Sådan bruges JavaScript-samlinger - kort og sæt

Introduktion

I JavaScript objectsbruges til at gemme flere værdier som en kompleks datastruktur.

Et objekt oprettes med krøllede seler {…}og en liste over egenskaber. En egenskab er et nøgleværdipar, hvor det keyskal være en streng, og det valuekan være af enhver type.

På den anden side arrayser der en ordnet samling, der kan indeholde data af enhver type. I JavaScript oprettes arrays med firkantede parenteser [...]og tillader duplikatelementer.

Indtil ES6 (ECMAScript 2015) var JavaScript objectsog arraysde vigtigste datastrukturer til håndtering af dataindsamlinger. Udviklerfællesskabet havde ikke mange valg uden for det. Alligevel var en kombination af objekter og arrays i stand til at håndtere data i mange scenarier.

Der var dog et par mangler,

  • Objektnøgler kan kun være af typen string.
  • Objekter opretholder ikke rækkefølgen af ​​elementerne, der er indsat i dem.
  • Objekter mangler nogle nyttige metoder, hvilket gør dem vanskelige at bruge i nogle situationer. For eksempel kan du ikke beregne størrelsen ( length) af et objekt let. At tælle et objekt er heller ikke så ligetil.
  • Arrays er samlinger af elementer, der tillader duplikater. Understøttende arrays, der kun har forskellige elementer, kræver ekstra logik og kode.

Med introduktionen af ​​ES6 fik vi to nye datastrukturer, der løser ovennævnte mangler, Mapog Set. I denne artikel vil vi se på begge nøje og forstå, hvordan man bruger dem i forskellige situationer.

Kort

Maper en samling af nøgleværdipar, hvor nøglen kan være af enhver type. Maphusker den oprindelige indsætningsrækkefølge for dets elementer, hvilket betyder, at data kan hentes i samme rækkefølge som de blev indsat i.

Med andre ord Maphar karakteristika for begge Objectog Array:

  • Ligesom et objekt understøtter det nøgleværdiparstrukturen.
  • Ligesom et array husker det indsættelsesrækkefølgen.

Sådan oprettes og initialiseres et kort

Et nyt Mapkan oprettes som dette:

const map = new Map();

Hvilket returnerer en tom Map:

Map(0) {}

En anden måde at skabe en Maper med indledende værdier. Sådan oprettes en Mapmed tre nøgleværdipar:

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

Hvilket returnerer a Mapmed tre elementer:

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

Sådan tilføjes værdier til et kort

Brug set(key, value)metoden for at tilføje værdi til et kort .

Den set(key, value)metode tager to parametre, keyog value, hvor nøglen og værdi kan være af enhver type, en primitiv ( boolean, string, number, etc.) eller et 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');

Produktion:

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

Bemærk, at hvis du bruger den samme nøgle til at tilføje en værdi Mapflere gange, erstatter den altid den tidligere værdi:

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

Så output ville være:

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

Sådan får du værdier fra et kort

For at få en værdi fra a Mapskal du bruge get(key)metoden:

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

Alt om kortnøgler

Map keys can be of any type, a primitive, or an object. This is one of the major differences between Map and regular JavaScript objects where the key can only be a string:

// 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);

Here is the output:

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

A regular JavaScript object always treats the key as a string. Even when you pass it a primitive or object, it internally converts the key into a string:

// 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']);

Map Properties and Methods

JavaScript's Map has in-built properties and methods that makes it easy to use. Here are some of the common ones:

  • Use the size property to know how many elements are in a Map:
console.log('size of the map is', map.size);
  • Search an element with the has(key) method:
// 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')); 
  • Remove an element with the delete(key) method:
map.delete('Sam'); // removes the element with key, 'Sam'.
  • Use the clear() method to remove all the elements from the Map at once:
// Clear the map by removing all the elements map.clear(); map.size // It will return, 0 

MapIterator: keys(), values(), and entries()

The methods keys(), values() and entries() methods return a MapIterator, which is excellent because you can use a for-of or forEach loop directly on it.

First, create a simple Map:

const ageMap = new Map([ ['Jack', 20], ['Alan', 34], ['Bill', 10], ['Sam', 9] ]);
  • Get all the keys:
console.log(ageMap.keys()); // Output: // MapIterator {"Jack", "Alan", "Bill", "Sam"}
  • Get all the values:
console.log(ageMap.values()); // Output // MapIterator {20, 34, 10, 9}
  • Get all the entries (key-value pairs):
console.log(ageMap.entries()); // Output // MapIterator {"Jack" => 20, "Alan" => 34, "Bill" => 10, "Sam" => 9}

How to Iterate Over a Map

You can use either the forEach or for-of loop to iterate over a 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!`); }

The output is going to be the same in both cases:

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

How to Convert an Object into a Map

You may encounter a situation where you need to convert an object to a Map-like structure. You can use the method, entries of Object to do that:

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

How to Convert a Map into an Object

If you want to do the reverse, you can use the fromEntries method:

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:

  • Mine foretrukne JavaScript-tip og -tricks
  • JavaScript-lighed og lighed med ==, === og Object.is ()

Hvis denne artikel var nyttig, bedes du dele den, så andre også kan læse den. Du kan @ mig på Twitter (@tapasadhikary) med kommentarer eller være velkommen til at følge mig.