When is memoization a good performance strategy in JS, and when does it hurt?

Short Interview Answer

Memoize pure expensive functions with stable inputs; avoid memoizing cheap functions or unbounded caches that leak memory.

Detailed Explanation

Memoization trades memory for CPU. Good: parsing, complex derived data with repeated identical inputs. Bad: functions with unique inputs each call (cache thrash), impure functions, or caches without eviction. In UI frameworks, over-memoization adds comparison overhead and complexity. Profile first. Prefer algorithmic improvements over micro-memo. Use WeakMap when keys are objects that should not retain memory.

Example

function memoize(fn) {
  const cache = new Map();
  return (key) => {
    if (cache.has(key)) return cache.get(key);
    const v = fn(key);
    cache.set(key, v);
    return v;
  };
}

Interview Tip

Say 'measure first' — avoids premature optimization stigma.

Common Mistake

Unbounded memo caches on user-generated keys (memory leak).