Pure Functions & Immutability — Series 3

Preview — 3 of 10 questions

Given these four functions, which one is pure?

javascript

let total = 0;

const a = (n) => n * 2;
const b = (n) => (total += n);
const c = (n) => { console.log(n); return n; };
const d = () => Math.random();

This function has no side effects, yet it is still impure. Why?

javascript

function greet(name) {
  return `Hello ${name}, it is ${new Date().getFullYear()}`;
}

AIt is pure — it never modifies anything outside itself.

BTemplate literals are impure because string building allocates memory.

CIt reads external state (the clock), so the same input can produce different output over time.

DIt is impure because it takes a parameter instead of reading a constant.

What is logged?

javascript

const base = Object.freeze([1, 2]);
const extended = base.concat(3);

console.log(base.length, extended.length, Object.isFrozen(extended));

A3 3 true

B2 3 false

C2 2 true

DTypeError: Cannot add property 2, object is not extensible

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