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Introduction to Maps in JavaScript - All You Need to Know

alexdevero profile image Alex Devero Originally published at blog.alexdevero.com ・11 min read

Have you ever heard about maps in JavaScript? Maps are a new object type that was introduced in ES2015. In this tutorial, you will learn all you need to know about this lesser known object type. You will learn about what maps in JavaScript are, how they work and how to use them.

Quick introduction to maps

As a JavaScript developer you probably know JavaScript objects. Objects allow you to store data in the form of a key-value pair. Maps are very similar to JavaScript objects. When you want to store some data in maps, you store those data also in the form of a key-value pair.

Also just like with objects, you can add new keys and delete existing, and retrieve their values from maps. When you compare maps and objects, there are some differences you should know about. Let's take a look at those differences before moving further.

Maps vs Objects

One of the most important differences is that, when it comes to maps in JavaScript, you can use any data type to create keys. You can use even objects or functions. Objects will allow you to use only strings or a symbols. Another important difference is the order of keys-value pairs.

In maps, keys are ordered based on the order you've added them to the map. If you iterate over a map, you will get its keys in the same order in which you've created them. In case of objects, this is true since ES2015 and only for JavaScript engines that support this specification. Before ES2015, keys in objects were not ordered.

Another difference is how easy it is to get the size of a map. Like set, every map has a size property that says how many key-value pairs it contains. With objects, you would have to use keys() or values() to get an array of keys or values. Then, use length to get the length of this array to finally get the size of an object.

Another nice thing is that maps, like array, are iterable. You don't have to get the keys or values first in order to iterate over them. You can do it right away. For example, you can use forEach() method, just like with array. You can also use for...of loop, just like with objects.

The last difference, that is good to know, is that maps are optimized for adding and removing of key-value pairs. Objects are not. This may not matter if you don't need to manipulate with data often. If you do using maps may help you improve performance of your JavaScript code.

Creating maps in JavaScript

Maps are similar to objects. One thing that is different between them, among the things we just discussed, is how you create them. When you want to create a new object, there are multiple options to do that. For example, you can use new Object(), Object.create(), object literal or object constructor.

When you want to create a new map, there are two ways to do it. Well, theoretically. The first option to create new maps is by creating new empty Map object using new Map() and assigning it values later.

// Creating new map
let myMap = new Map()

From array to map

The second option is also about using new Map() to create new Map object. However, you can also pass in an array. To make this work, this array has to be structured in a specific way. It has to contain nested array for each key-value pair. Each array (key-value pair) has to contain two items, the key and the value.

// Create new map and assign it some values right away
const myMap = new Map([
  ['name',  'Jackie'],
  ['gender', 'female'],
  ['age', 23]
])

// Log the content of "myMap" map
console.log(myMap)
// Output:
// Map { 'name' => 'Jackie', 'gender' => 'female', 'age' => 23 }

From object to map

You can use the second option also with objects. You can take existing object and get all its entries with entries() method. The entries() method returns all entries in the same format as the array you saw in the previous example. So, you can pass the result of calling entries() method to the Map() object.

// Create new object
const myObj = {
  subject: 'Math',
  level: '1',
  difficulty: 'Medium'
}

// Create new map from "myObj"
const myMap = new Map(Object.entries(myObj))

// Log the content of "myMap" map
console.log(myMap)
// Outputs:
// Map { 'subject' => 'Math', 'level' => '1', 'difficulty' => 'Medium' }


// Or, a bit longer
// Create new object
const myObj = {
  subject: 'Math',
  level: '1',
  difficulty: 'Medium'
}

// Get all entries
const myObjEntries = Object.entries(myObj)

// Create new map from "myObjEntries"
const myMap = new Map(myObjEntries)

// Log the content of "myMap" map
console.log(myMap)
// Outputs:
// Map { 'subject' => 'Math', 'level' => '1', 'difficulty' => 'Medium' }

Adding values to maps

When you want to add values, key-value pairs, to an object there are two ways to do that. Well, three, if you count adding values during object initialization. The first is using do notation. The second is using square brackets. The second way, square brackets, works also with maps.

That said, adding values to maps using square brackets is not a good practice. The problem is that when you do this you will lose performance optimizations maps have. The right way to add values to maps is by using set() method. This method accepts two parameters. First is for the key and second for the value.

// Create new map
const myMap = new Map()

// Create simple function
function sayHi() {
  return 'Hello!'
}

// Add some values (key-value pairs) to "myMap"
myMap.set('name', 'James Reacher')
myMap.set('bio', { age: 35, height: 189, weight: 82 })
myMap.set(sayHi, 'Function as a key?')

// Log the content of "myMap"
console.log(myMap)
// Output:
// Map {
//   'name' => 'James Reacher',
//   'bio' => { age: 35, height: 189, weight: 82 },
//   [Function: sayHi] => 'Function as a key?'
// }

When you want to add multiple key-value pairs to a map you can do it only one at the time. One interesting thing is that maps support chaining. So, yes, you have to use set() method for every key-value pair you want to add. However, you can chain these methods so you don't have to use the name of the map over and over again.

// Create new map
const myMap = new Map()

// Add some values using chaining
myMap.set('Language', 'JavaScript')
  .set('Author', 'Brendan Eich')
  .set('First appeared', '1995')

// Log the content of "myMap"
console.log(myMap)
// Output:
// Map {
//   'Language' => 'JavaScript',
//   'Author' => 'Brendan Eich',
//   'First appeared' => '1995'
// }

Removing values from maps

When you want to remove values from maps the process is simple. There is a method you have to use called delete(). This method accepts one parameter, the key of the key-value pair you want to remove. If the deletion is successful, the delete() method will return true. If the key doesn't exist, it will return false.

One thing to remember about delete() method is that it works only with one key at the time. You can't pass in multiple keys. If you try it the delete() method will remove only the first key. It will ignore the rest.

// Create new map
const myMap = new Map()

// Add some values to "myMap"
myMap.set('name', 'Joe')
myMap.set('age', 25)

// Log the content of "myMap"
console.log(myMap)
// Output:
// Map { 'name' => 'Joe', 'age' => 25 }

// Remove "name" from "myMap"
myMap.delete('name')

// Log the content of "myMap" again
console.log(myMap)
// Output:
// Map { 'age' => 25 }


// This will not work
// Create new map
const myMap = new Map()

// Add some values to "myMap"
myMap.set('name', 'Joe')
myMap.set('age', 25)

// Try to remove "name" and "age" at the same time
myMap.delete('name', 'age')

// Log the content of "myMap" again
// Hint: only the "name" will be removed
// because it was the first parameter
console.log(myMap)
// Output:
// Map { 'age' => 25 }

Removing all values from maps

Removing values with delete() method is handy when you want to remove just one or few values. If you want to remove all values in a map at once, there is a better and faster way to do it. Aside from overwriting the map, you can do this also with clear() method. This method doesn't accept any parameters.

// Create new map
const myMap = new Map()

// Add some values to "myMap"
myMap.set('The Lean Startup', 'Eric Ries')
myMap.set('Measure What Matters', 'John Doerr')
myMap.set('The Startup Owner\'s Manual', 'Steve Blank')

// Log the content of "myMap"
console.log(myMap)
// Output:
// Map {
//   'The Lean Startup' => 'Eric Ries',
//   'Measure What Matters' => 'John Doerr',
//   "The Startup Owner's Manual" => 'Steve Blank'
// }

// Remove all values from "myMap"
myMap.clear()

// Log the content of "myMap"
console.log(myMap)
// Output:
// Map {}

Retrieving values from maps

Adding and removing values from maps is straightforward. You can say the same about retrieving them. When you want to retrieve a specific value from a map you can use get() method. This method accepts one parameter, a key that is associated with the value you want to retrieve.

If the key its value you want to retrieve exists, it will return the value. If it doesn't exist it will return undefined.

// Create new map
const myMap = new Map()

// Add some values to "myMap"
myMap.set('front-end', 'React')
myMap.set('back-end', 'Node.js')
myMap.set('database', 'MongoDB')

// Get the value of "back-end" key
myMap.get('back-end')
// Output:
// 'Node.js'

// Try to get the value of non-existent key "cms"
myMap.get('cms')
// Output:
// undefined

Checking if value exists in map

In some sense, the get() method can also help you check if some key exists in a map. However, there is a method dedicated for this. This method is called has(). Similarly to get(), the has() method also accepts one parameter, the key you are looking for. If the key exists has() returns true. If not, it returns false.

// Create new map
const myMap = new Map()

// Add some values to "myMap"
myMap.set('language', 'English')

// Check if "myMap" has "language" key
myMap.get('language')
// Output:
// true

// Check if "myMap" has "compiler" key
myMap.get('compiler')
// Output:
// false

Getting the size of the map

In the "Maps vs Objects" we discussed that one of the benefits of maps is that it is easy to find out their size. This is true. Every Map object has its own size property. This property is analogous to the length property that exists on arrays. Using this size property will quickly tell you how many key-value pairs are there in a specific map.

// Create new map
const myMap = new Map()

// Log the size of "myMap"
console.log(myMap.size)
// Output:
// 0

// Add some values to "myMap"
myMap.set('Tony Stark', 'Iron Man')
  .set('Steve Rogers', 'Captain America')
  .set('Black Widow', 'Natasha Romanoff')
  .set('Bruce Banner', 'Hulk')

// Log the size of "myMap" again
console.log(myMap.size)
// Output:
// 4

Iterating over maps

You know how to add values to maps and how to remove them. You also know how to retrieve values, one by one. Question is, what if you want to retrieve all values from a map? There are four options you can choose from. These options are keys(), values(), entries() and forEach() methods.

Map.keys(), Map.values() and Map.entries()

The first three methods all return Iterator object hat contain specific data. The first method, keys(), returns an Iterator with keys, one key for each pair in the Map. The second method, values() Iterator with values, also one value for each pair in the Map. The third method entries() returns an iterable for all entries.

These entries are returned in the form of a [key, value]. When you use these three methods you can then iterate over the returned Iterator object with next() method and its value property. Every use of next() method, along with value, will return the next value in the iterator, next value, key or entry in the map.

// Create new map
const myMap = new Map()

// Add some values
myMap.set('First name', 'Joshua Doer')
myMap.set('Email', 'joshua@doer.io')
myMap.set('username', 'josh1234')


// Example no.1: Map.keys()
// Create iterator for keys
const myKeysIterator = myMap.keys()

// Log the first key
console.log(myKeysIterator.next().value)
// Output:
// 'First name'

// Log the second key
console.log(myKeysIterator.next().value)
// Output:
// 'Email'

// Log the third key
console.log(myKeysIterator.next().value)
// Output:
// 'username'


// Example no.2: Map.values()
// Create iterator for values
const myValuesIterator = myMap.values()

// Log the first value
console.log(myValuesIterator.next().value)
// Output:
// 'Joshua Doer'

// Log the second value
console.log(myValuesIterator.next().value)
// Output:
// 'joshua@doer.io'

// Log the third value
console.log(myValuesIterator.next().value)
// Output:
// 'josh1234'


// Example no.3: Map.entries()
// Create iterator for entries
const myEntriesIterator = myMap.entries()

// Log the first entry
console.log(myEntriesIterator.next().value)
// Output:
// [ 'First name', 'Joshua Doer' ]

// Log the second entry
console.log(myEntriesIterator.next().value)
// Output:
// [ 'Email', 'joshua@doer.io' ]

// Log the third entry
console.log(myEntriesIterator.next().value)
// Output:
// [ 'username', 'josh1234' ]

Maps, iterators and for...of loop

Using the next() and value will not be the best tool if you want to get all data from Iterator object at once. For this, a better option will be for...of loop. This loop allows you to iterate over an Iterator and get all data inside, without the need to use next() multiple times.

// Create new map
const myMap = new Map()

// Add some values
myMap.set('First name', 'Joshua Doer')
myMap.set('Email', 'joshua@doer.io')
myMap.set('username', 'josh1234')


// Create iterator for entries
// NOTE: this will work in the same way
// also for keys() and values()
const myEntriesIterator = myMap.entries()

// Loop over the iterate object "myEntriesIterator"
for (let iteratorItem of myEntriesIterator) {
  // Log each item in the iterator
  console.log(iteratorItem)
}
// Output:
// [ 'First name', 'Joshua Doer' ]
// [ 'Email', 'joshua@doer.io' ]
// [ 'username', 'josh1234' ]

Map.forEach()

The forEach() method is a bit different. It doesn't return Iterator object like keys(), values() and entries() and lets you iterate over those values manually. Instead, forEach() iterates over the map directly It also automatically iterates over all key-value pairs.

When it iterates over the pairs it executes a callback function for each of them. This callback function accepts three parameters. All these parameters are optional. These parameters are value, key and map. The value allows you to access current value in each iteration.

The key allows you to access current key in the iteration. The last one, the map, allows you to access the whole map you are iterating over.

// Create new map
const myMap = new Map()

// Add some values
myMap.set('title', 'JavaScript: The Definitive Guide')
myMap.set('author', 'David Flanagan')
myMap.set('publisher', 'O\'Reilly Media')

// Loop over "myMap" map directly
myMap.forEach((value, key) => {
  // Log key and value in the map
  console.log(`${key}: ${value}`)
})
// Output:
// 'title: JavaScript: The Definitive Guide'
// 'author: David Flanagan'
// "publisher: O'Reilly Media"

From maps to objects

You know that you can create maps from objects. You can also do the opposite. You can take existing map and use it to create new object. This can be done with fromEntries() method. In the beginning, you used entries() method to create an array from entries that exist inside an object.

The fromEntries() method does the opposite thing. It takes an array, in the form of [key, value], and transforms it into an object. The entries() method that exists on map will help you transform any map into an array you need. You can then use that array with fromEntries() method to create new object.

// Create new map
const myMap = new Map()

// Add some values
myMap.set('spanish', 'Buenos dias')
myMap.set('french', 'Bonjour')
myMap.set('russian', 'Доброе утро')

// Transform the map into an array
const myTransformedMap = myMap.entries()

// Create new object from transformed map
const myObj = Object.fromEntries(myTransformedMap)

// Log the content of "myObj"
console.log(myObj)
// Output:
// {
//   spanish: 'Buenos dias',
//   french: 'Bonjour',
//   russian: 'Доброе утро'
// }

Conclusion: Introduction to maps in JavaScript

Maps are one of the lesser known, and less frequently used, object types in JavaScript. I hope this tutorial helped you learn what maps in JavaScript are, how they work and how to use them. I also hope it helped you distinguish when they can be a better choice than objects.

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