Key differences

Object pitfalls

const o = {};
o["__proto__"] = "x";       // doesn't set a string key; touches prototype
o.constructor = "y";        // shadow built-in property

Mitigate with Object.create(null):

const safe = Object.create(null);
safe.__proto__ = "x";       // now just a regular string key

Map advantages

const m = new Map();
m.set({ id: 1 }, "alice");
m.set(NaN, "weird");
m.size;                     // 2
m.get(NaN);                 // "weird" (Map treats NaN consistently)

• Object keys store user-by-object: no stringification.
• Iteration order is insertion order, strictly.
• size is O(1).
• Frequent add/remove is faster than delete obj[k] (which can deoptimize shape).

When to use which

WeakMap

Like Map but keys are objects only, weakly held. When the key object is GC'd, the entry vanishes — great for "private state per object" patterns and memoization keyed on object identity.

Performance reality

For small fixed-shape objects (records), object literals are faster than Maps thanks to V8's hidden-class optimization. For dynamic, keyed-by-anything collections that change a lot, Map wins.

Serialization

const m = new Map([[1, "a"], [2, "b"]]);
JSON.stringify(m);             // "{}" — doesn't work
JSON.stringify([...m]);        // "[[1,"a"],[2,"b"]]"
JSON.stringify(Object.fromEntries(m));    // "{\"1\":\"a\",\"2\":\"b\"}"

Map needs help to serialize.

Iteration

for (const [k, v] of m) ...
m.forEach((v, k) => ...);

Plus m.keys(), m.values(), m.entries().

Interview framing

"Object: string/symbol keys, prototype chain (collision hazards like __proto__), good for fixed-shape data and JSON. Map: any keys including objects, no prototype chain, preserves insertion order strictly, has O(1) .size, and is faster for frequent add/remove. Use Object for records and JSON. Use Map when keys can be objects, you need ordered iteration, you'll add/remove a lot, or you need .size. WeakMap for object-keyed entries that should die with the key. Object.create(null) if you want object semantics but no prototype chain."