Recursion on Arrays and Strings in DSA Javascript - Practice Problems & Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of Recursive Array Sum

What is the output of the following JavaScript code that sums elements of an array recursively?

DSA Javascript

function recursiveSum(arr, index = 0) {
  if (index === arr.length) return 0;
  return arr[index] + recursiveSum(arr, index + 1);
}

console.log(recursiveSum([1, 2, 3, 4]));

A 10

B 24

C 0

D undefined

💡 Hint

Think about how the function adds each element starting from index 0 until the end.

The function adds the current element to the sum of the rest of the array recursively. For [1,2,3,4], the sum is 1+2+3+4 = 10.

❓ Predict Output

intermediate

Output of Recursive String Reverse

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("code"));

A "edoc"

B "code"

C "odec"

D ""

💡 Hint

The function takes the first character and puts it at the end after reversing the rest.

The function calls itself on the substring excluding the first character, then adds the first character at the end, effectively reversing the string.

❓ Predict Output

advanced

Output of Recursive Array Flattening

What is the output of the following code that flattens a nested array recursively?

DSA Javascript

function flatten(arr) {
  let result = [];
  for (let i = 0; i < arr.length; i++) {
    if (Array.isArray(arr[i])) {
      result = result.concat(flatten(arr[i]));
    } else {
      result.push(arr[i]);
    }
  }
  return result;
}

console.log(flatten([1, [2, [3, 4], 5], 6]));

A undefined

B TypeError: arr.concat is not a function

C RangeError: Maximum call stack size exceeded

D [1, 2, 3, 4, 5, 6]

💡 Hint

Check how the function handles nested arrays and concatenates results.

The function correctly flattens the nested array by recursively concatenating inner arrays. The output is a flat array with all elements.

❓ Predict Output

advanced

Output of Recursive Palindrome Check

What is the output of this recursive function that checks if a string is a palindrome?

DSA Javascript

function isPalindrome(str) {
  if (str.length <= 1) return true;
  if (str[0] !== str[str.length - 1]) return false;
  return isPalindrome(str.slice(1, -1));
}

console.log(isPalindrome("racecar"));

A undefined

B false

C true

D TypeError

💡 Hint

The function compares first and last characters and checks the middle recursively.

The function returns true because "racecar" reads the same forwards and backwards.

🧠 Conceptual

expert

Recursion Depth for Large Arrays

What is the main risk of using simple recursion to process very large arrays in JavaScript?

A Memory leak because recursion stores all elements permanently

B Stack overflow error due to too many recursive calls

C Infinite loop because recursion never ends

D Syntax error because recursion is not allowed on arrays

💡 Hint

Think about how JavaScript handles function calls and memory for recursion.

JavaScript has a limited call stack size. Too many recursive calls cause a stack overflow error.

Practice

1. What does a recursive function on an array typically do to process the array elements?

easy

A. It uses loops instead of calling itself.

B. It processes one element and calls itself on the rest of the array.

C. It processes all elements at once without calling itself.

D. It only processes the first and last elements.

Solution

1. Step 1: Understand recursion on arrays

   Recursive functions handle one element and then call themselves on the smaller part (rest of the array).

2. Step 2: Identify the typical recursive pattern

   This pattern reduces the problem size by one element each call until the array is empty.

3. Final Answer:

   It processes one element and calls itself on the rest of the array. -> Option B

4. Quick Check:

   Recursion = process one + recurse rest [OK]

Hint: Recursion breaks problem into smaller parts [OK]

Common Mistakes:

• Thinking recursion processes all elements at once
• Confusing recursion with loops
• Ignoring the base case

2. Which of the following is the correct syntax to define a recursive function in JavaScript that takes an array as input?

easy

A. function recurse(arr) { if(arr.length === 0) return; recurse(arr.slice(1)); }

B. function recurse(arr) { while(arr.length > 0) recurse(arr); }

C. function recurse(arr) { for(let i=0; i

D. function recurse(arr) { if(arr.length === 0) return; recurse(arr.pop()); }

Solution

1. Step 1: Check base case and recursive call

   function recurse(arr) { if(arr.length === 0) return; recurse(arr.slice(1)); } correctly checks if array is empty and calls itself with the rest of the array using slice(1).

2. Step 2: Identify incorrect syntax in other options

   The version using while(arr.length > 0) recurse(arr); causes infinite recursion because it passes the same array; the version using for loop calls recurse on individual elements; the version using arr.pop() passes a single element, not an array.

3. Final Answer:

   function recurse(arr) { if(arr.length === 0) return; recurse(arr.slice(1)); } -> Option A

4. Quick Check:

   Base case + recurse smaller array = correct syntax [OK]

Hint: Base case + smaller input slice needed [OK]

Common Mistakes:

• Missing base case causing infinite recursion
• Passing wrong argument type in recursive call
• Using loops inside recursion incorrectly

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

function sumArray(arr) {
  if (arr.length === 0) return 0;
  return arr[0] + sumArray(arr.slice(1));
}
console.log(sumArray([1, 2, 3, 4]));

medium

A. 1234

B. Error

C. undefined

D. 10

Solution

1. Step 1: Understand the recursive sum logic

   The function adds the first element to the sum of the rest recursively until array is empty.

2. Step 2: Calculate sum step-by-step

   1 + sumArray([2,3,4]) = 1 + (2 + sumArray([3,4])) = 1 + 2 + 3 + 4 = 10

3. Final Answer:

   10 -> Option D

4. Quick Check:

   Sum of [1,2,3,4] = 10 [OK]

Hint: Sum = first element + sum of rest [OK]

Common Mistakes:

• Concatenating numbers as strings
• Forgetting base case returns 0
• Using arr.pop() which mutates array

4. Find the error in this recursive function that reverses a string:

function reverseStr(str) {
  if (str.length === 1) return str;
  return reverseStr(str.slice(1)) + str[0];
}
console.log(reverseStr('abc'));

medium

A. Base case should check for empty string, not length 1.

B. The recursive call should use str.slice(0, -1) instead of str.slice(1).

C. The function should return str[0] + reverseStr(str.slice(1)) instead.

D. No error, the function works correctly.

Solution

1. Step 1: Analyze base case condition

   Base case checks if string length is 1, but for empty string it should return empty string to avoid errors.

2. Step 2: Understand recursion on empty string

   If input is empty, function should return empty string to stop recursion safely.

3. Final Answer:

   Base case should check for empty string, not length 1. -> Option A

4. Quick Check:

   Base case empty string needed for safe recursion [OK]

Hint: Base case must handle empty string [OK]

Common Mistakes:

• Using wrong base case length
• Confusing slice indexes
• Assuming function works without base case

5. Given a nested array like [1, [2, [3, 4], 5], 6], which recursive approach correctly flattens it into [1, 2, 3, 4, 5, 6]?

hard

A. Return the original array as is without changes.

B. Use a loop to add elements without recursion.

C. Check if element is array, recursively flatten it, else add element to result.

D. Only flatten one level using slice and concat.

Solution

1. Step 1: Identify recursive flattening logic

   To flatten nested arrays, check each element: if array, recurse; else add to result.

2. Step 2: Understand why other options fail

   Loops alone can't flatten nested arrays fully; one-level flattening misses deeper nesting; returning original array does nothing.

3. Final Answer:

   Check if element is array, recursively flatten it, else add element to result. -> Option C

4. Quick Check:

   Recursive flatten = check array + recurse + add element [OK]

Hint: Recursively flatten nested arrays by checking element type [OK]

Common Mistakes:

• Flattening only one level
• Not checking if element is array
• Ignoring nested deeper arrays