JavaScript Maps

[Guest]

JavaScript Maps


A Map holds key-value pairs where the keys can be any datatype.
A Map remembers the original insertion order of the keys.
A Map has a property that represents the size of the map.

Map Methods

MethodDescription
new Map()Creates a new Map object
set()Sets the value for a key in a Map
get()Gets the value for a key in a Map
clear()Removes all the elements from a Map
delete()Removes a Map element specified by a key
has()Returns true if a key exists in a Map
forEach()Invokes a callback for each key/value pair in a Map
entries()Returns an iterator object with the [key, value] pairs in a Map
keys()Returns an iterator object with the keys in a Map
values()Returns an iterator object of the values in a Map


PropertyDescription
sizeReturns the number of Map elements


How to Create a Map
You can create a JavaScript Map by:

Passing an Array to new Map()
Create a Map and use Map.set()


new Map()
You can create a Map by passing an Array to the new Map() constructor:

Example

// Create a Map
const fruits = new Map([
  ["apples", 500],
  ["bananas", 300],
  ["oranges", 200]
]);

Try it Yourself »


Map.set()
You can add elements to a Map with the set() method:

Example

// Create a Map
const fruits = new Map();

// Set Map Values
fruits.set("apples", 500);
fruits.set("bananas", 300);
fruits.set("oranges", 200);

Try it Yourself »

The set() method can also be used to change existing Map values:

Example

fruits.set("apples", 500);

Try it Yourself »


Map.get()
The get() method gets the value of a key in a Map:

Example

fruits.get("apples");    // Returns 500

Try it Yourself »








Map.size
The size property returns the number of elements in a Map:

Example

fruits.size;

Try it Yourself »


Map.delete()
The delete() method removes a Map element:

Example

fruits.delete("apples");

Try it Yourself »


Map.clear()
The clear() method removes all the elements from a Map:

Example

fruits.clear();

Try it Yourself »


Map.has()
The has() method returns true if a key exists in a Map:

Example

fruits.has("apples");

Try it Yourself »
Try This:

fruits.delete("apples");
fruits.has("apples");

Try it Yourself »


Maps are Objects
typeof returns object:

Example

// Returns object:
typeof fruits;

Try it Yourself »

instanceof Map returns true:

Example

// Returns true:
fruits instanceof Map;

Try it Yourself »


JavaScript Objects vs Maps
Differences between JavaScript Objects and Maps:

ObjectMap

Not directly iterable
Directly iterable

Do not have a size property
Have a size property

Keys must be Strings (or Symbols)
Keys can be any datatype

Keys are not well ordered
Keys are ordered by insertion

Have default keys
Do not have default keys


Map.forEach()
The forEach() method invokes a callback for each key/value pair in a Map:

Example

// List all entries
let text = "";
fruits.forEach (function(value, key) {
  text += key + ' = ' + value;
})

Try it Yourself »


Map.entries()
The entries() method returns an iterator object with the [key,values] in a Map:

Example

// List all entries
let text = "";
for (const x of fruits.entries()) {
  text += x;
}

Try it Yourself »


Map.keys()
The keys() method returns an iterator object with the keys in a Map:

Example

// List all keys
let text = "";
for (const x of fruits.keys()) {
  text += x;
}

Try it Yourself »


Map.values()
The values() method returns an iterator object with the values in a Map:

Example

// List all values
let text = "";
for (const x of fruits.values()) {
  text += x;
}

Try it Yourself »

You can use the values() method to sum the values in a Map:

Example

// Sum all values
let total = 0;
for (const x of fruits.values()) {
  total += x;
}

Try it Yourself »


Objects as Keys

Being able to use objects as keys is an important Map feature.


Example

// Create Objects
const apples = {name: 'Apples'};
const bananas = {name: 'Bananas'};
const oranges = {name: 'Oranges'};

// Create a Map
const fruits = new Map();

// Add new Elements to the Map
fruits.set(apples, 500);
fruits.set(bananas, 300);
fruits.set(oranges, 200);

Try it Yourself »

Remember: The key is an object (apples), not a string ("apples"):

Example

fruits.get("apples");  // Returns undefined

Try it Yourself »


Browser Support
JavaScript Maps are supported in all browsers, except Internet Explorer:










Chrome
Edge
Firefox
Safari
Opera
















★
+1

Reference: https://www.w3schools.com/js/js_object_maps.asp