Common array operations in Javascript - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Array filter and map combination

What is the output of the following JavaScript code?

Javascript

const numbers = [1, 2, 3, 4, 5];
const result = numbers.filter(n => n % 2 === 0).map(n => n * 3);
console.log(result);

A [3, 9, 15]

B [6, 12]

C [2, 4]

D [1, 3, 5]

💡 Hint

Remember filter keeps only even numbers, then map multiplies each by 3.

The filter keeps only even numbers: 2 and 4. Then map multiplies each by 3, resulting in [6, 12].

❓ Predict Output

intermediate

Result of Array reduce for sum

What is the value of total after running this code?

Javascript

const values = [10, 20, 30];
const total = values.reduce((acc, val) => acc + val, 5);
console.log(total);

A 60

B SyntaxError

C 30

D 65

💡 Hint

The initial value for reduce is 5, so sum starts from 5.

Reduce sums all values starting from 5: 5 + 10 + 20 + 30 = 65.

❓ Predict Output

advanced

Output of Array splice method

What will be logged to the console after running this code?

Javascript

const arr = [1, 2, 3, 4, 5];
const removed = arr.splice(2, 2, 8, 9);
console.log(arr);
console.log(removed);

A arr: [1, 2, 8, 9], removed: [5]

B arr: [1, 2, 3, 4, 5], removed: []

C arr: [1, 2, 8, 9, 5], removed: [3, 4]

D arr: [8, 9, 5], removed: [1, 2, 3, 4]

💡 Hint

splice removes 2 elements starting at index 2 and inserts 8 and 9 there.

splice(2, 2, 8, 9) removes elements at index 2 and 3 (3 and 4), inserts 8 and 9 at index 2. The array becomes [1, 2, 8, 9, 5]. The removed elements are [3, 4].

❓ Predict Output

advanced

Output of Array sort with compare function

What is the output of this code?

Javascript

const fruits = ['banana', 'apple', 'cherry'];
fruits.sort((a, b) => b.localeCompare(a));
console.log(fruits);

A ['cherry', 'banana', 'apple']

B ['apple', 'banana', 'cherry']

C ['banana', 'apple', 'cherry']

D TypeError

💡 Hint

localeCompare returns positive if a is after b alphabetically. b.localeCompare(a) sorts descending.

The sort with b.localeCompare(a) sorts strings in descending alphabetical order: cherry, banana, apple.

🧠 Conceptual

expert

Which option causes a runtime error?

Which of the following array operations will cause a runtime error when executed?

A const arr = [1, 2, 3]; arr.forEach(x => { if(x > 1) throw new Error('Stop'); });

B const arr = [1, 2, 3]; const filtered = arr.filter(x => x > 1);

C const arr = [1, 2, 3]; const result = arr.map(x => x * 2);

D const arr = [1, 2, 3]; const first = arr.find(x => x > 3);

💡 Hint

Look for explicit error throwing inside the callback.

Option A throws an error explicitly inside forEach, causing a runtime error. Others run without errors.