When to Prefer Map Over Object in JavaScript

JavaScript objects are versatile, allowing you to group data together easily. But since ES6, the Map object has entered the scene, offering unique capabilities for certain use cases. While many developers default to using objects for everything, this can lead to performance and security issues.

In this article, we’ll explore the differences between Object and Map, their use cases, performance characteristics, and when you should choose one over the other.

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TL;DR

• Use Object for fixed, finite records like configuration objects or one-time use data structures.
• Use Map for dynamic dictionaries or hash maps with frequent updates, especially when keys aren't known ahead of time.
• Map is generally faster than Object for insertion, deletion, and iteration, and it consumes less memory.

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Why Object Falls Short as a Hash Map

Key Limitations

1. String-Only Keys
   Object keys must be strings or symbols. Non-string keys are converted to strings, leading to unexpected behavior:

   const foo = [];
   const obj = { [foo]: "value" };
   
   console.log(obj); // { "": "value" }

2. Inheritance Issues
   By default, objects inherit from Object.prototype, leading to unwanted properties and potential prototype pollution:

   const hashMap = {}; // inherits from Object.prototype
   
   console.log(hashMap.toString); // [Function: toString]

   This can also cause name collisions when a user-defined property shadows an inherited one.

3. Cumbersome API
   Common tasks like determining size, clearing properties, or checking existence are more complicated with objects compared to Map:

   const obj = { a: undefined };
   
   console.log(Object.keys(obj).length); // 1 (size workaround)
   console.log(obj.hasOwnProperty("a")); // true

4. Performance Overhead
   Objects require additional workarounds for tasks like iterating over keys or deleting properties.

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Why Map Excels for Hash Maps

Key Advantages

1. Flexible Key Types
   Map supports any data type as keys, including objects and functions:

   const map = new Map();
   const key = {};
   map.set(key, "value");
   
   console.log(map.get(key)); // "value"

2. Better Ergonomics
   Map offers dedicated APIs for common tasks:

   • size: Easily get the number of entries.
   • clear: Remove all entries in one operation.
   • has: Check if a key exists.
   • get: Retrieve a value associated with a key.

3. Performance Benefits
   Benchmarks show that Map is faster than Object for most operations, especially as the number of entries grows.

   Operation	Object	Map
   Insertion	Slower	Faster
   Deletion	Moderate	Faster
   Iteration	Slower	Faster

4. Memory Efficiency
   Map consumes less memory than an object of the same size.

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Performance Benchmarks

String Keys

For non-numeric string keys, Map outperforms Object in all operations:

• Insertion: Map is initially twice as fast as Object but converges as the size grows.
• Iteration: Map maintains a consistent advantage.

Integer Keys

For integer keys (e.g., 1, 2), objects can sometimes outperform Map due to V8 optimizations, particularly for small-sized objects. However, as size increases, Map regains its advantage.

Memory Usage

Map consistently uses 20–50% less memory than Object.

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When to Use Map Over Object

Use Object When:

• The structure has a fixed set of properties (e.g., a configuration object).
• You prioritize simplicity over performance.
• Keys are known and limited to strings or symbols.

Use Map When:

• You need a dynamic key-value store with frequent updates.
• Keys are of diverse types (e.g., objects, numbers, or functions).
• Iteration order and performance are critical.
• You need a reliable size API or frequent key checks.

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Browser Compatibility

Map is widely supported in modern browsers, including IE 11 (though IE 11 is officially obsolete). Unless you're targeting legacy environments, you can safely use Map.

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Conclusion

Map is a better choice than Object for hash maps, offering more features, better performance, and fewer pitfalls. However, Object still excels as a simple record structure for finite, predefined fields. Understanding their differences will help you make more informed decisions in your JavaScript development.

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