The new Map() Method
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]
]);
Map.get()
You get the value of a key in a map with the get() method
Example
fruits.get("apples");
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);
The set() method can also be used to change existing map values:
Example
fruits.set("apples", 500);
Map.size
The size property returns the number of elements in a map:
Example
fruits.size;
Map.delete()
The delete() method removes a map element:
Example
fruits.delete("apples");
Map.clear()
The clear() method removes all the elements from a map:
Example
fruits.clear();
Map.has()
The has() method returns true if a key exists in a map:
Example
fruits.has("apples");
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;
})
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;
}
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;
}
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;
}
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;
}
Objects as Keys
Note: 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);
Remember: The key is an object (apples), not a string ("apples"):
Example
fruits.get("apples"); // Returns undefined
JavaScript Map.groupBy()
ES2024 added the Map.groupBy() method to JavaScript.
The Map.groupBy() method groups the elements of an iterable according to the values returned from a callback function.
The Map.groupBy() method does not change the original object.
The return value is a Map with a key for each group, where each key is assigned to an Array of the associated elements.
Example
// Create an Array
const fruits = [
{name:"apples", quantity:300},
{name:"bananas", quantity:500},
{name:"oranges", quantity:200},
{name:"kiwi", quantity:150}
];
// Callback function to Group Elements
function myCallback({ quantity }) {
return quantity > 200 ? "ok" : "low";
}
// Group by Quantity
const result = Map.groupBy(fruits, myCallback);
Browser Support
Map.groupBy() is an ECMAScript 2024 feature.
JavaScript 2024 is supported in new browsers since March 2024:
Warning: ES2024 features are relatively new. Older browsers may need an alternative code (Polyfill)
Object.groupBy() vs Map.groupBy()
The difference between Object.groupBy() and Map.groupBy() is:
Object.groupBy() groups elements into a JavaScript object.
Map.groupBy() groups elements into a Map object.
Note: Learn More: JavaScript Maps JavaScript Weak Maps JavaScript Map Reference JavaScript Sets
