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Map vs Object

Map vs Object

Introduction

Both Map and Object store key-value pairs, but they have significant differences in key types, iteration, and performance.

Comparison Table

Feature	Map	Object
Key types	Any (primitives, objects, functions)	String or Symbol only
Order	Insertion order	Integer keys first, then insertion
Size	`.size` property	Manual (`Object.keys().length`)
Iteration	Directly iterable (`for...of`)	Needs `Object.keys/values/entries`
Prototype	None (no inherited keys)	Has Object.prototype
JSON	No native serialization	`JSON.stringify()` works
Performance	Better with frequent add/delete	Better for property access

When to Use Map

// ✅ Map for dynamic key collections
const userRoles = new Map();
userRoles.set(user1, 'admin');
userRoles.set(user2, 'user');
userRoles.set(user3, 'editor');

// ✅ Map for frequent add/delete
const cache = new Map();
function memoized(fn) {
  return function(key) {
    if (cache.has(key)) return cache.get(key);
    const result = fn(key);
    cache.set(key, result);
    return result;
  };
}

// ✅ Map when keys are objects or mixed types
const mixed = new Map();
mixed.set(1, 'number');
mixed.set('1', 'string');
mixed.set(true, 'boolean');
mixed.set({ obj: true }, 'object');

When to Use Object

// ✅ Object for fixed known keys
const config = {
  theme: 'dark',
  lang: 'en',
  debug: false
};

// ✅ Object when JSON serialization is needed
const data = { id: 1, name: 'Alice' };
const json = JSON.stringify(data);

// ✅ Object for a simple record/struct
const point = { x: 10, y: 20 };

Key Type Demonstration

// Object: keys are coerced to strings
const obj = {};
obj[42] = 'number';
obj['42'] = 'string'; // overwrites!
console.log(obj); // { '42': 'string' }

// Map: keys retain their type
const map = new Map();
map.set(42, 'number');
map.set('42', 'string');
console.log(map.get(42));  // 'number'
console.log(map.get('42')); // 'string'

Summary

Use Map for dynamic keys, object keys, frequent add/delete, and iteration
Use Object for fixed keys, JSON serialization, and simple records
Map has better performance for collections that change frequently
Object is more natural for configuration and data transfer

Further Reading