Accessor Properties

Description

Accessor properties do not contain a data value. They use a combination of a getter/setter function.

When an accessor property is read , the getter function is called. When an accessor property is written to, the setter function is called and acts on the new value.

Accessor properties have four attributes:

• Configurable - Indicates if the property may be redefined by removing the property via delete, changing the property's attributes, or changing the property into a data property. Default to true.
• Enumerable - Indicates if the property will be returned in a for-in loop. Default to true.
• Get - Marks the getter function. Default to undefined.
• Set - Marks setter function. The default value is undefined.

It is not possible to define an accessor property explicitly; you must use Object.defineProperty().

Example

var book = {
  _year: 2004,/* w ww.  j  a  v  a2s .com*/
  edition: 1
};

Object.defineProperty(book, "year", {
    get: function(){
       return this._year;
    },
    set: function(newValue){
       if (newValue > 2004) {
          this._year = newValue;
          this.edition += newValue - 2004;
       }
    }
});

book.year = 2005;
console.log(book.year);  
console.log(book.edition);  

book.year = 2000;
console.log(book.year);  
console.log(book.edition);  

An object is created with two properties: _year and edition. The underscore on _year is a practice to indicate that a property is not intended to be accessed from outside of the object's methods.

The code above generates the following result.

Note

It's not necessary to assign both a getter and a setter.

A property with only getter method cannot be written to. In strict mode, trying to write to a property with only a getter throws an error.

A property with only a setter cannot be read and will return the value undefined in nonstrict mode, while doing so throws an error in strict mode.