🚀 Frontend DSA 5 - 📊 How to Sum Values in a Singly Linked List in JavaScript (Recursive Approach)

Singly linked lists are a fundamental data structure every JavaScript developer should understand. They’re used in everything from memory-efficient data storage to implementing queues and stacks. 🚀

In this article, you'll learn how to traverse and sum all values in a singly linked list using a recursive approach in JavaScript. 🧠

📌 What’s a Singly Linked List?

A singly linked list is a collection of nodes where:

• Each node has a value.
• Each node has a reference (pointer) to the next node.
• The last node's next is null.

Example:

1 → 2 → 3 → 4 → null

Each node "points" to the next, just like a chain 🔗.

🧠 Problem: Calculate the Total Sum of All Values in the Linked List

You’re given a linked list:

let list = {
  value: 1,
  next: {
    value: 2,
    next: {
      value: 3,
      next: {
        value: 4,
        next: null
      }
    }
  }
};

You need to write a function that sums all the values in the list and returns the total ➕.

✅ JavaScript Recursive Solution

// 📌 Function to calculate sum of linked list values recursively
const printList = (inputList) => {
    // 🛑 Base case: If no next node, return current node's value
    if (inputList.next === null) {
        return inputList.value;
    } else {
        // 🔁 Recursive case: Add current value to sum of rest of list
        return inputList.value + printList(inputList.next);
    }
};

🔢 Sample Input and Output

🧪 Example 1

let list1 = {
  value: 1,
  next: {
    value: 2,
    next: {
      value: 3,
      next: {
        value: 4,
        next: null
      }
    }
  }
};

console.log(printList(list1)); // ✅ Output: 10

🧪 Example 2

let list2 = {
  value: 5,
  next: {
    value: 10,
    next: {
      value: 15,
      next: null
    }
  }
};

console.log(printList(list2)); // ✅ Output: 30

🧪 Example 3

let list3 = {
  value: 42,
  next: null
};

console.log(printList(list3)); // ✅ Output: 42

🧠 JavaScript Notes

• Recursion is when a function calls itself. In our case, each function call processes one node and passes the rest down.
• The base case (if(inputList.next === null)) ensures the recursion stops.
• This function returns a sum, not logs — great for unit tests! ✅
• Works well for small to medium-sized lists. For very large lists, consider an iterative version to avoid a stack overflow. ⚠️

🔄 Iterative Version (Bonus)

const printListIterative = (inputList) => {
  let sum = 0;
  let current = inputList;
  
  while (current !== null) {
    sum += current.value;
    current = current.next;
  }

  return sum;
};

✨ Final Thoughts

Learning to work with linked lists is 🔑 for improving your data structure and algorithm skills in JavaScript. Whether you're preparing for interviews or building custom data models, recursive and iterative traversal will always be handy.

🧑‍💻 Want to visualize the list traversal or convert this to a doubly linked list? Let me know — we’ll break it down step by step!

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