Métodos de objeto JavaScript ES5

Índice

ECMAScript 5 (2009) adicionou muitos novos métodos de objeto ao JavaScript.

Gerenciando objetos

// Create object with an existing object as prototype
Object.create()

// Adding or changing an object property
Object.defineProperty(object, property, descriptor)

// Adding or changing object properties
Object.defineProperties(object, descriptors)

// Accessing Properties
Object.getOwnPropertyDescriptor(object, property)

// Returns all properties as an array
Object.getOwnPropertyNames(object)

// Accessing the prototype
Object.getPrototypeOf(object)

// Returns enumerable properties as an array
Object.keys(object)

Protegendo Objetos

// Prevents adding properties to an object
Object.preventExtensions(object)

// Returns true if properties can be added to an object
Object.isExtensible(object)

// Prevents changes of object properties (not values)
Object.seal(object)

// Returns true if object is sealed
Object.isSealed(object)

// Prevents any changes to an object
Object.freeze(object)

// Returns true if object is frozen
Object.isFrozen(object)

Alterando um valor de propriedade

Sintaxe

Object.defineProperty(object, property, {value : value})

Este exemplo altera um valor de propriedade:

Exemplo

const person = {
  firstName: "John",
  
  lastName : "Doe",
  language : "EN" 
};
// Change a property
Object.defineProperty(person, "language", 
  {value : "NO"});

Experimente você mesmo →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object.defineProperty()</h2>

<p>This example changes a property value.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "EN" 
};

// Change a property
Object.defineProperty(person, "language", {value:"NO"})
document.getElementById("demo").innerHTML = person.language;
</script>

</body>
</html>

Alterando Metadados

ES5 permite que os seguintes metadados de propriedade sejam alterados:

writable : true      // Property value can be changed
enumerable : true    // Property can be enumerated
configurable : true  // Property can be reconfigured

writable : false     // Property value can not be changed
enumerable : false   // Property can be not enumerated
configurable : false // Property can be not reconfigured

ES5 permite que getters e setters sejam alterados:

// Defining a getter
get: function() { return language }
// Defining a setter
set: function(value) { language = value }

Este exemplo torna a linguagem somente leitura:

Object.defineProperty(person, "language", {writable:false});

Este exemplo torna a linguagem não enumerável:

Object.defineProperty(person, "language", {enumerable:false});

Listando todas as propriedades

Este exemplo lista todas as propriedades de um objeto:

Exemplo

const person = {
  firstName: "John",
  
  lastName : "Doe",
  language : "EN" 
};

  Object.defineProperty(person, "language", {enumerable:false});
Object.getOwnPropertyNames(person);  // Returns an array of 
  properties

Experimente você mesmo →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object.getOwnPropertyNames()</h2>

<p>This example list the properties of an object.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "EN"
}

// Change Property
Object.defineProperty(person, "language", {enumerable:false});

// Display Properties
document.getElementById("demo").innerHTML = Object.getOwnPropertyNames(person);
</script>

</body>
</html>

Listando propriedades enumeráveis

Este exemplo lista apenas as propriedades enumeráveis de um objeto:

Exemplo

const person = {
  firstName: "John",
  
  lastName : "Doe",
  language : "EN" 
};

  Object.defineProperty(person, "language", {enumerable:false});
Object.keys(person);  // Returns an array 
  of enumerable  
  properties

Experimente você mesmo →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object.keys()</h2>

<p>This example list the enumerable properties of an object.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "EN"
}

// Change Property
Object.defineProperty(person, "language", {enumerable:false});

// Display Properties
document.getElementById("demo").innerHTML = Object.keys(person);
</script>

</body>
</html>

Adicionando uma propriedade

Este exemplo adiciona uma nova propriedade a um objeto:

Exemplo

 // Create an object:
const person = {
  firstName: "John",
  lastName : 
  "Doe",
  language : "EN"
};
// Add a property
Object.defineProperty(person, "year", 
  {value:"2008"});

Experimente você mesmo →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object.defineProperty()</h2>

<p>This example adds a new property to an object.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {
  firstName: "John",
  lastName: "Doe",
  language: "EN"
};

// Add a property
Object.defineProperty(person, "year", {value:"2008"})
document.getElementById("demo").innerHTML = person.year;
</script>

</body>
</html>

Adicionando Getters e Setters

O método Object.defineProperty() também pode ser usado para adicionar Getters e Configuradores:

Exemplo

//Create an object
const person = {firstName:"John", lastName:"Doe"};

// Define a getter
Object.defineProperty(person, "fullName", {
  get: 
function () {return this.firstName + " " + this.lastName;}
});

Experimente você mesmo →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Object.defineProperty()</h2>

<p>This example uses the defineProperty() method to add a getter and a setter.</p>

<p id="demo"></p>

<script>
// Create an object:
const person = {firstName:"John", lastName:"Doe"};

// Define a getter
Object.defineProperty(person, "fullName", {
  get: function () {return this.firstName + " " + this.lastName;}
});

// Display full name
document.getElementById("demo").innerHTML = person.fullName;
</script>

</body>
</html>

Um exemplo de contador

Exemplo

// Define object
const obj = {counter:0};

// Define setters
Object.defineProperty(obj, "reset", {
  get : function () {this.counter = 0;}
});
Object.defineProperty(obj, "increment", {
  get : function () {this.counter++;}
});
Object.defineProperty(obj, "decrement", {
  get : function () {this.counter--;}
});
Object.defineProperty(obj, "add", {
  set : function (value) {this.counter += value;}
});
Object.defineProperty(obj, "subtract", {
  set : function (i) {this.counter -= i;}
});

// Play with the counter:
obj.reset;
obj.add = 5;
obj.subtract = 1;
obj.increment;
obj.decrement;

Experimente você mesmo →

<!DOCTYPE html>
<html>
<body>

<h2>JavaScript Getters and Setters</h2>

<p>Perfect for creating counters:</p>

<p id="demo"></p>

<script>
// Define an object
const obj = {counter:0};

// Define Setters and Getters
Object.defineProperty(obj, "reset", {
  get : function () {this.counter = 0;}
});
Object.defineProperty(obj, "increment", {
  get : function () {this.counter++;}
});
Object.defineProperty(obj, "decrement", {
  get : function () {this.counter--;}
});
Object.defineProperty(obj, "add", {
  set : function (value) {this.counter += value;}
});
Object.defineProperty(obj, "subtract", {
  set : function (value) {this.counter -= value;}
});

// Play with counter:
obj.reset;
obj.add = 5;
obj.subtract = 1;
obj.increment;
obj.decrement;
document.getElementById("demo").innerHTML = obj.counter;
</script>

</body>
</html>