Functional Programming Interview Questions in JavaScript

Functional Programming Interview Questions in JavaScript

Functional programming is a common interview topic. Here are the most asked questions with answers and code.

Q1: What is a pure function?

A function that given the same input always returns the same output and has no side effects. It does not modify external state or read mutable external state.

Q2: What is immutability in JavaScript?

Not mutating data. Instead of changing an object, you create a new copy with the changes. Use spread: { ...obj, x: 2 } or [...arr, 4].

Q3: Implement a compose function.

function compose(...fns) {
  return (x) => fns.reduceRight((acc, fn) => fn(acc), x);
}
``
### Q4: Implement a curry function.

```js
function curry(fn) {
  return function curried(...args) {
    if (args.length >= fn.length) return fn(...args);
    return (...next) => curried(...args, ...next);
  };
}
``
### Q5: Implement a memoize function.

```js
function memoize(fn) {
  const cache = {};
  return (...args) => {
    const key = JSON.stringify(args);
    if (!(key in cache)) cache[key] = fn(...args);
    return cache[key];
  };
}
``
### Q6: What is the difference between map and forEach?

`map` returns a new array (use for transformation). `forEach` returns `undefined` (use for side effects). Do not use `map` if you do not need the return value.

### Q7: What is a higher-order function?

A function that takes a function as an argument, returns a function, or both. Examples: `map`, `filter`, `reduce`, `compose`, `curry`, `setTimeout`.

### Q8: What is the difference between declarative and imperative code?

Imperative describes how (loops, mutations). Declarative describes what (`map`, `filter`, `reduce`). FP is declarative.

### Q9: What is function composition?

Chaining functions so each function's output is the next function's input. `compose(f, g)(x) = f(g(x))`. It builds complex behavior from simple functions.

### Q10: What is a side effect in JavaScript?

Anything that interacts with the outside world: modifying external state, I/O, network calls, DOM manipulation, console.log. Pure functions have no side effects.

### The Takeaway

FP interview questions test: pure functions, immutability, `compose`/`curry`/`memoize` implementations, `map` vs `forEach`, HOF definition, declarative vs imperative, function composition, and side effects. Practice implementing each utility from scratch.