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what does mean a 'get' statement in a function? [duplicate]

I've been trying to get my head around getters and setters and its not sinking in. I've read JavaScript Getters and Setters and Defining Getters and Setters and just not getting it.
Can someone clearly state:
What a getter and setter are meant to do, and
Give some VERY simple examples?

In addition to #millimoose's answer, setters can also be used to update other values.
function Name(first, last) {
this.first = first;
this.last = last;
}
Name.prototype = {
get fullName() {
return this.first + " " + this.last;
},
set fullName(name) {
var names = name.split(" ");
this.first = names[0];
this.last = names[1];
}
};
Now, you can set fullName, and first and last will be updated and vice versa.
n = new Name('Claude', 'Monet')
n.first # "Claude"
n.last # "Monet"
n.fullName # "Claude Monet"
n.fullName = "Gustav Klimt"
n.first # "Gustav"
n.last # "Klimt"

Getters and Setters in JavaScript
Overview
Getters and setters in JavaScript are used for defining computed properties, or accessors. A computed property is one that uses a function to get or set an object value. The basic theory is doing something like this:
var user = { /* ... object with getters and setters ... */ };
user.phone = '+1 (123) 456-7890'; // updates a database
console.log( user.areaCode ); // displays '123'
console.log( user.area ); // displays 'Anytown, USA'
This is useful for automatically doing things behind-the-scenes when a property is accessed, like keeping numbers in range, reformatting strings, triggering value-has-changed events, updating relational data, providing access to private properties, and more.
The examples below show the basic syntax, though they simply get and set the internal object value without doing anything special. In real-world cases you would modify the input and/or output value to suit your needs, as noted above.
get/set Keywords
ECMAScript 5 supports get and set keywords for defining computed properties. They work with all modern browsers except IE 8 and below.
var foo = {
bar : 123,
get bar(){ return bar; },
set bar( value ){ this.bar = value; }
};
foo.bar = 456;
var gaz = foo.bar;
Custom Getters and Setters
get and set aren't reserved words, so they can be overloaded to create your own custom, cross-browser computed property functions. This will work in any browser.
var foo = {
_bar : 123,
get : function( name ){ return this[ '_' + name ]; },
set : function( name, value ){ this[ '_' + name ] = value; }
};
foo.set( 'bar', 456 );
var gaz = foo.get( 'bar' );
Or for a more compact approach, a single function may be used.
var foo = {
_bar : 123,
value : function( name /*, value */ ){
if( arguments.length < 2 ){ return this[ '_' + name ]; }
this[ '_' + name ] = value;
}
};
foo.value( 'bar', 456 );
var gaz = foo.value( 'bar' );
Avoid doing something like this, which can lead to code bloat.
var foo = {
_a : 123, _b : 456, _c : 789,
getA : function(){ return this._a; },
getB : ..., getC : ..., setA : ..., setB : ..., setC : ...
};
For the above examples, the internal property names are abstracted with an underscore in order to discourage users from simply doing foo.bar vs. foo.get( 'bar' ) and getting an "uncooked" value. You can use conditional code to do different things depending on the name of the property being accessed (via the name parameter).
Object.defineProperty()
Using Object.defineProperty() is another way to add getters and setters, and can be used on objects after they're defined. It can also be used to set configurable and enumerable behaviors. This syntax also works with IE 8, but unfortunately only on DOM objects.
var foo = { _bar : 123 };
Object.defineProperty( foo, 'bar', {
get : function(){ return this._bar; },
set : function( value ){ this._bar = value; }
} );
foo.bar = 456;
var gaz = foo.bar;
__defineGetter__()
Finally, __defineGetter__() is another option. It's deprecated, but still widely used around the web and thus unlikely to disappear anytime soon. It works on all browsers except IE 10 and below. Though the other options also work well on non-IE, so this one isn't that useful.
var foo = { _bar : 123; }
foo.__defineGetter__( 'bar', function(){ return this._bar; } );
foo.__defineSetter__( 'bar', function( value ){ this._bar = value; } );
Also worth noting is that in the latter examples, the internal names must be different than the accessor names to avoid recursion (ie, foo.bar calling foo.get(bar) calling foo.bar calling foo.get(bar)...).
See Also
MDN get, set,
Object.defineProperty(), __defineGetter__(), __defineSetter__()
MSDN
IE8 Getter Support

You'd use them for instance to implement computed properties.
For example:
function Circle(radius) {
this.radius = radius;
}
Object.defineProperty(Circle.prototype, 'circumference', {
get: function() { return 2*Math.PI*this.radius; }
});
Object.defineProperty(Circle.prototype, 'area', {
get: function() { return Math.PI*this.radius*this.radius; }
});
c = new Circle(10);
console.log(c.area); // Should output 314.159
console.log(c.circumference); // Should output 62.832
(CodePen)

Sorry to resurrect an old question, but I thought I might contribute a couple of very basic examples and for-dummies explanations. None of the other answers posted thusfar illustrate syntax like the MDN guide's first example, which is about as basic as one can get.
Getter:
var settings = {
firstname: 'John',
lastname: 'Smith',
get fullname() { return this.firstname + ' ' + this.lastname; }
};
console.log(settings.fullname);
... will log John Smith, of course. A getter behaves like a variable object property, but offers the flexibility of a function to calculate its returned value on the fly. It's basically a fancy way to create a function that doesn't require () when calling.
Setter:
var address = {
set raw(what) {
var loc = what.split(/\s*;\s*/),
area = loc[1].split(/,?\s+(\w{2})\s+(?=\d{5})/);
this.street = loc[0];
this.city = area[0];
this.state = area[1];
this.zip = area[2];
}
};
address.raw = '123 Lexington Ave; New York NY 10001';
console.log(address.city);
... will log New York to the console. Like getters, setters are called with the same syntax as setting an object property's value, but are yet another fancy way to call a function without ().
See this jsfiddle for a more thorough, perhaps more practical example. Passing values into the object's setter triggers the creation or population of other object items. Specifically, in the jsfiddle example, passing an array of numbers prompts the setter to calculate mean, median, mode, and range; then sets object properties for each result.

Getters and setters really only make sense when you have private properties of classes. Since Javascript doesn't really have private class properties as you would normally think of from Object Oriented Languages, it can be hard to understand. Here is one example of a private counter object. The nice thing about this object is that the internal variable "count" cannot be accessed from outside the object.
var counter = function() {
var count = 0;
this.inc = function() {
count++;
};
this.getCount = function() {
return count;
};
};
var i = new Counter();
i.inc();
i.inc();
// writes "2" to the document
document.write( i.getCount());
If you are still confused, take a look at Crockford's article on Private Members in Javascript.

I think the first article you link to states it pretty clearly:
The obvious advantage to writing JavaScript in this manner is that you can use it obscure values that you don't want the user to directly access.
The goal here is to encapsulate and abstract away the fields by only allowing access to them thru a get() or set() method. This way, you can store the field/data internally in whichever way you want, but outside components are only away of your published interface. This allows you to make internal changes without changing external interfaces, to do some validation or error-checking within the set() method, etc.

Although often we are used to seeing objects with public properties without any access
control, JavaScript allows us to accurately describe properties. In fact, we can use
descriptors in order to control how a property can be accessed and which logic we can
apply to it. Consider the following example:
var employee = {
first: "Boris",
last: "Sergeev",
get fullName() {
return this.first + " " + this.last;
},
set fullName(value) {
var parts = value.toString().split(" ");
this.first = parts[0] || "";
this.last = parts[1] || "";
},
email: "boris.sergeev#example.com"
};
The final result:
console.log(employee.fullName); //Boris Sergeev
employee.fullName = "Alex Makarenko";
console.log(employee.first);//Alex
console.log(employee.last);//Makarenko
console.log(employee.fullName);//Alex Makarenko

You can define instance method for js class, via prototype of the constructor.
Following is the sample code:
// BaseClass
var BaseClass = function(name) {
// instance property
this.name = name;
};
// instance method
BaseClass.prototype.getName = function() {
return this.name;
};
BaseClass.prototype.setName = function(name) {
return this.name = name;
};
// test - start
function test() {
var b1 = new BaseClass("b1");
var b2 = new BaseClass("b2");
console.log(b1.getName());
console.log(b2.getName());
b1.setName("b1_new");
console.log(b1.getName());
console.log(b2.getName());
}
test();
// test - end
And, this should work for any browser, you can also simply use nodejs to run this code.

If you're referring to the concept of accessors, then the simple goal is to hide the underlying storage from arbitrary manipulation. The most extreme mechanism for this is
function Foo(someValue) {
this.getValue = function() { return someValue; }
return this;
}
var myFoo = new Foo(5);
/* We can read someValue through getValue(), but there is no mechanism
* to modify it -- hurrah, we have achieved encapsulation!
*/
myFoo.getValue();
If you're referring to the actual JS getter/setter feature, eg. defineGetter/defineSetter, or { get Foo() { /* code */ } }, then it's worth noting that in most modern engines subsequent usage of those properties will be much much slower than it would otherwise be. eg. compare performance of
var a = { getValue: function(){ return 5; }; }
for (var i = 0; i < 100000; i++)
a.getValue();
vs.
var a = { get value(){ return 5; }; }
for (var i = 0; i < 100000; i++)
a.value;

What's so confusing about it... getters are functions that are called when you get a property, setters, when you set it.
example, if you do
obj.prop = "abc";
You're setting the property prop, if you're using getters/setters, then the setter function will be called, with "abc" as an argument.
The setter function definition inside the object would ideally look something like this:
set prop(var) {
// do stuff with var...
}
I'm not sure how well that is implemented across browsers. It seems Firefox also has an alternative syntax, with double-underscored special ("magic") methods. As usual Internet Explorer does not support any of this.

I was also somewhat confused by the explanation I read, because I was trying to add a property to an existing prototype that I did not write, so replacing the prototype seemed like the wrong approach. So, for posterity, here's how I added a last property to Array:
Object.defineProperty(Array.prototype, "last", {
get: function() { return this[this.length - 1] }
});
Ever so slightly nicer than adding a function IMHO.

You can also use __defineGetter__:
function Vector2(x,y) {
this.x = x;
this.y = y;
}
Vector2.prototype.__defineGetter__("magnitude", function () {
return Math.sqrt(this.x*this.x+this.y*this.y);
});
console.log(new Vector2(1,1).magnitude)
Or, if you prefer:
function Vector2(x,y) {
this.x = x;
this.y = y;
this.__defineGetter__("magnitude", function () {
return Math.sqrt(this.x*this.x+this.y*this.y);
});
}
console.log(new Vector2(1,1).magnitude)
But this function has been flagged as "legacy" recently, being dropped in favor of Object.defineProperty().

There's no example here with ES6 class (which is not even 'new' now, it's the norm):
class Student {
contructor(firstName, lastName){
this.firstName = firstName
this.lastName = lastName
this.secretId = Math.random()
}
get fullName() {
return `${this.firstName} ${this.lastName}`; // this is backtick in js, u can check it out here: https://stackoverflow.com/a/27678299/12056841
}
set firstName(newFirstName) {
// validate that newFirstName is a string (and maybe limit length)
this.firstName = newFirstName
}
get studentId() { return this.secretId }
}
and no setter for secretId because we don't want anyone to change it.
** if secretId shouldn't be changed at all, a nice approach is to declare it as 'private' to this class by adding a '#' to it
(e.g: this.#secretId = Math.random(), and return this.#secretId
Update: about backing fields
You might need to rename your field - or your setter function but it makes more sense to me to change your field name. One option is like I mentioned above (using a # for declaring the field as 'private'). Another way is to just rename it (_firstName, firstName_...)

I've got one for you guys that might be a little ugly, but it does get'er done across platforms
function myFunc () {
var _myAttribute = "default";
this.myAttribute = function() {
if (arguments.length > 0) _myAttribute = arguments[0];
return _myAttribute;
}
}
this way, when you call
var test = new myFunc();
test.myAttribute(); //-> "default"
test.myAttribute("ok"); //-> "ok"
test.myAttribute(); //-> "ok"
If you really want to spice things up.. you can insert a typeof check:
if (arguments.length > 0 && typeof arguments[0] == "boolean") _myAttribute = arguments[0];
if (arguments.length > 0 && typeof arguments[0] == "number") _myAttribute = arguments[0];
if (arguments.length > 0 && typeof arguments[0] == "string") _myAttribute = arguments[0];
or go even crazier with the advanced typeof check: type.of() code at codingforums.com

Should one use getters and setters for private variables?

I'm using JavaScript objects like this:
var obj = new Foob;
should I pretend like there is a private way and do:
obj.get('foo');
or should I just try to access directly as :
obj.foo

You can actually have variables which can only be accessed through setters and getters in Javascript:
function Foob(){
var foo = 5;
this.getFoo = function(){
return foo;
}
this.setFoo = function(newFoo){
foo = newFoo;
return this;
}
}
var obj = new Foob;
obj.getFoo(); // 5
obj.foo; // undefined
Or if you want a generic getter/setter:
function Foob(){
// You can set default values in the data object
var data = {};
this.get = function(key){
return data[key];
}
this.set = function(key, value){
data[key] = value;
return this;
}
}

One least known feature of Javascript is that it supports getters and setters natively.
When defining a property, you can either define it simply by :
someObj.prop = 12;
OR you can define getters and setters , using Object.defineProperty and the reserved words get and set :
Object.defineProperty ( someObj, "prop" ,
{ get : function () { return ??; } ,
set : function (val) { /* store val */ } } ;
A way of using such getters/setters inside a 'class' to get a private variable would be :
var MyClass = function() {
var _private = 5;
Object.defineProperty(this, "public", {
get : function() { return _private; },
set : function(val) { _private=val; }
});
return this;
};
var anInstance = new MyClass();
anInstance.public = 12 ;
console.log(anInstance.public) ; // writes 12
console.log(anInstance._private) ; // writes undefined.
so you have a truly private variable, armed with a getter and a setter.
Obviously, there is not much interest to use getters/setters code , unless you want to make bound-checking/type-checking or have a another specific reason.

I used to like the idea of getters and setters when I started using object-oriented JavaScript heavily, but that is because I was coming from a Java background.
ES5 supports getters and setters through a special syntax
See John Resig's explanation:
http://ejohn.org/blog/javascript-getters-and-setters/
My take is to think about why getters/setters are useful. It's so one has a way to encapsulate a variable's access so that it can be intercepted / controlled. If calling code directly mutates another object's instances variables, then you can't change it transparently. Needing to catch all of these mutations requires changing variable scope, adding a getter and setter and altering all calling code.
However, this syntax is transparent to calling code. Therefore, you can simply allow a property to be directly controlled and then if you need to intercept it, to say add a console.log for debugging, you can ADD a getter and setter and it will just work.
function Foob() {
}
Foob.prototype = {
get foo() {
return this._foo;
},
set foo(foo) {
this._foo = foo;
}
};
var o = new Foob();
console.log(o.foo);
The downside to the getter/setter syntax is that it doesn't actually make your variable private, it only "hides" it so that you can use some secret internal name, unless you define it within the constructor as Vincent indicated.
To get truly private:
function Foob() {
var foo;
this.__defineGetter__('foo', function () {
return foo;
});
this.__defineSetter__('foo', function (_foo) {
foo = _foo;
});
}
var o = new Foob();
console.log(o.foo);

OO Javascript: good way to combine prototypal inheritance with private vars?

In OO Javascript constructor pattern: neo-classical vs prototypal, I learned that constructors using prototypal inheritance can be 10x faster (or more) than constructors using the so-called neo-classical pattern with closures as proposed by Crockford in his "Good Parts" book and presentations.
For that reason it seems like preferring prototypal inheritance seems like the right thing, in general.
Question Is there a way to combine prototypal inheritance with the module pattern to allow private variables when necessary?
What I am thinking is:
// makeClass method - By John Resig (MIT Licensed)
function makeClass(){
return function(args){
if ( this instanceof arguments.callee ) {
if ( typeof this.init == "function" )
this.init.apply( this, args.callee ? args : arguments );
} else
return new arguments.callee( arguments );
};
}
// =======================================================
var User = makeClass();
// convention; define an init method and attach to the prototype
User.prototype.init = function(first, last){
this.name = first + " " + last;
};
User.prototype.doWork = function (a,b,c) {/* ... */ };
User.prototype.method2= (function (a,b,c) {
// this code is run once per class
return function(a,b,c) {
// this code gets run with each call into the method
var _v2 = 0;
function inc() {
_v2++;
}
var dummy = function(a,b,c) {
/* ... */
inc();
WScript.echo("doOtherWork(" + this.name + ") v2= " + _v2);
return _v2;
};
var x = dummy(a,b,c);
this.method2 = dummy; // replace self
return x;
};
})();
That isn't quite right. But it illustrates the point.
Is there a way to do this and is it worth it?

preferring prototypal inheritance seems like the right thing, in general
Well... it's the more natural, native-feeling thing to do in JavaScript, certainly. But so much JavaScript does is wrong that this isn't necessarily a compliment!
Certainly when performance isn't an issue, objects that get their own bound copies of each method are easier to cope with than objects that share their methods, because you can just pass on a reference to object.method without having to make a closure-delegate or function.bind.
Is there a way to combine prototypal inheritance with the module pattern to allow private variables when necessary?
What do you want from private variables? If it's some Java-style idea of security by encapsulation, I'd give that up and just do it the Python way: put an underline on the start of the member name and anyone who wants to use from the outside will be suitably warned that it's unsupported and may screw up. There is never a security boundary inside JavaScript code executing on the same page that would warrant keeping your privates really private.
If what you want is to avoid having to locate the right copy of this when the method is called, you could manually bind methods methods in the initialiser:
var Thing= makeClass();
Thing.prototype.init= function(a) {
this._a= a;
this.showA= this.showA.bind(this);
};
Thing.prototype.showA= function() {
alert(this._a);
};
thing= new Thing(3);
setTimeout(thing.showA, 1000); // will work as `thing` has its own bound copy of `showA`
(function.bind is future-JavaScript that you can hack into the Function.prototype now until browsers support it.)
This naturally loses some of the lightweight nature of prototype-based objects, but at least you can still have them share members that aren't methods, and methods that are never going to be used as delegates, as long as it's clear and you can always remember which methods are the ones that you can use this way.
If you simply want to be able to type a private variable name without having to put this. all the time then, yeah, you'd have to do it with a closure. Your example world maybe be a little clearer called from the initialiser rather than using the first-time-self-writing:
var User= makeClass();
User.prototype.init= function(first, last){
this.name= first+' '+last;
this.method2= this._method2factory();
};
User.prototype._method2factory= function() {
var _v2= 0;
function inc() {
_v2++;
}
return function method2(a,b,c) {
/* ... */
inc();
WScript.echo('doOtherWork('+this.name+') v2= '+_v2);
return _v2;
};
};
But I'm not really sure this gets you much in comparison to just writing this._v2 and this._inc().

I'm not totally sure I understand your question. But going from what I think I do understand...
function Foo () { /*constructor*/
var counter = 0; /* private variable */
this.incrementCounter=function () { /*privileged function */
counter++
}
this.getCounter=function () { /*privileged function */
return counter;
}
}
/*public functions. Note: this pattern destroys constructor property.
Lesson: Don't depend on the constructor property! */
Foo.prototype = {
toString: function () {
var string = "";
var count = this.getCounter();
while(count--) {
string+="*"
}
return string;
}
}
var bar = new Foo();
bar.incrementCounter();
bar.incrementCounter();
bar.toString(); /* in theory, this returns "**".. haven't tested code */

You may take a look at
https://github.com/riga/jclass
I think that's what you're looking for.

Personally, I prefer the following syntax :
var keyValueStore = (function() {
// Singleton private properties
var count = 0;
var kvs = function() {
// Instance private / privileged properties
count++;
};
kvs.prototype = {
// Instance public properties
'data' : {},
'get' : function(key) { return this.data[key]; },
'set' : function(key, value) { this.data[key] = value; },
'delete' : function(key) { delete this.data[key]; },
'getLength' : function() {
var l = 0;
for (p in this.data) l++;
return l;
}
};
return {
// Singleton public properties
'create' : function() { return new kvs(); },
'count' : function() { return count; }
};
})();
With this syntax, you have a singleton object, the possibility to create instances with prototype inheritance and the possibility to define private properties at several levels.
You use it like this :
kvs = keyValueStore.create();
kvs.set('Tom', "Baker");
kvs.set('Daisy', "Hostess");
var profession_of_daisy = kvs.get('Daisy');
kvs.delete('Daisy');
console.log(keyValueStore.count());

Getters \ setters for dummies

I've been trying to get my head around getters and setters and its not sinking in. I've read JavaScript Getters and Setters and Defining Getters and Setters and just not getting it.
Can someone clearly state:
What a getter and setter are meant to do, and
Give some VERY simple examples?

In addition to #millimoose's answer, setters can also be used to update other values.
function Name(first, last) {
this.first = first;
this.last = last;
}
Name.prototype = {
get fullName() {
return this.first + " " + this.last;
},
set fullName(name) {
var names = name.split(" ");
this.first = names[0];
this.last = names[1];
}
};
Now, you can set fullName, and first and last will be updated and vice versa.
n = new Name('Claude', 'Monet')
n.first # "Claude"
n.last # "Monet"
n.fullName # "Claude Monet"
n.fullName = "Gustav Klimt"
n.first # "Gustav"
n.last # "Klimt"

Getters and Setters in JavaScript
Overview
Getters and setters in JavaScript are used for defining computed properties, or accessors. A computed property is one that uses a function to get or set an object value. The basic theory is doing something like this:
var user = { /* ... object with getters and setters ... */ };
user.phone = '+1 (123) 456-7890'; // updates a database
console.log( user.areaCode ); // displays '123'
console.log( user.area ); // displays 'Anytown, USA'
This is useful for automatically doing things behind-the-scenes when a property is accessed, like keeping numbers in range, reformatting strings, triggering value-has-changed events, updating relational data, providing access to private properties, and more.
The examples below show the basic syntax, though they simply get and set the internal object value without doing anything special. In real-world cases you would modify the input and/or output value to suit your needs, as noted above.
get/set Keywords
ECMAScript 5 supports get and set keywords for defining computed properties. They work with all modern browsers except IE 8 and below.
var foo = {
bar : 123,
get bar(){ return bar; },
set bar( value ){ this.bar = value; }
};
foo.bar = 456;
var gaz = foo.bar;
Custom Getters and Setters
get and set aren't reserved words, so they can be overloaded to create your own custom, cross-browser computed property functions. This will work in any browser.
var foo = {
_bar : 123,
get : function( name ){ return this[ '_' + name ]; },
set : function( name, value ){ this[ '_' + name ] = value; }
};
foo.set( 'bar', 456 );
var gaz = foo.get( 'bar' );
Or for a more compact approach, a single function may be used.
var foo = {
_bar : 123,
value : function( name /*, value */ ){
if( arguments.length < 2 ){ return this[ '_' + name ]; }
this[ '_' + name ] = value;
}
};
foo.value( 'bar', 456 );
var gaz = foo.value( 'bar' );
Avoid doing something like this, which can lead to code bloat.
var foo = {
_a : 123, _b : 456, _c : 789,
getA : function(){ return this._a; },
getB : ..., getC : ..., setA : ..., setB : ..., setC : ...
};
For the above examples, the internal property names are abstracted with an underscore in order to discourage users from simply doing foo.bar vs. foo.get( 'bar' ) and getting an "uncooked" value. You can use conditional code to do different things depending on the name of the property being accessed (via the name parameter).
Object.defineProperty()
Using Object.defineProperty() is another way to add getters and setters, and can be used on objects after they're defined. It can also be used to set configurable and enumerable behaviors. This syntax also works with IE 8, but unfortunately only on DOM objects.
var foo = { _bar : 123 };
Object.defineProperty( foo, 'bar', {
get : function(){ return this._bar; },
set : function( value ){ this._bar = value; }
} );
foo.bar = 456;
var gaz = foo.bar;
__defineGetter__()
Finally, __defineGetter__() is another option. It's deprecated, but still widely used around the web and thus unlikely to disappear anytime soon. It works on all browsers except IE 10 and below. Though the other options also work well on non-IE, so this one isn't that useful.
var foo = { _bar : 123; }
foo.__defineGetter__( 'bar', function(){ return this._bar; } );
foo.__defineSetter__( 'bar', function( value ){ this._bar = value; } );
Also worth noting is that in the latter examples, the internal names must be different than the accessor names to avoid recursion (ie, foo.bar calling foo.get(bar) calling foo.bar calling foo.get(bar)...).
See Also
MDN get, set,
Object.defineProperty(), __defineGetter__(), __defineSetter__()
MSDN
IE8 Getter Support

You'd use them for instance to implement computed properties.
For example:
function Circle(radius) {
this.radius = radius;
}
Object.defineProperty(Circle.prototype, 'circumference', {
get: function() { return 2*Math.PI*this.radius; }
});
Object.defineProperty(Circle.prototype, 'area', {
get: function() { return Math.PI*this.radius*this.radius; }
});
c = new Circle(10);
console.log(c.area); // Should output 314.159
console.log(c.circumference); // Should output 62.832
(CodePen)

Sorry to resurrect an old question, but I thought I might contribute a couple of very basic examples and for-dummies explanations. None of the other answers posted thusfar illustrate syntax like the MDN guide's first example, which is about as basic as one can get.
Getter:
var settings = {
firstname: 'John',
lastname: 'Smith',
get fullname() { return this.firstname + ' ' + this.lastname; }
};
console.log(settings.fullname);
... will log John Smith, of course. A getter behaves like a variable object property, but offers the flexibility of a function to calculate its returned value on the fly. It's basically a fancy way to create a function that doesn't require () when calling.
Setter:
var address = {
set raw(what) {
var loc = what.split(/\s*;\s*/),
area = loc[1].split(/,?\s+(\w{2})\s+(?=\d{5})/);
this.street = loc[0];
this.city = area[0];
this.state = area[1];
this.zip = area[2];
}
};
address.raw = '123 Lexington Ave; New York NY 10001';
console.log(address.city);
... will log New York to the console. Like getters, setters are called with the same syntax as setting an object property's value, but are yet another fancy way to call a function without ().
See this jsfiddle for a more thorough, perhaps more practical example. Passing values into the object's setter triggers the creation or population of other object items. Specifically, in the jsfiddle example, passing an array of numbers prompts the setter to calculate mean, median, mode, and range; then sets object properties for each result.

Getters and setters really only make sense when you have private properties of classes. Since Javascript doesn't really have private class properties as you would normally think of from Object Oriented Languages, it can be hard to understand. Here is one example of a private counter object. The nice thing about this object is that the internal variable "count" cannot be accessed from outside the object.
var counter = function() {
var count = 0;
this.inc = function() {
count++;
};
this.getCount = function() {
return count;
};
};
var i = new Counter();
i.inc();
i.inc();
// writes "2" to the document
document.write( i.getCount());
If you are still confused, take a look at Crockford's article on Private Members in Javascript.

I think the first article you link to states it pretty clearly:
The obvious advantage to writing JavaScript in this manner is that you can use it obscure values that you don't want the user to directly access.
The goal here is to encapsulate and abstract away the fields by only allowing access to them thru a get() or set() method. This way, you can store the field/data internally in whichever way you want, but outside components are only away of your published interface. This allows you to make internal changes without changing external interfaces, to do some validation or error-checking within the set() method, etc.

Although often we are used to seeing objects with public properties without any access
control, JavaScript allows us to accurately describe properties. In fact, we can use
descriptors in order to control how a property can be accessed and which logic we can
apply to it. Consider the following example:
var employee = {
first: "Boris",
last: "Sergeev",
get fullName() {
return this.first + " " + this.last;
},
set fullName(value) {
var parts = value.toString().split(" ");
this.first = parts[0] || "";
this.last = parts[1] || "";
},
email: "boris.sergeev#example.com"
};
The final result:
console.log(employee.fullName); //Boris Sergeev
employee.fullName = "Alex Makarenko";
console.log(employee.first);//Alex
console.log(employee.last);//Makarenko
console.log(employee.fullName);//Alex Makarenko

You can define instance method for js class, via prototype of the constructor.
Following is the sample code:
// BaseClass
var BaseClass = function(name) {
// instance property
this.name = name;
};
// instance method
BaseClass.prototype.getName = function() {
return this.name;
};
BaseClass.prototype.setName = function(name) {
return this.name = name;
};
// test - start
function test() {
var b1 = new BaseClass("b1");
var b2 = new BaseClass("b2");
console.log(b1.getName());
console.log(b2.getName());
b1.setName("b1_new");
console.log(b1.getName());
console.log(b2.getName());
}
test();
// test - end
And, this should work for any browser, you can also simply use nodejs to run this code.

If you're referring to the concept of accessors, then the simple goal is to hide the underlying storage from arbitrary manipulation. The most extreme mechanism for this is
function Foo(someValue) {
this.getValue = function() { return someValue; }
return this;
}
var myFoo = new Foo(5);
/* We can read someValue through getValue(), but there is no mechanism
* to modify it -- hurrah, we have achieved encapsulation!
*/
myFoo.getValue();
If you're referring to the actual JS getter/setter feature, eg. defineGetter/defineSetter, or { get Foo() { /* code */ } }, then it's worth noting that in most modern engines subsequent usage of those properties will be much much slower than it would otherwise be. eg. compare performance of
var a = { getValue: function(){ return 5; }; }
for (var i = 0; i < 100000; i++)
a.getValue();
vs.
var a = { get value(){ return 5; }; }
for (var i = 0; i < 100000; i++)
a.value;

What's so confusing about it... getters are functions that are called when you get a property, setters, when you set it.
example, if you do
obj.prop = "abc";
You're setting the property prop, if you're using getters/setters, then the setter function will be called, with "abc" as an argument.
The setter function definition inside the object would ideally look something like this:
set prop(var) {
// do stuff with var...
}
I'm not sure how well that is implemented across browsers. It seems Firefox also has an alternative syntax, with double-underscored special ("magic") methods. As usual Internet Explorer does not support any of this.

I was also somewhat confused by the explanation I read, because I was trying to add a property to an existing prototype that I did not write, so replacing the prototype seemed like the wrong approach. So, for posterity, here's how I added a last property to Array:
Object.defineProperty(Array.prototype, "last", {
get: function() { return this[this.length - 1] }
});
Ever so slightly nicer than adding a function IMHO.

You can also use __defineGetter__:
function Vector2(x,y) {
this.x = x;
this.y = y;
}
Vector2.prototype.__defineGetter__("magnitude", function () {
return Math.sqrt(this.x*this.x+this.y*this.y);
});
console.log(new Vector2(1,1).magnitude)
Or, if you prefer:
function Vector2(x,y) {
this.x = x;
this.y = y;
this.__defineGetter__("magnitude", function () {
return Math.sqrt(this.x*this.x+this.y*this.y);
});
}
console.log(new Vector2(1,1).magnitude)
But this function has been flagged as "legacy" recently, being dropped in favor of Object.defineProperty().

There's no example here with ES6 class (which is not even 'new' now, it's the norm):
class Student {
contructor(firstName, lastName){
this.firstName = firstName
this.lastName = lastName
this.secretId = Math.random()
}
get fullName() {
return `${this.firstName} ${this.lastName}`; // this is backtick in js, u can check it out here: https://stackoverflow.com/a/27678299/12056841
}
set firstName(newFirstName) {
// validate that newFirstName is a string (and maybe limit length)
this.firstName = newFirstName
}
get studentId() { return this.secretId }
}
and no setter for secretId because we don't want anyone to change it.
** if secretId shouldn't be changed at all, a nice approach is to declare it as 'private' to this class by adding a '#' to it
(e.g: this.#secretId = Math.random(), and return this.#secretId
Update: about backing fields
You might need to rename your field - or your setter function but it makes more sense to me to change your field name. One option is like I mentioned above (using a # for declaring the field as 'private'). Another way is to just rename it (_firstName, firstName_...)

I've got one for you guys that might be a little ugly, but it does get'er done across platforms
function myFunc () {
var _myAttribute = "default";
this.myAttribute = function() {
if (arguments.length > 0) _myAttribute = arguments[0];
return _myAttribute;
}
}
this way, when you call
var test = new myFunc();
test.myAttribute(); //-> "default"
test.myAttribute("ok"); //-> "ok"
test.myAttribute(); //-> "ok"
If you really want to spice things up.. you can insert a typeof check:
if (arguments.length > 0 && typeof arguments[0] == "boolean") _myAttribute = arguments[0];
if (arguments.length > 0 && typeof arguments[0] == "number") _myAttribute = arguments[0];
if (arguments.length > 0 && typeof arguments[0] == "string") _myAttribute = arguments[0];
or go even crazier with the advanced typeof check: type.of() code at codingforums.com

JavaScript automatic getter/setters (John Resig book)

I'm reading "Pro JavaScript Techniques" by John Resig, and I'm confused with an example. This is the code:
// Create a new user object that accepts an object of properties
function User( properties ) {
// Iterate through the properties of the object, and make sure
// that it's properly scoped (as discussed previously)
for ( var i in properties ) { (function(){
// Create a new getter for the property
this[ "get" + i ] = function() {
return properties[i];
};
// Create a new setter for the property
this[ "set" + i ] = function(val) {
properties[i] = val;
};
})(); }
}
// Create a new user object instance and pass in an object of
// properties to seed it with
var user = new User({
name: "Bob",
age: 44
});
// Just note that the name property does not exist, as it's private
// within the properties object
alert( user.name == null );
// However, we're able to access its value using the new getname()
// method, that was dynamically generated
alert( user.getname() == "Bob" );
// Finally, we can see that it's possible to set and get the age using
// the newly generated functions
user.setage( 22 );
alert( user.getage() == 22 );
Now running that in the Firebug console (on Firefox 3) throws that user.getname() is not a function. I tried doing this:
var other = User
other()
window.getname() // --> This works!
And it worked!
Why?
Doing:
var me = this;
seems to work a bit better, but when executing "getname()" it returns '44' (the second property)...
Also I find it strange that it worked on the window object without modification...
And a third question, what's the difference between PEZ solution and the original? (He doesn't use an anonymous function.)

I think it's best not to use the new keyword at all when working in JavaScript.
This is because if you then instantiate the object without using the new keyword (ex: var user = User()) by mistake, *very bad things will happen...*reason being that in the function (if instantiated without the new keyword), the this will refer to the global object, ie the window...
So therefore, I suggest a better way on how to use class-like objects.
Consider the following example :
var user = function (props) {
var pObject = {};
for (p in props) {
(function (pc) {
pObject['set' + pc] = function (v) {
props[pc] = v;
return pObject;
}
pObject['get' + pc] = function () {
return props[pc];
}
})(p);
}
return pObject;
}
In the above example, I am creating a new object inside of the function, and then attaching getters and setters to this newly created object.
Finally, I am returning this newly created object. Note that the the this keyword is not used anywhere
Then, to 'instantiate' a user, I would do the following:
var john = user({name : 'Andreas', age : 21});
john.getname(); //returns 'Andreas'
john.setage(19).getage(); //returns 19
The best way to avoid falling into pitfalls is by not creating them in the first place...In the above example, I am avoiding the new keyword pitfall (as i said, not using the new keyword when it's supposed to be used will cause bad things to happen) by not using new at all.

Adapting Jason's answer, it works:
We need to make a closure for the values. Here's one way:
function bindAccessors(o, property, value) {
var _value = value;
o["get" + property] = function() {
return _value;
};
o["set" + property] = function(v) {
_value = v;
};
}
Then the User constructor looks like this:
function User( properties ) {
for (var i in properties ) {
bindAccessors(this, i, properties[i]);
}
}

You probably want something like this, which is more readable (closures are easy to learn once you get some practice):
function User( properties ) {
// Helper function to create closures based on passed-in arguments:
var bindGetterSetter = function(obj, p, properties)
{
obj["get" + p] = function() { return properties[p]; }
obj["set" + p] = function(val) { properties[p]=val; return this; }
};
for (var p in properties)
bindGetterSetter(this, p, properties);
}
I also added "return this;", so you can do:
u = new User({a: 1, b:77, c:48});
u.seta(3).setb(20).setc(400)

I started this post with the sole purpose of learning why that things happened, and I finally did. So in case there's someone else interested in the "whys", here they are:
Why does 'this' changes inside the anonymous function?
A new function, even if it is an anonymous, declared inside an object or another function, always changes the scope, in this case returning to the global scope (window).
Solution: all stated in the post, I think the clearer is executing the anonymous function with .call(this).
Why does getname() always return the age?
While the anonymous function gets executed right away, the getters/setters get executed for the first time when they are called. In that moment, the value of i will always be the last, because it has already iterated for all the properties... and it will always return properties[i] which is the last value, in this case the age.
Solution: save the i value in a variable like this
for ( i in properties ) { (function(){
var j = i
// From now on, use properties[j]

As written in the OP, this in the loop is not referring to the User object as it should be. If you capture that variable outside the loop, you can make it work:
function User( properties ) {
// Iterate through the properties of the object, and make sure
// that it's properly scoped (as discussed previously)
var me = this;
for ( i in properties ) { (function(){
// Create a new getter for the property
me[ "get" + i ] = function() {
return properties[i];
};
// Create a new setter for the property
me[ "set" + i ] = function(val) {
properties[i] = val;
};
// etc

I just modified the code a bit like this.. This one should work.. This is same as setting me=this; But a closure is required to set the value of each property properly, else the last value will be assigned to all properties.
// Create a new user object that accepts an object of properties
var User = function( properties ) {
// Iterate through the properties of the object, and make sure
// that it's properly scoped (as discussed previously)
var THIS = this;
for ( var i in properties ) { (function(i){
// Create a new getter for the property
THIS[ "get" + i ] = function() {
return properties[i];
};
// Create a new setter for the property
THIS[ "set" + i ] = function(val) {
properties[i] = val;
};
})(i); }
}
// Create a new user object instance and pass in an object of
// properties to seed it with
var user = new User({
name: "Bob",
age: 44
});
// Just note that the name property does not exist, as it's private
// within the properties object
alert( user.name == null );
// However, we're able to access its value using the new getname()
// method, that was dynamically generated
alert( user.getname() == "Bob" );
// Finally, we can see that it's possible to set and get the age using
// the newly generated functions
user.setage( 22 );
alert( user.getage() == 22 );

Maybe the variable i is "closured" with the last value in the iteration ("age")? Then all getters and setters will access properties["age"].

I found something that seems to be the answer; it’s all about context. Using the anonymous function inside the for loop, changes the context, making 'this' refer to the window object. Strange isn't it?
So:
function User(properties) {
for (var i in properties) {
// Here this == User Object
(function(){
// Inside this anonymous function, this == window object
this["get" + i] = function() {
return properties[i];
};
this["set" + i] = function(val) {
properties[i] = val;
};
})();
}
}
I don't know why that function changes the context of execution, and I'm not sure it should do that. Anyway, you can test it running the code there and trying window.getname(). It magically works! :S
The solution, as stated before, is changing the context. It can be done like J Cooper said, passing the variable 'me' and making the function a closure or you can do this:
(function(){
// Inside this anonymous function this == User
// because we called it with 'call'
this[ "get" + i ] = function() {
return properties[i];
};
this["set" + i] = function(val) {
properties[i] = val;
};
}).call(this);
Anyway, I'm still getting 44 when running 'getname'... What could it be?