Recursion on Arrays and Strings in DSA Javascript - Build from Scratch

Recursion on Arrays and Strings

📖 Scenario: Imagine you have a list of words from a story. You want to find out if a certain word appears in the list using a method that checks one word at a time by calling itself again and again until it finds the word or finishes the list.

🎯 Goal: Build a recursive function in JavaScript that checks if a word exists in an array of words by looking at one word at a time.

📋 What You'll Learn

Create an array called words with the exact values: ['apple', 'banana', 'cherry', 'date', 'fig']

Create a variable called targetWord and set it to the string 'cherry'

Write a recursive function called findWord that takes two parameters: arr (array) and index (number)

The function should return true if targetWord is found in arr, otherwise false

Use recursion to check each word starting from index 0

Print the result of calling findWord(words, 0)

💡 Why This Matters

🌍 Real World

Searching for items in lists is common in apps like contact lists, shopping carts, or word games.

💼 Career

Understanding recursion helps in solving problems that involve breaking down tasks into smaller parts, useful in coding interviews and algorithm design.

1

Create the array of words

Create an array called words with these exact values: 'apple', 'banana', 'cherry', 'date', 'fig'

DSA Javascript

// Create the array words with the given values
// Your code here

Hint

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

2

Set the target word to find

Create a variable called targetWord and set it to the string 'cherry'

DSA Javascript

const words = ['apple', 'banana', 'cherry', 'date', 'fig'];
// Create the variable targetWord with value 'cherry'
// Your code here

Hint

Use const to create a variable and assign the string 'cherry'.

3

Write the recursive function to find the word

Write a recursive function called findWord that takes parameters arr and index. It should return true if targetWord is found at arr[index] or in the rest of the array by calling itself with index + 1. Return false if index is beyond the last element.

DSA Javascript

const words = ['apple', 'banana', 'cherry', 'date', 'fig'];
const targetWord = 'cherry';
// Write the recursive function findWord(arr, index) below
// Your code here

Hint

Check if index is outside the array to stop recursion. Compare arr[index] with targetWord. Call findWord again with index + 1 if not found.

4

Print the result of the recursive search

Print the result of calling findWord(words, 0) using console.log

DSA Javascript

const words = ['apple', 'banana', 'cherry', 'date', 'fig'];
const targetWord = 'cherry';

function findWord(arr, index) {
    if (index >= arr.length) {
        return false;
    }
    if (arr[index] === targetWord) {
        return true;
    }
    return findWord(arr, index + 1);
}
// Print the result of findWord(words, 0)
// Your code here

Hint

Use console.log to show the result of findWord(words, 0).

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