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Neste artigo, veremos a sintaxe fundamental de objetos JavaScript e revisitaremos alguns recursos JavaScript vistos anteriormente no curso, reiterando o fato de que muitos dos recursos que você já utilizou são objetos.

Pré-requisitos: Interação básica com o computador, entendimento básico de HTML e CSS, familiaridade com o básico de JavaScript (ver Primeiros passos e Elementos construtivos).
Objetivo: Entender a teoria básica por trás da programação orientada a objetos, como isso se relaciona com o JavaScript ("quase tudo é objeto"), e como começar a trabalhar com objetos JavaScript.

Objeto, noções básicas

Um objeto é uma coleção de dados e/ou funcionalidades relacionadas (que geralmente consistem em diversas variáveis e funções — que são chamadas de propriedades e métodos quando estão dentro de objetos). Vamos trabalhar com um exemplo para entender como eles são.

Para começar, faça uma cópia do nosso arquivo oojs.html. Isto contém muito pouco — um elemento <script> para escrevermos nosso código-fonte. Vamos usar isto como base para explorar a sintaxe básica do objeto. Ao trabalhar com este exemplo, você deve ter seu console de ferramentas de desenvolvedor JavaScript aberto e pronto para digitar alguns comandos.

Como acontece com muitas coisas em JavaScript, a criação de um objeto geralmente começa com a definição e a inicialização de uma variável. Tente digitar o código a seguir no arquivo que você baixou, salve e atualize:

var pessoa = {};

Se você inserir pessoa no seu console JS e pressionar o botão, deverá obter o seguinte resultado:

[object Object]

Parabéns, você acabou de criar seu primeiro objeto. Tarefa concluída! Mas este é um objeto vazio, então não podemos fazer muita coisa com isso. Vamos atualizar nosso objeto para ficar assim:

var pessoa = {
  nome: ['Bob', 'Smith'],
  idade: 32,
  sexo: 'masculino',
  interesses: ['música', 'esquiar'],
  bio: function() {
    alert(this.nome[0] + ' ' + this.nome[1] + ' tem ' + this.idade + ' anos de idade. Ele gosta de ' + this.interesses[0] + ' e ' + this.interesses[1] + '.');
  },
  saudacao: function() {
    alert('Oi! Eu sou ' + this.nome[0] + '.');
  }
};

Depois de salvar e atualizar, tente inserir alguns dos itens a seguir no console JavaScript no devtools do seu navegador:

pessoa.nome
pessoa.nome[0]
pessoa.idade
pessoa.interesses[1]
pessoa.bio()
pessoa.saudacao()

Agora você tem alguns dados e funcionalidades dentro de seu objeto e é capaz de acessá-los com uma sintaxe simples e agradável!

Nota: Se você está tendo problemas para fazer isto funcionar, tente comparar seu código com a nossa versão — veja oojs-finished.html (ou veja um exemplo funcionando). O exemplo lhe dará uma tela em branco, mas tudo bem — novamente, abra seu devtools e tente digitar os comandos acima para ver a estrutura do objeto.

So what is going on here? Well, an object is made up of multiple members, each of which has a name (e.g. name and age above), and a value (e.g. ['Bob', 'Smith'] and 32). Each name/value pair must be separated by a comma, and the name and value in each case are separated by a colon. The syntax always follows this pattern:

var objectName = {
  member1Name: member1Value,
  member2Name: member2Value,
  member3Name: member3Value
};

The value of an object member can be pretty much anything — in our person object we've got a string, a number, two arrays, and two functions. The first four items are data items, and are referred to as the object's properties. The last two items are functions that allow the object to do something with that data, and are referred to as the object's methods.

An object like this is referred to as an object literal — we've literally written out the object contents as we've come to create it. This is in contrast to objects instantiated from classes, which we'll look at later on.

It is very common to create an object using an object literal when you want to transfer a series of structured, related data items in some manner, for example sending a request to the server to be put into a database. Sending a single object is much more efficient than sending several items individually, and it is easier to work with than an array, when you want to identify individual items by name.

Dot notation

Above, you accessed the object's properties and methods using dot notation. The object name (person) acts as the namespace — it must be entered first to access anything encapsulated inside the object. Next you write a dot, then the item you want to access — this can be the name of a simple property, an item of an array property, or a call to one of the object's methods, for example:

person.age
person.interests[1]
person.bio()

Sub-namespaces

It is even possible to make the value of an object member another object. For example, try changing the name member from

name: ['Bob', 'Smith'],

to

name : {
  first: 'Bob',
  last: 'Smith'
},

Here we are effectively creating a sub-namespace. This sounds complex, but really it's not — to access these items you just need to chain the extra step onto the end with another dot. Try these in the JS console:

person.name.first
person.name.last

Important: At this point you'll also need to go through your method code and change any instances of

name[0]
name[1]

to

name.first
name.last

Otherwise your methods will no longer work.

Bracket notation

There is another way to access object properties — using bracket notation. Instead of using these:

person.age
person.name.first

You can use

person['age']
person['name']['first']

This looks very similar to how you access the items in an array, and it is basically the same thing — instead of using an index number to select an item, you are using the name associated with each member's value. It is no wonder that objects are sometimes called associative arrays — they map strings to values in the same way that arrays map numbers to values.

Setting object members

So far we've only looked at retrieving (or getting) object members — you can also set (update) the value of object members by simply declaring the member you want to set (using dot or bracket notation), like this:

person.age = 45;
person['name']['last'] = 'Cratchit';

Try entering the above lines, and then getting the members again to see how they've changed, like so:

person.age
person['name']['last']

Setting members doesn't just stop at updating the values of existing properties and methods; you can also create completely new members. Try these in the JS console:

person['eyes'] = 'hazel';
person.farewell = function() { alert("Bye everybody!"); }

You can now test out your new members:

person['eyes']
person.farewell()

One useful aspect of bracket notation is that it can be used to set not only member values dynamically, but member names too. Let's say we wanted users to be able to store custom value types in their people data, by typing the member name and value into two text inputs? We could get those values like this:

var myDataName = nameInput.value;
var myDataValue = nameValue.value;

we could then add this new member name and value to the person object like this:

person[myDataName] = myDataValue;

To test this, try adding the following lines into your code, just below the closing curly brace of the person object:

var myDataName = 'height';
var myDataValue = '1.75m';
person[myDataName] = myDataValue;

Now try saving and refreshing, and entering the following into your text input:

person.height

Adding a property to an object using the method above isn't possible with dot notation, which can only accept a literal member name, not a variable value pointing to a name.

What is "this"?

You may have noticed something slightly strange in our methods. Look at this one for example:

greeting: function() {
  alert('Hi! I\'m ' + this.name.first + '.');
}

You are probably wondering what "this" is. The this keyword refers to the current object the code is being written inside — so in this case this is equivalent to person. So why not just write person instead? As you'll see in the Object-oriented JavaScript for beginners article when we start creating constructors, etc., this is very useful — it will always ensure that the correct values are used when a member's context changes (e.g. two different person object instances may have different names, but will want to use their own name when saying their greeting).

Let's illustrate what we mean with a simplified pair of person objects:

var person1 = {
  name: 'Chris',
  greeting: function() {
    alert('Hi! I\'m ' + this.name + '.');
  }
}

var person2 = {
  name: 'Brian',
  greeting: function() {
    alert('Hi! I\'m ' + this.name + '.');
  }
}

In this case, person1.greeting() will output "Hi! I'm Chris."; person2.greeting() on the other hand will output "Hi! I'm Brian.", even though the method's code is exactly the same in each case. As we said earlier, this is equal to the object the code is inside — this isn't hugely useful when you are writing out object literals by hand, but it really comes into its own when you are dynamically generating objects (for example using constructors). It will all become clearer later on.

You've been using objects all along

As you've been going through these examples, you have probably been thinking that the dot notation you've been using is very familiar. That's because you've been using it throughout the course! Every time we've been working through an example that uses a built-in browser API or JavaScript object, we've been using objects, because such features are built using exactly the same kind of object structures that we've been looking at here, albeit more complex ones than our own basic custom examples.

So when you used string methods like:

myString.split(',');

You were using a method available on an instance of the String class. Every time you create a string in your code, that string is automatically created as an instance of String, and therefore has several common methods/properties available on it.

When you accessed the document object model using lines like this:

var myDiv = document.createElement('div');
var myVideo = document.querySelector('video');

You were using methods available on an instance of the Document class. For each webpage loaded, an instance of Document is created, called document, which represents the entire page's structure, content, and other features such as its URL. Again, this means that it has several common methods/properties available on it.

The same is true of pretty much any other built-in object/API you've been using — Array, Math, etc.

Note that built in Objects/APIs don't always create object instances automatically. As an example, the Notifications API — which allows modern browsers to fire system notifications — requires you to instantiate a new object instance using the constructor for each notification you want to fire. Try entering the following into your JavaScript console:

var myNotification = new Notification('Hello!');

Again, we'll look at constructors in a later article.

Note: It is useful to think about the way objects communicate as message passing — when an object needs another object to perform some kind of action often it will send a message to another object via one of its methods, and wait for a response, which we know as a return value.

Summary

Congratulations, you've reached the end of our first JS objects article — you should now have a good idea of how to work with objects in JavaScript — including creating your own simple objects. You should also appreciate that objects are very useful as structures for storing related data and functionality — if you tried to keep track of all the properties and methods in our person object as separate variables and functions, it would be inefficient and frustrating, and we'd run the risk of clashing with other variables and functions that have the same names. Objects let us keep the information safely locked away in their own package, out of harm's way.

In the next article we'll start to look at object-oriented programming (OOP) theory, and how such techniques can be used in JavaScript.

In this module

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