JavaScript objects can have getter and setter functions that are called when a property is read from or written to.

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For example, if you'd like an object that has a virtual person.fullName attribute that dynamically composes person.firstName and person.lastName:

var person = {

  firstName: 'Guybrush',

  lastName: 'Threepwood',

  get fullName() {
    return this.firstName + " " + this.lastName;
  },
  
  set fullName(name) {
    var parts = name.split(" ");
    this.firstName = parts[0];
    this.lastName = parts[1];
  }

};

The code below shows how this object can be used. Notice the absence of parentheses ("()"):

person.fullName = 'Anna Pearson';
person.firstName; // => "Anna"
person.lastName; // => "Pearson"
person.firstName = 'Nick';
person.fullName; // => "Nick Pearson"

If you'd like to define getters and setters on an existing object, use Object.defineProperty() instead:

var person = {
  firstName: 'Guybrush',
  lastName: 'Threepwood'
};

Object.defineProperty(person, "fullName", {
  get: function() {
    return this.firstName + " " + this.lastName;
  },
  set: function(name) {
    var parts = name.split(" ");
    this.firstName = parts[0];
    this.lastName = parts[1];
  }
});

Browser compatibility

Works with IE9+ and real browsers.

CoffeeScript

CoffeeScript doesn't support getter/setter functions, and probably never will Show archive.org snapshot .

However, you can define getters/setters using Object.defineProperty Show archive.org snapshot :

object = {}

fooValue = undefined

Object.defineProperty object, 'foo', 
  get: -> fooValue
  set: (newValue) -> fooValue = newValue

Some people are also doing some crazy things Show archive.org snapshot to allow easier definition of getters/setters in class definitions.

Performance considerations

Read and write accesses have the same performance characteristics of regular function calls, so it's slower than a dumb property.