The old way

__defineGetter and __defineSetter__ have been deprecated in the time since this question was initially posted.

Accessing those accessor and mutator functions was handled with __lookupGetter__ and __lookupSetter__ respectively.

Example

function MyClass() {
 var MyField
 this.__defineGetter__("MyField",function(){
  return MyField
 })
 this.__defineSetter__("MyField",function(value){
  MyField = value
 })
}

var obj = new MyClass()
obj.__lookupSetter__("MyField")("MyValue")
console.log(obj.MyField)

The new way

The current standard way to create a dynamic accessor or mutator is to use Object.defineProperty or Object.defineProperties.

Accessing accessor and mutator functions can be done using Object.getOwnPropertyDescriptor and Object.getOwnPropertyDescriptors.

Example

function MyClass() {
 let MyField
 Object.defineProperty(this, 'MyField', {
   get: () => MyField,
   set: value => {
     MyField = value
   }
 })
}

var obj = new MyClass()
Object.getOwnPropertyDescriptor(obj, 'MyField').set('MyValue')
console.log(obj.MyField)

ES6 Classes

Note that Object.getOwnPropertyDescriptor works even when using get and set keywords, however there are a couple gotchas if you're using the class keyword.

Accessors and mutators defined on a class are not "own properties" of the instances of that class. Instead they belong to the prototype for that class (class is just syntactic sugar for declaring a prototype).

Because the functions added to the class are added to the prototype you will need to be careful about context when calling the getter or setter.

class MyClass {
  get MyField() {
    return this._myField
  }
  set MyField(value) {
    this._myField = value
  }
}

const obj = new MyClass()
Object.getOwnPropertyDescriptor(MyClass.prototype, 'MyField').set.call(obj, 'MyValue')
console.log(obj.MyField)