String Reversal Approaches in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of reversing string using split, reverse, join

What is the output of the following JavaScript code that reverses a string?

DSA Javascript

const str = "hello";
const reversed = str.split("").reverse().join("");
console.log(reversed);

A hello

B olleh

C undefined

D Error

💡 Hint

Think about what split, reverse, and join do to the string.

The string is split into an array of characters, reversed, then joined back into a string, resulting in the reversed string.

❓ Predict Output

intermediate

Output of manual string reversal with for loop

What is the output of this code that reverses a string manually using a for loop?

DSA Javascript

const str = "world";
let reversed = "";
for(let i = str.length - 1; i >= 0; i--) {
  reversed += str[i];
}
console.log(reversed);

A Error

B world

C undefined

D dlrow

💡 Hint

The loop starts from the end of the string and builds the reversed string.

The loop appends characters from the end to the start, creating the reversed string.

❓ Predict Output

advanced

Output of recursive string reversal function

What is the output of this recursive function that reverses a string?

DSA Javascript

function reverseString(str) {
  if (str === "") return "";
  return reverseString(str.substr(1)) + str[0];
}
console.log(reverseString("abcde"));

A abcde

B Error

C edcba

D edcbe

💡 Hint

The function calls itself with the string minus the first character, then adds the first character at the end.

The recursion builds the reversed string by moving the first character to the end after reversing the rest.

❓ Predict Output

advanced

Output of reversing string using reduce method

What is the output of this code that reverses a string using the reduce method?

DSA Javascript

const str = "reduce";
const reversed = str.split("").reduce((acc, char) => char + acc, "");
console.log(reversed);

A ecuder

B reduce

C Error

D edcuer

💡 Hint

Reduce accumulates characters by adding the current character before the accumulator.

The reduce method builds the reversed string by prepending each character to the accumulator.

❓ Predict Output

expert

Output of reversing string with spread operator and map

What is the output of this code that reverses a string using the spread operator and map?

DSA Javascript

const str = "maptest";
const reversed = [...str].map((_, i, arr) => arr[arr.length - 1 - i]).join("");
console.log(reversed);

A tsetpam

B maptest

C Error

D tsetmap

💡 Hint

The map function accesses characters from the end to the start using the index.

The map creates a new array by picking characters from the end to the start, then joins them to form the reversed string.

Practice

1. What does the JavaScript method split('') do when applied to a string?

easy

A. It reverses the string characters.

B. It removes all spaces from the string.

C. It joins array elements into a string.

D. It converts the string into an array of individual characters.

Solution

1. Step 1: Understand the split method

   The split('') method splits a string into an array by each character because the separator is an empty string.

2. Step 2: Effect on string

   For example, 'hello'.split('') becomes ['h', 'e', 'l', 'l', 'o'].

3. Final Answer:

   It converts the string into an array of individual characters. -> Option D

4. Quick Check:

   split('') = array of chars [OK]

Hint: split('') breaks string into chars array [OK]

Common Mistakes:

• Thinking split('') reverses the string
• Confusing split('') with join()
• Assuming split('') removes spaces

2. Which of the following is the correct syntax to reverse a string str in JavaScript using built-in methods?

easy

A. str.reverse().join('')

B. str.split('').reverse().join('')

C. reverse(str).split('').join('')

D. str.join('').reverse()

Solution

1. Step 1: Understand string and array methods

   Strings do not have a reverse() method, but arrays do. So first convert string to array using split('').

2. Step 2: Reverse array and join back

   Use reverse() on the array, then join('') to convert back to string.

3. Final Answer:

   str.split('').reverse().join('') -> Option B

4. Quick Check:

   split + reverse + join = reversed string [OK]

Hint: Split string, reverse array, join back [OK]

Common Mistakes:

• Trying to call reverse() directly on string
• Using join() before reverse()
• Using incorrect method order

3. What is the output of the following code?

const str = 'abc';
const reversed = str.split('').reverse().join('');
console.log(reversed);

medium

A. Error: reverse is not a function

B. 'abc'

C. 'cba'

D. ['c', 'b', 'a']

Solution

1. Step 1: Split string into array

   'abc'.split('') becomes ['a', 'b', 'c'].

2. Step 2: Reverse array and join

   ['a', 'b', 'c'].reverse() becomes ['c', 'b', 'a'], then .join('') makes it 'cba'.

3. Final Answer:

   'cba' -> Option C

4. Quick Check:

   split + reverse + join = 'cba' [OK]

Hint: Split, reverse, join outputs reversed string [OK]

Common Mistakes:

• Expecting original string output
• Expecting array output instead of string
• Confusing method order causing errors

4. Identify the error in this string reversal code:

function reverseString(s) {
  return s.reverse().join('');
}

medium

A. Strings do not have reverse() method.

B. join() cannot be used on strings.

C. Function missing return statement.

D. split('') is missing before reverse().

Solution

1. Step 1: Check string methods

   Strings do not have reverse(), so calling s.reverse() causes error.

2. Step 2: Fix by splitting string

   We must convert string to array first using s.split('') before reversing.

3. Final Answer:

   Strings do not have reverse() method. -> Option A

4. Quick Check:

   No reverse() method on strings [OK]

Hint: Strings do not have reverse() method [OK]

Common Mistakes:

• Trying to reverse string directly
• Ignoring need to split string
• Misunderstanding join() usage

5. Given the function below, what will reverseString('hello world') return?

function reverseString(s) {
  let reversed = '';
  for (let i = s.length - 1; i >= 0; i--) {
    reversed += s[i];
  }
  return reversed;
}

hard

A. 'dlrow olleh'

B. 'hello world'

C. 'helloworld'

D. Error: Cannot read property 'length' of undefined

Solution

1. Step 1: Loop from end to start

   The for loop starts at last character index and moves backward to 0, appending each character to reversed.

2. Step 2: Build reversed string

   Characters are added in reverse order, so 'hello world' becomes 'dlrow olleh'.

3. Final Answer:

   'dlrow olleh' -> Option A

4. Quick Check:

   Loop backward appends chars reversed [OK]

Hint: Loop backward to build reversed string [OK]

Common Mistakes:

• Looping forward instead of backward
• Not appending characters correctly
• Confusing string concatenation