Base Case
Recursion is when a function calls itself.
A base case stops the recursion to prevent infinite calls.
JavaScript Tutorial
JavaScript Recursion
Recursion is a technique where a function calls itself to solve a smaller version of a problem.
It is powerful for nested structures and divide-and-conquer problems.
Why We Need It
Some problems are naturally recursive, such as tree traversal and hierarchical data.
Understanding recursion helps you choose the right approach for complex tasks.
Syntax
function fn(n) {
if (base) return value;
return fn(n - 1);
}Basic Example
1. Factorial
function factorial(n) {
if (n <= 1) return 1;
return n * factorial(n - 1);
}
console.log(factorial(5));Base case stops at n <= 1.
Real World Example
2. Sum array
function sum(arr, i = 0) {
if (i === arr.length) return 0;
return arr[i] + sum(arr, i + 1);
}
console.log(sum([1, 2, 3]));Recursively accumulate array values.
Multiple Use Cases
Divide and Conquer
Recursion solves problems by reducing them into smaller versions.
Tree traversal, factorial, and Fibonacci are classic examples.
Performance Considerations
Recursion can be less efficient due to call stack overhead.
Use iteration for simple loops, recursion for hierarchical structures.
More Examples
3. Countdown
function countdown(n) {
if (n === 0) return;
console.log(n);
countdown(n - 1);
}
countdown(3);Reduce the input until the base case.
4. Tree traversal
const tree = { value: 1, children: [{ value: 2 }, { value: 3 }] };
function walk(node) {
console.log(node.value);
if (!node.children) return;
node.children.forEach(walk);
}
walk(tree);Recursion fits nested structures naturally.
Comparison
Without
let total = 0;
for (let i = 1; i <= 5; i++) {
total *= i;
}With
function factorial(n) {
if (n <= 1) return 1;
return n * factorial(n - 1);
}Common Mistakes and Fixes
Missing base case
Always define a clear stopping condition.
Stack overflow
Avoid deep recursion or use iteration where possible.
Mutating shared state
Prefer passing data through parameters.
Interview Questions
What is a base case?
The condition that stops recursion.
Why can recursion be slow?
Each call adds stack overhead.
When is recursion useful?
For trees, graphs, and divide-and-conquer problems.
Practice Problem
Practice: Write a recursive function to compute the sum of numbers from 1 to n.
// TODO: sumTo(n)
One Possible Solution
function sumTo(n) {
if (n <= 1) return 1;
return n + sumTo(n - 1);
}
console.log(sumTo(4));Frequently Asked Questions
What is recursion?
A function calling itself to solve smaller subproblems.
Why is a base case important?
It stops recursion and prevents infinite calls.
When should I avoid recursion?
For very deep loops or performance-critical paths.
Try It Yourself
Try different values to see recursive calls in action.