Sets_and_Maps

Sets and maps

In ex6 ,two more data structures were finally introduced.

Set is iterable

Sets

Basic usage

Use new Set() to creat a new set.

1 2	const set = new Set(['a', 'b', 'c', 'a', 'b', 'c', 'a', 'b', 'c']); console.log(set);//Set(3) { 'a', 'b', 'c' }

1 2	const set = new Set('aniya'); console.log(set);//Set(4) { 'a', 'n', 'i', 'y' }

The input must be iterable.
And the elements in a set is unique.
And there is no order of the elements. No index.

So there is no need to get a value form sets.

Methods of set

.size

1 2	const set //4= new Set('aniya'); console.log(set.size);//4

.has()

1 2	const set = new Set('aniya'); console.log(set.has('a'), set.has('0'));//true false

To find out whether the set has the thing you want to know.

.add()

const set = new Set();
console.log(set);//Set(0) {}
set.add('Tommy idiot');
console.log(set);//Set(1) { 'Tommy idiot' }

You can just add one at one time.

.delete

const set = new Set();
console.log(set); //Set(0) {}
set.add('Tommy idiot');
console.log(set); //Set(1) { 'Tommy idiot' }
set.delete('Tommy idiot');
console.log(set);//Set(0) {}

.clear()

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const set = new Set('aniya');
set.clear();
console.log(set);//Set(0) {}

To clear out the set.

Loop over the set

const set = new Set('aniya');
for (const item of set) {
  console.log(item);
}
//a
//n
//i
//y

Some usecases of sets

To remove duplicated values of arrarys.

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const arr = [1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4, 1, 2, 3, 4];
const arrUnique = [...new Set(arr)];
console.log(arrUnique);//[ 1, 2, 3, 4 ]

And we can konw its size by the size method.

Maps

Maps is alike to objects,data are also stored as key-value pairs.
The biggest diffrence is that keys in maps can be any type,such as number, string ,object and even another map.
While the keys in objects can only be string.

Creat a map

const question = new Map([
  ['key', 'value'],
  ['question', "What's the best programming language in the world?"],
  [1, 'C'],
  [2, 'Java'],
  [3, 'Javascript'],
  ['correct', 3],
  [true, 'Congratulations! 🤣'],
  [false, 'Try again!😅'],
]);
console.log(question);

The argument is a big arrary which contains many small arrarys which are compose of key-value pairs.

.set()

let o = {
  name: 'jonas',
};
const map = new Map();
map.set(1, 'the first');
map.set('key', 2);
map.set(o, o);
console.log(map.set(o, o));
console.log(map);
//Map(3) {
  1 => 'the first',
  'key' => 2,
  { name: 'jonas' } => { name: 'jonas' }
}

Calling the set method like this does not only update the map that’s called on,but it also returns the map.

So you can just do that like this:

map.set(1, 'the first').set(o, o).set('key', 2);
console.log(map);
//Map(3) {
  1 => 'the first',
  'key' => 2,
  { name: 'jonas' } => { name: 'jonas' }
}

.get()

1	console.log(map.get('key'));//2

.has()

.delete()

If you want to use an “Complex datatype” as an key ,you should store it in an variation first.

Wrong

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const map = new Map();
map.set([1, 2], 'this');
console.log(map.get([1, 2]));//undefined

Because the two [1,2] is not the same object that is stored in heap.

Right

const map = new Map();
const arr = [1, 2];
map.set(arr, 'this');
console.log(map.get(arr)); //this

Convert a object into a map.

1	newMap = new Map(Object.entries(objectName));

Loop over a map

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for(const [key,value] of mapName){
	console.log(key,value);
}