Using a dictionary data structure in JavaScript

Learn about how to use a dictionary data structure in JavaScript using the Map object.
February 20, 2021JavaScript
4 min read

If you're looking for a dictionary data structure in JavaScript, you can use the Map object. It was introduced in ECMAScript 6. It is very similar to the implementation of a dictionary data structure that you would find in other programming languages.

Getting a value from a Map

A Map doesn't contain any keys upon creation. It will only contain what we explicitly put into it using the set function.

const dictionary = new Map();
dictionary.set("Apple", "A fruit and a brand.");

console.log(dictionary.get("Apple")); // outputs "A fruit and a brand."

Map size

The number of items in a Map can be retrieved from its size property.

console.log(dictionary.size); // 1

Map entry checking

We can check if a Map contains a key using the has function.

console.log(dictionary.has("Apple")); // true

Deleting from a Map

We can delete an entry from a Map using the delete function.

console.log(dictionary.size); // 0

Adding objects and arrays to a Map

We can add objects or arrays to a Map entry.

dictionary.set('Mario', { color: "Red", sibling: "Luigi" });
console.log(dictionary.get("Mario")); // { color: "Red", sibling: "Luigi" }

dictionary.set('Toad', ['Rainbow Road', 'Shy Guy Falls', 'Yoshi Circuit']);
console.log(dictionary.get("Toad")); // ['Rainbow Road', 'Shy Guy Falls', 'Yoshi Circuit']

Map versus regular objects

A Map is a JavaScript object with special properties. It tends to perform better in situations that involve the frequent addition and deletion of key-value pairs. Regular objects are not optimized for this.

Map keys versus object keys

One of the Map object's special properties is that the keys of a Map can be any type of value, including functions and objects. This is unlike regular objects in JavaScript which cannot have any object as a key. Regular objects can only have string, number, or symbol keys.

const driver = { name: "Luigi", color: "Green" };
const kart = { type: "Sport Bike", tires: "Sport" };

const dictionary = new Map();
dictionary.set(driver, "1st place");
dictionary.set(kart, "No damage");

console.log(dictionary.get(driver)); // 1st place
console.log(dictionary.get(kart)); // No damage
console.log(dictionary.get({})); // undefined

The last output will return undefined because the empty object {} does not reference the driver or kart object keys that are in the Map.

Map iteration versus object iteration

A Map is directly iterable. The Map object will iterate entries, keys, and values in the order that they were entered. The items in the Map will be output in the order that they were inserted.

const dictionary = new Map();
dictionary.set("Yoshi", "2nd Place");
dictionary.set("Toad", "3rd Place");
dictionary.set("Luigi", "1st Place");

dictionary.forEach((value, key) => {
  console.log(`${key}: ${value}`);

// Yoshi: 2nd Place
// Toad: 3rd Place
// Luigi: 1st Place

You can also iterate over the Map using a for...of loop to get the same output.

for (let [key, value] of dictionary) {
 console.log(`${key}: ${value}`);

Regular JavaScript objects do not implement an iteration protocol. Therefore, they are not directly iterable using the forEach or the for...of loops. You could use Object.keys(), Object.values(), or Object.entries() to iterate over a regular object's keys, values, or entries. However, the object itself is not directly iterable like the Map is.

Keys and values of a Map

Just as with regular objects, you can also get the keys and values of a Map.

const dictionary = new Map();
dictionary.set("Yoshi", "2nd");
dictionary.set("Toad", "3rd");
dictionary.set("Luigi", "1st");

for (let key of dictionary.keys()) {
	console.log(key); // Yoshi, Toad, Luigi

for (let value of dictionary.values()){
	console.log(value); // 2nd, 3rd, 1st


You can now confidently use the Map object and data structure in JavaScript. If you are using a regular object for the frequent addition and deletion of key-value pairs, you will get better performance by converting that object into a Map.

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