What is a pure function and why does it matter?

A pure function satisfies two rules:

1. Referential transparency — given the same arguments, it always returns the same result.
2. No side effects — it does not mutate external state, I/O, DOM, globals, or arguments.

// Pure
const add = (a, b) => a + b

// Impure — depends on external state
let tax = 0.1
const withTax = (price) => price * (1 + tax)

// Impure — mutates input
const pushItem = (arr, item) => { arr.push(item); return arr }

Why interviewers care

• Predictable — easier to test and reason about.
• Cacheable — enables memoization.
• Parallel-safe — no hidden shared state.
• Works well with HOFs — map, filter, reduce assume non-mutating callbacks.

Pure functions pair naturally with immutability patterns (spread, map instead of in-place mutation). Real apps need side effects at the boundaries — the functional style pushes impure I/O to the edges and keeps core logic pure.