When should you use a Map instead of a plain object?

Objects and Maps both store key-value pairs, but they optimise for different use cases. A Map is a purpose-built collection; an object is a language primitive with a prototype.

const counts = new Map();

counts.set("js", 3);
counts.set("css", 2);
counts.set(42, "answer");

console.log(counts.get("js")); // 3
console.log(counts.has(42)); // true
console.log(counts.size); // 3

for (const [key, value] of counts) {
  console.log(key, value);
}

Key differences

• Map keys can be objects, functions or any primitive; object keys are strings or symbols
• Map preserves insertion order for iteration
• Map has a size property; object size needs Object.keys(obj).length
• Map is not serialized by JSON.stringify by default
• Object literals inherit from Object.prototype, which can collide with keys such as toString

Use a Map when the key is not a string, when you need frequent add/delete operations, or when you need a reliable iteration order. Use an object when the data is a record with a known shape or must be serialized to JSON.

const cache = new Map();

function memoize(fn) {
  return (key) => {
    if (!cache.has(key)) cache.set(key, fn(key));
    return cache.get(key);
  };
}