
  for (let i = 0; i < array.length; i++) {
    if (array[i] === x) {
      if (first === -1) first = i;
      last = i;
    }
  }
  return { first, last };
}

console.log(firstLastOccurrence(arr, target));

A {"first":-1,"last":-1}

B {"first":0,"last":0}

C {"first":null,"last":null}

D undefined

💡 Hint

Check what the initial values of first and last are and if they change when the element is not found.

Since the target 5 is not in the array, the loop never updates first or last, so they remain -1.

❓ Predict Output

advanced

Output of recursive first occurrence search

What is the output of this recursive function that finds the first occurrence of a target in an array?

DSA Javascript

function firstOccurrenceRecursive(arr, target, index = 0) {
  if (index === arr.length) return -1;
  if (arr[index] === target) return index;
  return firstOccurrenceRecursive(arr, target, index + 1);
}

const array = [4, 2, 7, 2, 9];
console.log(firstOccurrenceRecursive(array, 2));

A 3

B 1

C -1

D 2

💡 Hint

The function checks elements from the start and returns the first index where the target is found.

The first 2 appears at index 1, so the function returns 1.

❓ Predict Output

advanced

Output of last occurrence using reverse loop

What is the output of this code that finds the last occurrence of the target 5 in the array by looping backwards?

DSA Javascript

const arr = [5, 1, 5, 3, 5, 7];
const target = 5;

function lastOccurrence(array, x) {
  for (let i = array.length - 1; i >= 0; i--) {
    if (array[i] === x) return i;
  }
  return -1;
}

console.log(lastOccurrence(arr, target));

A 4

B -1

C 2

D 0

💡 Hint

The loop starts from the end and returns the first matching index it finds.

The last 5 is at index 4, so the function returns 4.

🚀 Application

expert

Count elements between first and last occurrence

Given an array and a target element, what is the output of the code that counts how many elements are between the first and last occurrence of the target (exclusive)?

DSA Javascript

const arr = [1, 3, 5, 3, 7, 3, 9];
const target = 3;

function countBetweenFirstLast(array, x) {
  let first = -1;
  let last = -1;
  for (let i = 0; i < array.length; i++) {
    if (array[i] === x) {
      if (first === -1) first = i;
      last = i;
    }
  }
  if (first === -1 || last === -1 || first === last) return 0;
  return last - first - 1;
}

console.log(countBetweenFirstLast(arr, target));

A 5

B 2

C 3

D 4

💡 Hint

Count the number of elements strictly between the first and last occurrence indexes.

First occurrence of 3 is at index 1, last at index 5. Elements between are at indexes 2,3,4 which is 3 elements. The formula last - first - 1 = 3.