HashMaps vs Objects in JavaScript: An In-depth Comparison

4 minutes read


Hello, developers! Today, we’re going to expose two central concepts in JavaScript, HashMaps and Objects. Whether you’re a beginner or an experienced developer, there’s something for everyone here. So, let’s dive in!

HashMaps and Objects are data structures that store data in the form of key-value pairs. They are part of a JavaScript developer’s everyday toolkit. But despite their similarities, they each come with their unique characteristics and use cases.


A HashMap in JavaScript is a data structure that provides an efficient way to map keys to values. It’s built upon the concept of a “hash function” that generates an index for each key.

JavaScript’s Map object is a close equivalent to HashMaps in other languages. A key feature of Maps is that they can use any data type for keys, including objects, whereas Object keys are limited to strings and symbols.

HashMaps bring some fascinating advantages to the table. For one, they maintain the insertion order of entries, providing you with a predictable traversal order. Plus, they perform well even when dealing with large sets of data.

Let’s see an example of a Map in action:

let map = new Map();

// Using a number as a key
map.set(1, 'number');
console.log(map.get(1)); // 'number'

// Using a string as a key
map.set('name', 'Alice');
console.log(map.get('name')); // Alice

// Using an object as a key
let objKey = {};
map.set(objKey, 'object');
console.log(map.get(objKey)); // 'object'


In JavaScript, an Object is an unordered collection of properties, each having a name (string or symbol) and value. Objects are powerful tools in JavaScript, used for storing data, creating custom data types, and organizing code.

However, Objects come with some limitations. Keys in an object are automatically converted to strings (or symbols), which can lead to unexpected behavior. For example, trying to use an object as a key would not work as expected:

let object = {};
let key = {};
object[key] = 'value';
console.log(object[key]); // 'value'
console.log(object['[object Object]']); // 'value'

Despite these limitations, Objects are incredibly flexible, allowing for methods, nesting, and many other features that make them indispensable.

 Here are some examples of using Objects:

// Simple object
let person = {
  name: 'Alice',
  age: 25

console.log(person.name); // 'Alice'
console.log(person.age); // 25

// Nested object
let nestedObject = {
  innerObject: {
    key: 'value'

console.log(nestedObject.innerObject.key); // 'value'

// Object with method
let objectWithMethod = {
  greet: function() {
    console.log('Hello, world!');

objectWithMethod.greet(); // 'Hello, world!'

Comparing Hash Maps and Objects

HashMaps and Objects share a lot in common – both store key-value pairs, allow retrieval of values and can be modified dynamically.

HashMaps shine when you need to maintain insertion order or when you’re dealing with large amounts of data. On the other hand, Objects are great when you need structure, methods, and the flexibility of using prototypes.

For example, you might use a Map when you’re dealing with high-volume data or when the keys are complex objects. Conversely, an Object could be a better fit when you’re creating a custom data type or when you want to leverage prototypical inheritance.

Pros and Cons of HashMaps


  • Flexibility with Key Types: HashMaps support any type of key, offering great flexibility.
  • Maintain Insertion Order: HashMaps preserve the order in which keys were first added.
  • Performance: They handle large data sets efficiently.


  • Lack of Inherent Features: Unlike objects, HashMaps lack features such as methods and prototypes.
  • Performance Overkill: For smaller data sets, the performance benefits of HashMaps might not be noticeable.

Pros and Cons of Objects

Objects are incredibly flexible, supporting methods, inheritance, and nesting. They’re also more memory-efficient for smaller data sets. However, they may need to improve when dealing with larger data sets, and their handling of keys can sometimes be counterintuitive.


  • Feature-Rich: Objects support methods, inheritance, and nesting.
  • Efficient with Small Data Sets: Objects handle smaller data sets more efficiently than HashMaps.


  • Key Restrictions: Keys in an object are converted to strings or symbols, which can lead to unexpected behavior.
  • Scalability: Objects may struggle with performance when dealing with larger data sets.

Time Complexity Analysis

Time Complexity Analysis for HashMaps

HashMaps (Maps in JavaScript) offer impressive time complexity for different operations due to their use of a hash function.

Search: HashMaps can locate a value by its key in constant time, O(1), regardless of the size of the data set.

let map = new Map();
map.set('key', 'value');
console.log(map.get('key')); // 'value' - O(1) time complexity

Insertion: Adding a new key-value pair is also a constant time operation, O(1).

let map = new Map();
map.set('key', 'value'); // O(1) time complexity

Deletion: Removing a key-value pair from the map occurs in constant time, O(1).

let map = new Map();
map.set('key', 'value');
map.delete('key'); // O(1) time complexity

Iteration: Due to maintaining the order of insertion, iterating through a Map also has a time complexity of O(n).

let map = new Map();
map.set(1, 'one');
map.set(2, 'two');
map.set(3, 'three');
for (let [key, value] of map) {
  console.log(`${key} = ${value}`);
} // O(n) time complexity

Time Complexity Analysis for Objects

Objects in JavaScript provide reasonable time complexity for their operations.

Search: Searching for a key in an object is usually a constant time operation, O(1).

let object = {key: 'value'};
console.log(object['key']); // 'value' - O(1) time complexity (in most cases)

Insertion: Adding a new property to an object also occurs in constant time, O(1).

let object = {};
object['key'] = 'value'; // O(1) time complexity

Deletion: Removing a property from an object is a constant time operation, O(1).

let object = {key: 'value'};
delete object.key; // O(1) time complexity

Iteration: Iterating over an object’s properties occurs in linear time, O(n), where n is the number of properties in the object.

let object = {1: 'one', 2: 'two', 3: 'three'};
for (let key in object) {
  console.log(`${key} = ${object[key]}`);
} // O(n) time complexity

Remember, the actual performance can vary depending on the JavaScript engine, so always profile your code when performance is a concern.

Further Reading


HashMaps and Objects in JavaScript each have unique strengths and weaknesses, making them suitable for different situations.

When choosing between HashMaps and Objects, consider your use case carefully. If you need advanced features like methods and inheritance, or if you’re dealing with a smaller data set, Objects might be your go-to. But if you’re working with large amounts of data or complex keys, or if insertion order matters to you, then HashMaps could be a better fit.

Like many things in programming, there’s no definitive ‘right’ or ‘wrong’ choice here – it all depends on your specific needs. Keep exploring and happy coding!

Note: JavaScript’s implementation of HashMaps (Maps) and Objects may vary between different JavaScript engines, and the actual time complexities may be different in practice. Always make sure to profile your code to ensure the best performance.

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