All 10 Functional Programming questions
More in Functional Programming
What is a pure function? 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? Why are recursion and immutable data often used together in functional style, instead of loops with mutable counters? 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

What does 'immutability' mean and why does functional code favor it?

Beginner

Answer

Immutability means not modifying data after it's created — instead, you produce new data structures with the desired changes. This avoids bugs from shared mutable state and makes it easier to track how and when data changes.
// Mutating (avoid)
const arr = [1,2,3];
arr.push(4);

// Immutable (preferred)
const arr2 = [...arr, 4];
Real-world example Avoiding accidental shared-state bugs in React, where mutating state directly can cause missed re-renders.

Common follow-ups: Does immutability have a performance cost, and how is it usually managed?

Destructuring Spread & Rest
All 10 Functional Programming questions