Object Accessors

Агуулга

JavaScript Accessors (Getters and Setters)
JavaScript Getter (The get Keyword)
- Example
JavaScript Setter (The set Keyword)
- Example
JavaScript Function or Getter?
- Example 1
- Example 2
Data Quality
- Example
- Example
Why Using Getters and Setters?
Object.defineProperty()
- Example
Browser Support

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;