String Reversal Approaches in DSA Javascript - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to reverse a string using built-in methods.

DSA Javascript

function reverseString(str) {
  return str.split('').[1]().join('');
}

Drag options to blanks, or click blank then click option'

Areverse

Bsort

Cmap

Dfilter

Attempts:

3 left

💡 Hint

Common Mistakes

Using sort() instead of reverse() changes order alphabetically, not reverses.

Using map() or filter() does not reverse the array.

The split('') method converts the string into an array of characters. The reverse() method reverses the array. Then join('') combines the characters back into a string.

2fill in blank

medium

Complete the code to reverse a string using a for loop.

DSA Javascript

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

Drag options to blanks, or click blank then click option'

A>=

B>

C<=

D<

Attempts:

3 left

💡 Hint

Common Mistakes

Using '<' or '<=' causes the loop to never run or run incorrectly.

Using '>' excludes the first character at index 0.

The loop starts from the last index (str.length - 1) and continues while i >= 0 to include the character at index 0.

3fill in blank

hard

Fix the error in the recursive string reversal function.

DSA Javascript

function reverseString(str) {
  if (str.length === 0) {
    return '';
  }
  return reverseString(str.slice(1)) + str[1](0);
}

Drag options to blanks, or click blank then click option'

Aslice

BcharAt

Csubstr

Dsubstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using slice(0) returns the entire string, causing infinite recursion.

substr and substring can work but are less clear for single character extraction.

To get the first character of the string, charAt(0) is the correct method. Using slice(0) returns the whole string, causing infinite recursion.

4fill in blank

hard

Fill both blanks to create a reversed string using reduce.

DSA Javascript

function reverseString(str) {
  return str.split('').[1]((acc, char) => char [2] acc, '');
}

Drag options to blanks, or click blank then click option'

Areduce

B+

Dconcat

Attempts:

3 left

💡 Hint

Common Mistakes

Using concat instead of '+' may cause errors since concat is for arrays.

Adding accumulator before char reverses the string incorrectly.

The reduce method accumulates characters in reverse order by adding the current character before the accumulator. Using '+' concatenates strings.

5fill in blank

hard

Fill the first two blanks to reverse a string using a while loop.

DSA Javascript

function reverseString(str) {
  let reversed = '';
  let i = str.length [1];
  while (i [2] 0) {
    reversed += str[i];
    i--;
  }
  return reversed;
}

Drag options to blanks, or click blank then click option'

A- 1

B>

D>=

Attempts:

3 left

💡 Hint

Common Mistakes

Starting i at str.length causes out-of-bounds access.

Using '>' excludes the character at index 0.

Incorrect indexing inside loop causes wrong characters.

Start i at str.length - 1 (last index). Loop while i >= 0 to include all characters including index 0.

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