JavaScript‌ ‌Map‌ ‌and‌ ‌Set

Introduction to JavaScript Maps

What is a JavaScript Map?

In JavaScript, a Map is a collection that stores key-value pairs. It allows the use of various data types as keys and maintains the order of its entries, differentiating it from regular objects that only support strings and symbols as keys.

Creating and Manipulating Maps

To create a Map, use `new Map()`. It supports several methods for data manipulation:

  • `map.set(key, value)`: Adds a key-value pair.
  • `map.get(key)`: Retrieves the value associated with a key.
  • `map.has(key)`: Checks if a key exists.
  • `map.delete(key)`: Removes a key-value pair.
  • `map.clear()`: Clears the Map.
  • `map.size`: Returns the number of key-value pairs.

Code Example:

let map = new Map(); map.set('key1', 'value1'); map.set('key2', 'value2'); console.log(map.get('key1')); // Output: value1

Iterating Over Maps

Maps can be iterated in various ways:
  • Using `map.keys()`, `map.values()`, and `map.entries()`.
  • Using `for...of` loops.
  • Using `map.forEach()` method.

Code Example:

let map = new Map(); map.set('apple', 1); map.set('banana', 2); for (let [key, value] of map) { console.log(key + ' = ' + value); }

Understanding JavaScript Sets

What is a JavaScript Set?

A Set in JavaScript is a collection of unique values. Unlike an array, each element can only occur once in a Set.

Working with Sets

To work with Sets:

  • Create a Set using `new Set(iterable)`.
  • Add an element using `set.add(value)`.
  • Check for an element using `set.has(value)`.
  • Remove an element using `set.delete(value)`.
  • Determine the number of elements using `set.size`.
  • Clear a Set using `set.clear()`.

Code Example:

let set = new Set(); set.add('apple'); set.add('banana'); set.add('apple'); // Duplicate: will not be added console.log(set.size); // Output: 2

Iterating Over Sets

Sets can be iterated using:

  • `for...of` loops.
  • `set.forEach()` method, which provides access to each value in the Set.

Code Example:

let set = new Set(['apple', 'banana', 'cherry']); set.forEach(function(value) { console.log(value); });
In addition to Map and Set, JavaScript also provides WeakMap and WeakSet, which manage memory differently by allowing their elements to be garbage-collected. Learn more about these specialized collections on our JavaScript WeakMap and WeakSet page.


JavaScript Maps and Sets offer powerful data structures for managing unique collections and key-value pairs. Understanding their functionalities and appropriate use cases enhances your JavaScript coding skills, leading to more efficient and effective code development.

Remember, JavaScript is a versatile language, and mastering these advanced concepts is crucial for tackling complex programming challenges. Keep practicing and exploring to deepen your understanding of JavaScript's capabilities.

Practice Your Knowledge

What are some properties of JavaScript Map and Set?

Quiz Time: Test Your Skills!

Ready to challenge what you've learned? Dive into our interactive quizzes for a deeper understanding and a fun way to reinforce your knowledge.

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