All 10 Functional Programming questions
More in Functional Programming
What is a pure function? What does 'immutability' mean and why does functional code favor it? What is a higher-order function? 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 is function composition and how do you implement it?

Intermediate

Answer

Composition combines multiple small functions into one, where the output of each becomes the input to the next — typically implemented with a compose() or pipe() helper that reduces over an array of functions.
const pipe = (...fns) => (x) => fns.reduce((acc, fn) => fn(acc), x);
const addOne = n => n + 1;
const double = n => n * 2;
const transform = pipe(addOne, double);
transform(3); // (3+1)*2 = 8
Real-world example Building a data-processing pipeline that trims, validates, then formats user input in clearly separated steps.

Common follow-ups: What's the difference between pipe() and compose() in terms of execution order?

Destructuring Spread & Rest
All 10 Functional Programming questions