Pure Functions — JS Technical Interview Questions & Answers

Same input → same output, no side effects.

Pure Functions

  • Pure Functions (js) is a JS topic. In plain words you use it for add(a, b) in a checkout total. Don’t start with a slogan — start with that picture.
  • Smallest example: function add / const add = () =>. Type it, run it, and say what you see. If you can do that from memory, you know Pure Functions (js).
  • From the example next to this theory: Same input → same output, no side effects.
  • Trap: this surprise in arrow vs function. Fix that before you talk about advanced JS.
  • Viva: what is Pure Functions (js)? Then show function add / const add = () =>. Then name the trap.

Technical Interview Tip: What is Pure Functions (js)? Show this: function add / const add = () =>. Trap: this surprise in arrow vs function.

Code Examples for Pure Functions

Pure Functions — working example

function greet(name = "friend") {
  return "Hi " + name;
}
const shout = function (msg) {
  return msg.toUpperCase();
};
(function () {
  console.log("IIFE ran");
})();

function mapNums(nums, fn) {
  const out = [];
  for (const n of nums) out.push(fn(n));
  return out;
}
console.log(greet("Ada"), shout("js"));
console.log(mapNums([1, 2], (n) => n * 2));

function factorial(n) {
  if (n <= 1) return 1;
  return n * factorial(n - 1);
}
console.log(factorial(4));

Pure Functions — functions take parameters, can be expressions/IIFEs/callbacks, and recurse with a base case. Pure functions avoid side effects.

How to Explain Pure Functions to an Interviewer

What is Pure Functions (js)?

Used for add(a, b) a checkout total Not just the heading

Give one small example of Pure Functions (js).

function add / const add = () => Same input → same output, no side effects

What mistake do beginners make with Pure Functions (js)?

this surprise in arrow vs function Show the fix

Where do you use Pure Functions (js)?

a checkout total add(a, b)