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Object Accessors

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JavaScript Accessors (Getters and Setters) #

ECMAScript 5 (2009) introduced Getter and Setters.

Getters and setters allow you to define Object Accessors (Computed Properties).


JavaScript Getter (The get Keyword) #

This example uses a lang property to get the value of the language property.

Example #

// Create an object: var person = { firstName: "John", lastName : "Doe", language : "en", get lang() { return this.language; } }; // Display data from the object using a getter: document.getElementById("demo").innerHTML = person.lang;

JavaScript Setter (The set Keyword) #

This example uses a lang property to set the value of the language property.

Example #

var person = { firstName: "John", lastName : "Doe", language : "", set lang(lang) { this.language = lang; } }; // Set an object property using a setter: person.lang = "en"; // Display data from the object: document.getElementById("demo").innerHTML = person.language;

JavaScript Function or Getter? #

What is the differences between these two examples?

Example 1 #

var person = { firstName: "John", lastName : "Doe", fullName : function() { return this.firstName + " " + this.lastName; } }; // Display data from the object using a method: document.getElementById("demo").innerHTML = person.fullName();

Example 2 #

var person = { firstName: "John", lastName : "Doe", get fullName() { return this.firstName + " " + this.lastName; } }; // Display data from the object using a getter: document.getElementById("demo").innerHTML = person.fullName;

Example 1 access fullName as a function: person.fullName().

Example 2 access fullName as a property: person.fullName.

The second example provides simpler syntax.


Data Quality #

JavaScript can secure better data quality when using getters and setters.

Using the lang property, in this example, returns the value of the language property in upper case:

Example #

// Create an object: var person = { firstName: "John", lastName : "Doe", language : "en", get lang() { return this.language.toUpperCase(); } }; // Display data from the object using a getter: document.getElementById("demo").innerHTML = person.lang;

Using the lang property, in this example, stores an upper case value in the language property:

Example #

var person = { firstName: "John", lastName : "Doe", language : "", set lang(lang) { this.language = lang.toUpperCase(); } }; // Set an object property using a setter: person.lang = "en"; // Display data from the object: document.getElementById("demo").innerHTML = person.language;

Why Using Getters and Setters? #

  • It gives simpler syntax
  • It allows equal syntax for properties and methods
  • It can secure better data quality
  • It is useful for doing things behind-the-scenes


Object.defineProperty() #

The Object.defineProperty() method can also be used to add Getters and Setters:

Example #

// Define object var 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 (value) {this.counter -= value;} }); // Play with the counter: obj.reset; obj.add = 5; obj.subtract = 1; obj.increment; obj.decrement;

Browser Support #

Getters and Setters are not supported in Internet Explorer 8 or earlier:

Yes 9.0 Yes Yes Yes

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