All you need to know about JavaScript Map Object

What is a javascript Map?

JavaScript map is a built-in data structure that allows you to store key-value pairs, where each key can be of any type. It is similar to an object, but with some key differences.

One key feature of JavaScript map is that it preserves the order of insertion of elements, so you can iterate through the elements in the order they were added.

Another benefit of using JavaScript map is that it provides a number of useful methods for working with the data it stores, such as forEach, keys, values, and entries.

Using a JavaScript map can be an efficient and powerful way to manage collections of data in your code. By using this data structure, you can easily access, manipulate, and iterate over data in a flexible and organized way.

How does a javascript Map work?

Now that you have a basic idea of what a Map object is, let’s see how this data structure works under the hood.

Map Object actually uses a hash table( hash map) data structure to store these key-value pairs, which provides fast access times and allows you to quickly lookup values by their associated keys.

The get() method is used to retrieve values from the map. When you call get() with a key, the map uses the key to compute a hash value, which is then used to look up the associated value in the hash table.

In addition to providing fast lookups, JavaScript map also maintains the order in which elements were added. This means that you can iterate through the elements in the same order they were inserted into the map, which can be useful in certain scenarios.

Overall, a JavaScript map is a flexible and efficient way to manage collections of data in your JavaScript code.

Why is a Javascript Map important?

A Map is an important data structure in JavaScript because it allows you to store key-value pairs where both the key and the value can be of any type. This makes Maps a more flexible alternative to regular JavaScript objects, which only allow string keys.

Maps are useful in many situations, such as:

  1. Storing complex data structures: Maps can store any type of object as a key or value, including other Maps or arrays. This makes them a good choice for storing complex data structures.
  2. Avoiding key conflicts: Since Maps can use any type of object as a key, you don’t have to worry about key conflicts like you do with regular JavaScript objects. This can make your code more robust and easier to maintain.
  3. Performance: In some cases, using a Map can be faster than using a regular JavaScript object, especially if you need to iterate over the keys or values of the object.

How to work with Javascript Map

As mentioned before, the keys and values in a Map object can be of any type, including objects, functions, and other Map objects. Additionally, the keys in a Map object are unique, so setting a key-value pair with an existing key will overwrite the existing value.

Creating a Map in JavaScript

you can create a Map object by calling the Map() constructor function. 

const myMap = new Map();

This creates an empty Map object called myMap. 

Adding Values to map object

You can add key-value pairs to the Map object using the set() method:

myMap.set("key1", "value1");

myMap.set("key2", "value2");

myMap.set("key3", "value3");

In this example, we add three key-value pairs to the myMap object.

You can add different types of key-value pairs to a Map( other than strings )

const myMap = new Map();

// Add key-value pairs of different types

myMap.set({ name: "John" }, "value1");
myMap.set(100, { name: "Jane", age: 25 });

Retrieving values from the Map object

You can retrieve the value associated with a key using the get() method.

const value1 = myMap.get("key1"); // "value1"

This retrieves the value associated with the key “key1” and stores it in the variable value1.

You can retrieve the values from the Map object( with various types of key-value pairs ) created above as:

// Get values using different types of keys
console.log(myMap.get({ name: "John" })); // Output: "value1"
console.log(myMap.get(100)); // Output: { name: "Jane", age: 25 }

Find values in the map object

You can also check if a key exists in the Map object using the has() method.

const hasKey2 = myMap.has("key2"); // true

const hasKey4 = myMap.has("key4"); // false

In this example, we check if the Map object has the keys “key2” and “key4”, respectively.

Delete values from the map object

Finally, you can delete a key-value pair from the Map object using the delete() method.


This deletes the key-value pair associated with the key “key3” from the Map object.

Other Javascript map Methods

set(key, value)Adds a new key-value pair to the map, or updates the value of an existing key.
get(key)Retrieves the value associated with a given key.
has(key)Returns a boolean indicating whether the map contains a given key.
delete(key)Removes a key-value pair from the map.
clear()Removes all key-value pairs from the map.
sizeReturns the number of key-value pairs in the map.
keys()Returns an iterator containing the keys of the map.
values()Returns an iterator containing the values of the map.
entries()Returns an iterator containing the key-value pairs of the map.

What can I use instead of a Map in JavaScript?

If you’re looking for an alternative to using a Map in JavaScript, there are a few different options depending on what you’re trying to accomplish. Here are a few possibilities:

  1. Plain JavaScript objects – if you don’t need the extra functionality that comes with a Map, such as maintaining order or handling non-string keys, you can use a plain JavaScript object. Objects in JavaScript are already key-value pairs, so they can be used to store and retrieve data in a similar way to a Map.
  2. Arrays – if you need to maintain order in your collection of data, you could use an array instead of a Map. You can use array methods like push, pop, shift, and unshift to add and remove elements from the array, and you can use index-based access to retrieve specific elements.
  3. Sets – if you don’t need the key-value functionality of a Map and are only interested in storing unique values, you could use a Set instead. Sets are similar to arrays but only allow unique values and have some additional methods like has, add, and delete.
  4. WeakMaps and WeakSets – if you’re dealing with a large number of objects and want to avoid memory leaks, you could use WeakMaps or WeakSets instead of regular Maps or Sets. WeakMaps and WeakSets allow you to store references to objects without preventing those objects from being garbage-collected if they’re no longer needed.

Javascript objects vs Map

It’s possible that you’ve encountered JavaScript objects before. Objects are a more frequently used data structure in programming compared to maps. Now, let’s explore the similarities and differences between JavaScript objects and JavaScript maps.

Similarities with JS objects

  • Both store collections of key-value pairs
  • Both allow values to be of any type
  • Both can be used to store and manipulate data in JavaScript
  • Both can be accessed and modified using similar syntax (e.g. obj[key] for Objects and map.get(key) for Maps)
  • Both can be used in loops and conditional statements to process their contents

Differences between Maps and Objects

  • Key types: Objects only allow strings or symbols as keys, while Maps can use any type of object as a key.
  • Key order: Objects do not guarantee the order of the keys, while Maps maintain the order in which keys were added.
  • Size: Maps have a size property to get the number of key-value pairs, while Objects do not have a built-in method for this.
  • Iteration: Maps have built-in methods like keys(), values(), and entries() for iteration, while Objects can be iterated over using a for…in loop.
  • Performance: Maps are generally slower than Objects for individual key-value access, but faster than Objects for iterating over all keys or values.

Pros and cons of using maps when compared to JS objects

Pros of MapsCons of Maps
Can use any type of object as a key, not just strings or symbolsSlower than Objects for individual key-value access
Maintains the order of the keys in which they were insertedMore memory-intensive than Objects
Built-in methods like keys(), values(), and entries() for iteration and manipulationCan be more complex and difficult to understand than Objects
Has a built-in size property to get the number of key-value pairsNot supported in older browsers that do not support ES6
Better performance for iterating over all keys or valuesLimited community support compared to Objects

in summary, Maps are a more powerful and flexible way to store collections of key-value pairs in JavaScript. They allow you to use any type of object as a key and maintain the order of the keys. However, if you only need to store string keys and do not care about the order of the keys, an Object may be a better choice because it is faster for individual key-value access.

What about JSON and Map?

JSON and Map are two different tools with different purposes and strengths. While JSON is ideal for data interchange between web applications, Map is a powerful data structure for storing and manipulating collections of key-value pairs within JavaScript code.

Used for data interchange between web applications and serversUsed for storing collections of key-value pairs within JavaScript code
Represented as text with a specific syntaxCreated using JavaScript syntax
Can represent a variety of data types, including numbers, strings, booleans, arrays, and objectsCan store any JavaScript value as a value, and any JavaScript object as a key
Highly flexible in terms of representing different data types and structuresHighly flexible in terms of the types of objects that can be used as keys
Optimized for data interchange and may not be as efficient for other tasksOptimized for efficient storage and retrieval of key-value pairs
Often used with AJAX and RESTful web servicesOften used for data manipulation, caching, and memoization

Can I store a Map in JSON

you can store a Map in JSON, but there are a few things to keep in mind. First, JSON only supports string keys, so if you want to store a Map in JSON, you’ll need to convert the keys to strings. One way to do this is to use the JSON.stringify() method with a custom replacer function that converts the Map keys to strings.

const myMap = new Map()
myMap.set({name: 'John'}, 42)
myMap.set({name: 'Jane'}, 37)

const mapToJson = (map) => {
  const obj = {}
  for (let [key, value] of map) {
    obj[JSON.stringify(key)] = value
  return obj

const json = JSON.stringify(mapToJson(myMap))
// Output: {"{\"name\":\"John\"}":42,"{\"name\":\"Jane\"}":37}

In this example, the mapToJson() function converts the Map to an object where the keys are strings that represent the original objects in the Map. The resulting JSON string can then be stored and retrieved as needed.

Second, keep in mind that not all JSON parsers may be able to handle non-string keys, so it’s a good idea to test your implementation thoroughly and ensure that it works with your specific use case. Finally, when you retrieve the JSON string and want to convert it back into a Map, you’ll need to use a custom reviver function with the JSON.parse() method to convert the string keys back into their original objects.

const jsonToMap = (json) => {
  const obj = JSON.parse(json);
  const map = new Map();
  for (let key in obj) {
    map.set(JSON.parse(key), obj[key]);
  return map;

const sampleJson = `{
  "{ \\"name\\": \\"John\\" }": 42,
  "{ \\"name\\": \\"Jane\\" }": 37
const myMap = jsonToMap(sampleJson);

In this example, the jsonToMap() function uses a custom reviver function to convert the string keys back into their original objects and then creates a new Map with the reconstructed key-value pairs.


In conclusion, the Map object in JavaScript provides a powerful and flexible way to store key-value pairs, making it an excellent alternative to traditional JavaScript objects. Maps offer several advantages over objects, such as the ability to use any type of key, support for built-in methods like iteration and size checking, and improved performance when working with large amounts of data. Additionally, Maps offer useful features such as ordering, which can be important when working with data that needs to be accessed in a specific order. Overall, the Map object is an essential tool for any JavaScript developer working with data structures and is well worth exploring in more depth.

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