- What is a Map
- Before ES6
- Enter Map
- Initialize a map with values
- Map keys
- Weird situations you’ll almost never find in real life
- Iterating over a map
- Convert to array
- WeakMap
What is a Map
A Map data structure allows to associate data to a key.
Before ES6
ECMAScript 6 (also called ES2015) introduced the Map data structure to the JavaScript world, along with Set
Before its introduction, people generally used objects as maps, by associating some object or value to a specific key value:
const car = {}
car['color'] = 'red'
car.owner = 'Flavio'
console.log(car['color']) //red
console.log(car.color) //red
console.log(car.owner) //Flavio
console.log(car['owner']) //Flavio
Enter Map
ES6 introduced the Map data structure, providing us a proper tool to handle this kind of data organization.
A Map is initialized by calling:
const m = new Map()
Add items to a Map
You can add items to the map by using the set method:
m.set('color', 'red')
m.set('age', 2)
Get an item from a map by key
And you can get items out of a map by using get:
const color = m.get('color')
const age = m.get('age')
Delete an item from a map by key
Use the delete() method:
m.delete('color')
Delete all items from a map
Use the clear() method:
m.clear()
Check if a map contains an item by key
Use the has() method:
const hasColor = m.has('color')
Find the number of items in a map
Use the size property:
const size = m.size
Initialize a map with values
You can initialize a map with a set of values:
const m = new Map([['color', 'red'], ['owner', 'Flavio'], ['age', 2]])
Map keys
Just like any value (object, array, string, number) can be used as the value of the key-value entry of a map item, any value can be used as the key, even objects.
If you try to get a non-existing key using get() out of a map, it will return undefined.
Weird situations you’ll almost never find in real life
const m = new Map()
m.set(NaN, 'test')
m.get(NaN) //test
const m = new Map()
m.set(+0, 'test')
m.get(-0) //test
Iterating over a map
Iterate over map keys
Map offers the keys() method we can use to iterate on all the keys:
for (const k of m.keys()) {
console.log(k)
}
Iterate over map values
The Map object offers the values() method we can use to iterate on all the values:
for (const v of m.values()) {
console.log(v)
}
Iterate over map key, value pairs
The Map object offers the entries() method we can use to iterate on all the values:
for (const [k, v] of m.entries()) {
console.log(k, v)
}
which can be simplified to
for (const [k, v] of m) {
console.log(k, v)
}
Convert to array
Convert the map keys into an array
const a = [...m.keys()]
Convert the map values into an array
const a = [...m.values()]
WeakMap
A WeakMap is a special kind of map.
In a map object, items are never garbage collected. A WeakMap instead lets all its items be freely garbage collected. Every key of a WeakMap is an object. When the reference to this object is lost, the value can be garbage collected.
Here are the main differences:
- you cannot iterate over the keys or values (or key-values) of a WeakMap
- you cannot clear all items from a WeakMap
- you cannot check its size
A WeakMap exposes those methods, which are equivalent to the Map ones:
get(k)set(k, v)has(k)delete(k)
The use cases of a WeakMap are less evident than the ones of a Map, and you might never find the need for them, but essentially it can be used to build a memory-sensitive cache that is not going to interfere with garbage collection, or for careful encapsualtion and information hiding.
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