JS Machine Coding Questions — Implementations You Must Know

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Memoize

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—Cache the result of a pure function so repeated calls with the same arguments skip recomputation.
—Key insight: Use a Map keyed by JSON.stringify(args). Works best for pure functions with primitive arguments.
—Gotcha: JSON.stringify fails for functions, undefined, circular refs — use a custom key for complex args.
—Gotcha: Each memoized function gets its own independent cache.
—Gotcha: Not useful for functions with side effects.

function memoize(fn) {
  const cache = new Map();
  return function (...args) {
    const key = JSON.stringify(args);
    if (cache.has(key)) return cache.get(key);
    const result = fn.apply(this, args);
    cache.set(key, result);
    return result;
  };
}

const factorial = memoize(function f(n) {
  return n <= 1 ? 1 : n * f(n - 1);
});
factorial(5); // computed → 120
factorial(5); // from cache → 120