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.

Practice

1. Which method adds a new item to the end of a JavaScript array?

easy

A. shift()

B. pop()

C. push()

D. unshift()

Solution

1. Step 1: Understand array methods for adding/removing items

   push() adds items to the end, pop() removes from the end, shift() removes from the start, unshift() adds to the start.

2. Step 2: Identify the method that adds to the end

   push() is the method that adds new items at the end of the array.

3. Final Answer:

   push() -> Option C

4. Quick Check:

   push() adds item at end [OK]

Hint: Remember push adds at end, pop removes from end [OK]

Common Mistakes:

• Confusing push() with pop()
• Using shift() to add items
• Using unshift() to add at end

2. Which of the following is the correct syntax to remove the first item from an array named arr?

easy

A. arr.shift()

B. arr.unshift()

C. arr.push()

D. arr.pop()

Solution

1. Step 1: Recall array methods for removing items

   pop() removes last item, shift() removes first item, push() adds last, unshift() adds first.

2. Step 2: Choose method that removes first item

   shift() removes the first item from the array.

3. Final Answer:

   arr.shift() -> Option A

4. Quick Check:

   shift() removes first item [OK]

Hint: shift() removes first, pop() removes last [OK]

Common Mistakes:

• Using pop() to remove first item
• Using push() or unshift() to remove items
• Confusing shift() with unshift()

3. What is the output of the following code?

const fruits = ['apple', 'banana', 'cherry'];
const index = fruits.indexOf('banana');
if (index !== -1) {
  fruits.splice(index, 1);
}
console.log(fruits);

medium

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

B. ['banana', 'cherry']

C. ['apple']

D. ['apple', 'cherry']

Solution

1. Step 1: Find the index of 'banana' in the array

   indexOf('banana') returns 1 because 'banana' is at position 1.

2. Step 2: Remove 'banana' using splice

   splice(1, 1) removes 1 item at index 1, so 'banana' is removed.

3. Final Answer:

   ['apple', 'cherry'] -> Option D

4. Quick Check:

   Removed 'banana' from array [OK]

Hint: indexOf finds position, splice removes item at that position [OK]

Common Mistakes:

• Not checking index before splice
• Using splice with wrong index
• Expecting splice to return new array

4. The following code is intended to remove 'orange' from the array, but it doesn't work as expected. What is the error?

const colors = ['red', 'green', 'blue'];
const index = colors.indexOf('orange');
colors.splice(index, 1);
console.log(colors);

medium

A. indexOf returns -1 if item not found, causing splice to remove last item

B. splice() is used incorrectly; it should be pop()

C. indexOf should be replaced with includes()

D. splice() cannot remove items by index

Solution

1. Step 1: Check indexOf result for 'orange'

   Since 'orange' is not in the array, indexOf returns -1.

2. Step 2: Understand splice behavior with negative index

   splice(-1, 1) removes the last item, so 'blue' is removed unexpectedly.

3. Final Answer:

   indexOf returns -1 if item not found, causing splice to remove last item -> Option A

4. Quick Check:

   Check index before splice to avoid removing wrong item [OK]

Hint: Check indexOf result before splice to avoid errors [OK]

Common Mistakes:

• Not checking if index is -1
• Using includes() instead of indexOf for index
• Assuming splice can't remove by index

5. Given an array nums = [1, 2, 3, 4, 5], which code snippet correctly removes all even numbers from the array?

hard

A. nums = nums.filter(n => n % 2 === 0);

B. nums = nums.filter(n => n % 2 !== 0);

C. nums.splice(1, 2);

D. nums.pop(); nums.pop();

Solution

1. Step 1: Understand filtering even numbers

   To remove even numbers, keep only numbers where n % 2 !== 0 (odd numbers).

2. Step 2: Check each option

   nums.filter(n => n % 2 !== 0) keeps odd numbers, removing all evens. nums.filter(n => n % 2 === 0) keeps evens. nums.splice(1, 2) removes items at fixed indexes. nums.pop() twice removes only the last two items.

3. Final Answer:

   nums = nums.filter(n => n % 2 !== 0); -> Option B

4. Quick Check:

   filter odd numbers to remove even [OK]

Hint: Use filter with condition to keep desired items [OK]

Common Mistakes:

• Using filter condition backwards
• Using splice without index knowledge
• Using pop to remove specific items