Array Reversal Techniques in DSA Javascript - Build from Scratch

Array Reversal Techniques

📖 Scenario: Imagine you have a list of your favorite fruits in order. Sometimes you want to see them in reverse order, like reading a list from bottom to top.

🎯 Goal: You will create an array of fruits, set up a helper variable for the reversed array, reverse the array using a loop, and finally print the reversed list.

📋 What You'll Learn

Create an array called fruits with these exact values: 'apple', 'banana', 'cherry', 'date', 'elderberry'

Create an empty array called reversedFruits

Use a for loop with variable i starting from the last index of fruits down to 0

Inside the loop, add fruits[i] to reversedFruits

Print the reversedFruits array

💡 Why This Matters

🌍 Real World

Reversing arrays is useful when you want to display data in the opposite order, like showing recent messages first or undoing a list.

💼 Career

Understanding array manipulation and loops is fundamental for programming jobs, especially when working with data or user interfaces.

1

Create the fruits array

Create an array called fruits with these exact values: 'apple', 'banana', 'cherry', 'date', 'elderberry'

DSA Javascript

// Create the fruits array
// Your code here

Hint

Use square brackets [] to create an array and separate items with commas.

2

Create an empty array for reversed fruits

Create an empty array called reversedFruits

DSA Javascript

const fruits = ['apple', 'banana', 'cherry', 'date', 'elderberry'];
// Create an empty array called reversedFruits
// Your code here

Hint

Use [] to create an empty array and assign it to reversedFruits.

3

Reverse the array using a for loop

Use a for loop with variable i starting from fruits.length - 1 down to 0. Inside the loop, add fruits[i] to reversedFruits using push

DSA Javascript

const fruits = ['apple', 'banana', 'cherry', 'date', 'elderberry'];
const reversedFruits = [];
// Use a for loop to add fruits in reverse order to reversedFruits
// Your code here

Hint

Start i at fruits.length - 1 and decrease it until 0. Use push to add items to reversedFruits.

4

Print the reversed array

Print the reversedFruits array using console.log

DSA Javascript

const fruits = ['apple', 'banana', 'cherry', 'date', 'elderberry'];
const reversedFruits = [];
for (let i = fruits.length - 1; i >= 0; i--) {
    reversedFruits.push(fruits[i]);
}
// Print reversedFruits
// Your code here

Hint

Use console.log(reversedFruits) to show the reversed array.

Practice

1. What does the reverse() method do to an array in JavaScript?

easy

A. It removes duplicate elements from the array.

B. It sorts the array in ascending order.

C. It reverses the order of elements in the original array.

D. It creates a new array with elements doubled.

Solution

1. Step 1: Understand the reverse() method

   The reverse() method changes the original array by reversing the order of its elements.

2. Step 2: Confirm the effect on the original array

   It does not create a new array or sort; it simply reverses the existing array in place.

3. Final Answer:

   It reverses the order of elements in the original array. -> Option C

4. Quick Check:

   reverse() = reverses array order [OK]

Hint: reverse() flips array order in place [OK]

Common Mistakes:

• Thinking reverse() sorts the array
• Assuming reverse() creates a new array
• Confusing reverse() with removing duplicates

2. Which of the following is the correct syntax to reverse an array named arr in JavaScript?

easy

A. arr.reverse();

B. reverse(arr);

C. arr = reverse();

D. arr.reverse(arr);

Solution

1. Step 1: Identify the method call syntax

   The reverse() method is called on the array object itself, so the syntax is arr.reverse();.

2. Step 2: Check other options for errors

   Options A, B, and C are incorrect because reverse is not a standalone function and does not take arguments.

3. Final Answer:

   arr.reverse(); -> Option A

4. Quick Check:

   Method called on array object = arr.reverse() [OK]

Hint: Use arrayName.reverse() to reverse in place [OK]

Common Mistakes:

• Calling reverse as a standalone function
• Passing the array as an argument to reverse
• Assigning reverse() without calling on array

3. What will be the output of the following code?

const arr = [1, 2, 3, 4];
const reversed = arr.reverse();
console.log(reversed);

medium

A. undefined

B. [1, 2, 3, 4]

C. [1, 4, 3, 2]

D. [4, 3, 2, 1]

Solution

1. Step 1: Understand reverse() return value

   The reverse() method reverses the original array and returns the same reversed array.

2. Step 2: Trace the code output

   After calling arr.reverse(), arr becomes [4, 3, 2, 1], and reversed holds this reversed array. Logging reversed prints [4, 3, 2, 1].

3. Final Answer:

   [4, 3, 2, 1] -> Option D

4. Quick Check:

   reverse() returns reversed array [OK]

Hint: reverse() returns the reversed array itself [OK]

Common Mistakes:

• Expecting original array unchanged
• Thinking reverse() returns undefined
• Confusing reverse() with sorting

4. Find the error in the following code that tries to reverse an array nums:

function reverseArray(nums) {
  for(let i = 0; i < nums.length / 2; i++) {
    let temp = nums[i];
    nums[i] = nums[nums.length - i];
    nums[nums.length - i] = temp;
  }
  return nums;
}

medium

A. The index nums.length - i is out of bounds; should be nums.length - i - 1.

B. The temp variable is not needed for swapping.

C. The loop should run while i < nums.length / 2, not <=.

D. The function should return a new array, not modify nums.

Solution

1. Step 1: Check index calculations for swapping

   The code uses nums.length - i as the index for the element to swap, but array indices go from 0 to nums.length - 1, so it should be nums.length - i - 1.

2. Step 2: Confirm loop condition and swapping logic

   The loop condition i < nums.length / 2 is acceptable, and temp is needed for swapping. Modifying the original array is allowed here.

3. Final Answer:

   The index nums.length - i is out of bounds; should be nums.length - i - 1. -> Option A

4. Quick Check:

   Array indices start at 0, so subtract 1 [OK]

Hint: Array last index = length - 1, not length [OK]

Common Mistakes:

• Using nums.length - i instead of nums.length - i - 1
• Removing temp variable causing swap errors
• Incorrect loop boundary causing extra swaps

5. Given an array arr, which code snippet correctly reverses the array without modifying the original array?

hard

A. const reversed = arr.reverse();

B. const reversed = arr.slice().reverse();

C. const reversed = [...arr].sort().reverse();

D. const reversed = arr.reverse().map(x => x);

Solution

1. Step 1: Understand how to avoid modifying original array

   Using arr.slice() creates a shallow copy of the array. Calling reverse() on this copy reverses it without changing the original.

2. Step 2: Analyze other options

   const reversed = arr.reverse(); reverses the original array. const reversed = [...arr].sort().reverse(); sorts before reversing, changing order. const reversed = arr.reverse().map(x => x); reverses the original array because reverse() is called on arr before mapping.

3. Final Answer:

   const reversed = arr.slice().reverse(); -> Option B

4. Quick Check:

   Copy then reverse to keep original safe [OK]

Hint: Use slice() to copy before reverse() [OK]

Common Mistakes:

• Using reverse() directly modifies original array
• Sorting before reversing changes order
• Thinking reverse().map(x => x) preserves original array