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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? 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

How do you implement a debounce function using closures, and how is it a functional programming technique?

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Answer

debounce() returns a new function that wraps a closure over a timer variable, delaying the original function's execution until a pause in calls occurs — it's a higher-order function that takes a function and returns an enhanced function, a core FP pattern.
function debounce(fn, delay) {
  let timer;
  return (...args) => {
    clearTimeout(timer);
    timer = setTimeout(() => fn(...args), delay);
  };
}
const debouncedSearch = debounce(runSearch, 300);
Real-world example Delaying an API search request until the user stops typing for 300ms, instead of firing on every keystroke.

Common follow-ups: How does debounce differ from throttle, and when would you choose each?

Performance Optimization: Debouncing Throttling & Memoization
All 10 Functional Programming questions