Kako se koriste JavaScript zbirke - karta i postavljanje

Programiranje

Uvod

U JavaScriptu objectsse koriste za pohranu više vrijednosti kao složene strukture podataka.

Objekt se kreira kovrčavim zagradama {…}i popisom svojstava. Svojstvo je par ključ / vrijednost gdje keymora biti niz, a valuemože biti bilo koje vrste.

S druge strane, arraysuređena su zbirka koja može sadržavati podatke bilo koje vrste. U JavaScriptu se nizovi izrađuju u uglatim zagradama [...]i omogućuju dvostruke elemente.

Do ES6 (ECMAScript 2015), JavaScript objectsi arraysbili su najvažnije strukture podataka za obradu zbirki podataka. Zajednica programera nije imala puno izbora izvan toga. Bez obzira na to, kombinacija objekata i nizova uspjela je obraditi podatke u mnogim scenarijima.

Međutim, bilo je nekoliko nedostataka,

  • Tipke objekta mogu biti samo tipa string.
  • Objekti ne održavaju redoslijed elemenata koji su u njih umetnuti.
  • Objektima nedostaju neke korisne metode, što ih otežava u nekim situacijama. Na primjer, ne možete lako izračunati veličinu ( length) objekta. Također, nabrajanje predmeta nije tako jednostavno.
  • Nizovi su zbirke elemenata koji omogućuju duplikate. Podržavanje nizova koji imaju samo različite elemente zahtijeva dodatnu logiku i kôd.

Uvođenjem ES6 dobili smo dvije nove podatkovne strukture koje rješavaju gore spomenute nedostatke Mapi Set. U ovom ćemo članku pomno pogledati oboje i razumjeti kako ih koristiti u različitim situacijama.

Karta

Mapje zbirka parova ključ / vrijednost gdje ključ može biti bilo koje vrste. Mappamti izvorni redoslijed umetanja njegovih elemenata, što znači da se podaci mogu dohvatiti istim redoslijedom u kojem su umetnuti.

Drugim riječima, Mapima obilježja Objecti Array: i

  • Poput objekta, podržava strukturu para ključ / vrijednost.
  • Poput niza, pamti redoslijed umetanja.

Kako stvoriti i inicijalizirati kartu

Novo Mapse može stvoriti ovako:

const map = new Map();

Koji vraća prazno Map:

Map(0) {}

Drugi način stvaranja a Mapje s početnim vrijednostima. Evo kako stvoriti a Maps tri para ključ / vrijednost:

const freeCodeCampBlog = new Map([ ['name', 'freeCodeCamp'], ['type', 'blog'], ['writer', 'Tapas Adhikary'], ]);

Koji vraća a Maps tri elementa:

Map(3) {"name" => "freeCodeCamp", "type" => "blog", "writer" => "Tapas Adhikary"}

Kako dodati vrijednosti na kartu

Da biste dodali vrijednost na kartu, upotrijebite set(key, value)metodu.

set(key, value)Postupak traje dva parametra, keyi value, gdje je ključ i vrijednosti mogu biti bilo kojeg tipa, primitivan ( boolean, string, number,) ili objekt:

// create a map const map = new Map(); // Add values to the map map.set('name', 'freeCodeCamp'); map.set('type', 'blog'); map.set('writer', 'Tapas Adhikary');

Izlaz:

Map(3) {"name" => "freeCodeCamp", "type" => "blog", "writer" => "Tapas Adhikary"}

Napominjemo, ako isti ključ koristite za dodavanje vrijednosti Mapviše puta, uvijek će zamijeniti prethodnu vrijednost:

// Add a different writer map.set('writer', 'Someone else!');

Tako bi izlaz bio:

Map(3)  {"name" => "freeCodeCamp", "type" => "blog", "writer" => "Someone else!"}

Kako dobiti vrijednosti s karte

Da biste dobili vrijednost iz a Map, upotrijebite get(key)metodu:

map.get('name'); // returns freeCodeCamp

Sve o ključevima karte

Maptipke mogu biti bilo koje vrste, primitivne ili objektne. Ovo je jedna od glavnih razlika između Mapi redovnih JavaScript objekata kod kojih ključ može biti samo niz:

// create a Map const funMap = new Map(); funMap.set(360, 'My House Number'); // number as key funMap.set(true, 'I write blogs!'); // boolean as key let obj = {'name': 'tapas'} funMap.set(obj, true); // object as key console.log(funMap);

Evo rezultata:

Map(3) { 360 => "My House Number", true => "I write blogs!", {…} => true }

Uobičajeni JavaScript objekt uvijek tretira ključ kao niz. Čak i kad mu predate primitiv ili objekt, on interno pretvara ključ u niz:

// Create an empty object const funObj = {}; // add a property. Note, passing the key as a number. funObj[360] = 'My House Number'; // It returns true because the number 360 got converted into the string '360' internally! console.log(funObj[360] === funObj['360']);

Svojstva i metode mape

JavaScript Mapima ugrađena svojstva i metode što ga čini jednostavnim za upotrebu. Evo nekoliko najčešćih:

  • Pomoću sizesvojstva saznajte koliko je elemenata u Map:
console.log('size of the map is', map.size);
  • Pretražite element pomoću has(key)metode:
// returns true, if map has an element with the key, 'John' console.log(map.has('John')); // returns false, if map doesn't have an element with the key, 'Tapas' console.log(map.has('Tapas'));
  • Uklonite element delete(key)metodom:
map.delete('Sam'); // removes the element with key, 'Sam'.
  • Upotrijebite clear()metodu za uklanjanje svih elemenata Mapodjednom:
// Clear the map by removing all the elements map.clear(); map.size // It will return, 0

MapIterator: ključevi (), vrijednosti () i unosi ()

Metode keys(), values()te entries()metode vraćaju MapIterator, što je izvrsno jer možete koristiti for-ofili forEachpetlju izravno na njega.

Prvo stvorite jednostavan Map:

const ageMap = new Map([ ['Jack', 20], ['Alan', 34], ['Bill', 10], ['Sam', 9] ]);
  • Uzmite sve ključeve:
console.log(ageMap.keys()); // Output: // MapIterator {"Jack", "Alan", "Bill", "Sam"}
  • Dobijte sve vrijednosti:
console.log(ageMap.values()); // Output // MapIterator {20, 34, 10, 9}
  • Dohvatite sve unose (parovi ključ / vrijednost):
console.log(ageMap.entries()); // Output // MapIterator {"Jack" => 20, "Alan" => 34, "Bill" => 10, "Sam" => 9}

Kako prelistavati kartu

Možete koristiti forEachili for-ofpetlju ili za itiriranje preko Map:

// with forEach ageMap.forEach((value, key) => { console.log(`${key} is ${value} years old!`); }); // with for-of for(const [key, value] of ageMap) { console.log(`${key} is ${value} years old!`); }

Izlaz će biti jednak u oba slučaja:

Jack is 20 years old! Alan is 34 years old! Bill is 10 years old! Sam is 9 years old!

Kako pretvoriti objekt u kartu

Možete se susresti sa situacijom kada trebate pretvoriti strukturu objectu Mapsličnu strukturu. Možete koristiti metodu, entriesod Objectza to:

const address = { 'Tapas': 'Bangalore', 'James': 'Huston', 'Selva': 'Srilanka' }; const addressMap = new Map(Object.entries(address));

Kako pretvoriti kartu u objekt

Ako želite napraviti obrnuto, možete koristiti fromEntriesmetodu:

Object.fromEntries(map)

How to Convert a Map into an Array

There are a couple of ways to convert a map into an array:

  • Using Array.from(map):
const map = new Map(); map.set('milk', 200); map.set("tea", 300); map.set('coffee', 500); console.log(Array.from(map));
  • Using the spread operator:
console.log([...map]);

Map vs. Object: When should you use them?

Map has characteristics of both object and array. However, Map is more like an object than array due to the nature of storing data in the key-value format.

The similarity with objects ends here though. As you've seen, Map is different in a lot of ways. So, which one should you use, and when? How do you decide?

Use Map when:

  • Your needs are not that simple. You may want to create keys that are non-strings. Storing an object as a key is a very powerful approach. Map gives you this ability by default.
  • You need a data structure where elements can be ordered. Regular objects do not maintain the order of their entries.
  • You are looking for flexibility without relying on an external library like lodash. You may end up using a library like lodash because we do not find methods like has(), values(), delete(), or a property like size with a regular object. Map makes this easy for you by providing all these methods by default.

Use an object when:

  • You do not have any of the needs listed above.
  • You rely on JSON.parse() as a Map cannot be parsed with it.

Set

A Set is a collection of unique elements that can be of any type. Set is also an ordered collection of elements, which means that elements will be retrieved in the same order that they were inserted in.

A Set in JavaScript behaves the same way as a mathematical set.

How to Create and Initialize a Set

A new Set can be created like this:

const set = new Set(); console.log(set);

And the output will be an empty Set:

Set(0) {}

Here's how to create a Set with some initial values:

const fruteSet = new Set(['?', '?', '?', '?']); console.log(fruteSet);

Output:

Set(4) {"?", "?", "?", "?"}

Set Properties and Methods

Set has methods to add an element to it, delete elements from it, check if an element exists in it, and to clear it completely:

  • Use the size property to know the size of the Set. It returns the number of elements in it:
set.size
  • Use the add(element) method to add an element to the Set:
// Create a set - saladSet const saladSet = new Set(); // Add some vegetables to it saladSet.add('?'); // tomato saladSet.add('?'); // avocado saladSet.add('?'); // carrot saladSet.add('?'); // cucumber console.log(saladSet); // Output // Set(4) {"?", "?", "?", "?"}

I love cucumbers! How about adding one more?

Oh no, I can't – Set is a collection of unique elements:

saladSet.add('?'); console.log(saladSet);

The output is the same as before – nothing got added to the saladSet.

  • Use the has(element) method to search if we have a carrot (?) or broccoli (?) in the Set:
// The salad has a ?, so returns true console.log('Does the salad have a carrot?', saladSet.has('?')); // The salad doesn't have a ?, so returns false console.log('Does the salad have broccoli?', saladSet.has('?'));
  • Use the delete(element) method to remove the avocado(?) from the Set:
saladSet.delete('?'); console.log('I do not like ?, remove from the salad:', saladSet);

Now our salad Set is as follows:

Set(3) {"?", "?", "?"}
  • Use the clear() method to remove all elements from a Set:
saladSet.clear();

How to Iterate Over a Set

Set has a method called values() which returns a SetIterator to get all its values:

// Create a Set const houseNos = new Set([360, 567, 101]); // Get the SetIterator using the `values()` method console.log(houseNos.values());

Output:

SetIterator {360, 567, 101}

We can use a forEach or for-of loop on this to retrieve the values.

Interestingly, JavaScript tries to make Set compatible with Map. That's why we find two of the same methods as Map, keys() and entries().

As Set doesn't have keys, the keys() method returns a SetIterator to retrieve its values:

console.log(houseNos.keys()); // Output // console.log(houseNos.keys());

With Map, the entries() method returns an iterator to retrieve key-value pairs. Again there are no keys in a Set, so entries() returns a SetIterator to retrieve the value-value pairs:

console.log(houseNos.entries()); // Output // SetIterator {360 => 360, 567 => 567, 101 => 101}

How to Enumerate over a Set

We can enumerate over a Set using forEach and for-of loops:

// with forEach houseNos.forEach((value) => { console.log(value); }); // with for-of for(const value of houseNos) { console.log(value); }

The output of both is:

360 567 101

Sets and Arrays

An array, like a Set, allows you to add and remove elements. But Set is quite different, and is not meant to replace arrays.

The major difference between an array and a Set is that arrays allow duplicate elements. Also, some of the Set operations like delete() are faster than array operations like shift() or splice().

Think of Set as an extension of a regular array, just with more muscles. The Set data structure is not a replacement of the array. Both can solve interesting problems.

How to Convert a Set into an array

Converting a Set into an array is simple:

const arr = [...houseNos]; console.log(arr);

Unique values from an array using the Set

Creating a Set is a really easy way to remove duplicate values from an array:

// Create a mixedFruit array with a few duplicate fruits const mixedFruit = ['?', '?', '?', '?', '?', '?', '?',]; // Pass the array to create a set of unique fruits const mixedFruitSet = new Set(mixedFruit); console.log(mixedFruitSet);

Output:

Set(4) {"?", "?", "?", "?"}

Set and Object

A Set can have elements of any type, even objects:

// Create a person object const person = { 'name': 'Alex', 'age': 32 }; // Create a set and add the object to it const pSet = new Set(); pSet.add(person); console.log(pSet);

Output:

No surprise here – the Set contains one element that is an object.

Let's change a property of the object and add it to the set again:

// Change the name of the person person.name = 'Bob'; // Add the person object to the set again pSet.add(person); console.log(pSet);

What do you think the output will be? Two person objects or just one?

Here is the output:

Set is a collection of unique elements. By changing the property of the object, we haven't changed the object itself. Hence Set will not allow duplicate elements.

Set is a great data structure to use in addition to JavaScript arrays. It doesn't have a huge advantage over regular arrays, though.

Use Set when you need to maintain a distinct set of data to perform set operations on like union, intersection, difference, and so on.

In Summary

Here is a GitHub repository to find all the source code used in this article. If you found it helpful, please show your support by giving it a star: //github.com/atapas/js-collections-map-set

You can read more about both the Map and Set data structures here:

  • Map (MDN)
  • Set (MDN)

You may also like some of my other articles:

  • My Favorite JavaScript Tips and Tricks
  • JavaScript equality and similarity with ==, === and Object.is()

If this article was useful, please share it so others can read it as well. You can @ me on Twitter (@tapasadhikary) with comments, or feel free to follow me.