All 10 Functional Programming questions
More in Functional Programming
What does 'immutability' mean and why does functional code favor it? What is a higher-order function? What is function composition and how do you implement it? What is currying and what problem does it solve? What does 'referential transparency' mean, and how does it enable optimizations like memoization? How do you implement a debounce function using closures, and how is it a functional programming technique? What are functors and monads, at a practical level, and does JavaScript have native support for them? How do you avoid deeply nested, mutation-heavy code when updating nested state immutably?
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JavaScript › Functional Programming

Why are recursion and immutable data often used together in functional style, instead of loops with mutable counters?

Intermediate

Answer

Recursion naturally avoids mutable loop counters and accumulator variables by passing updated state as arguments to each recursive call, keeping each step's data immutable and self-contained — matching the broader functional preference for avoiding shared mutable state.
function sum(arr, acc = 0) {
  if (arr.length === 0) return acc;
  const [first, ...rest] = arr;
  return sum(rest, acc + first); // no mutable counter
}
sum([1,2,3]); // 6
Real-world example Processing a linked-list-like or tree-like data structure where a recursive, immutable approach mirrors its shape naturally.

Common follow-ups: What's the risk of using deep recursion in JavaScript, given it lacks guaranteed tail-call optimization?

Error Handling
All 10 Functional Programming questions