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I recently read about the fact that there is a possibility of defining getters/setters in JavaScript. It seems extremely helpful - the setter is a kind of 'helper' which can parse the value to be set first, before actually setting it.
For example, I currently have this code:
var obj = function(value) {
var test = !!value; // 'test' has to be a boolean
return {
get test() { return test },
set test(value) { test = !!value }
};
};
var instance = new obj(true);
This code always converts value to a boolean. So if you code instance.test = 0, then instance.test === false.
However, for this to work you have to actually return an object, which means that the new instance is not of type obj but just is a plain object. This means that changing the prototype of obj has no effect on instances. For example, this does not work - instance.func is undefined:
obj.prototype.func = function() { console.log(this.value); };
because instance is not of type obj. To get the prototype functions work, I guess I should not return a plain object, but rather not return anything so that instance would just be of type obj, like a regular constructor works.
The problem then is how to implement getters/setters? I can only find articles describing how to add these to an object, not as being part of the constructor of a custom type.
So how do I implement getters/setters in the constructor so as to be able to both use getters/setters and extending the prototype?
You can't do that.
You can set setter/getters for properties of objects though. I advice you use ES5 Object.defineProperties though. of course this only works in modern browsers.
var obj = function() {
...
Object.defineProperties(this, {
"test": {
"get": function() { ... },
"set": function() { ... }
}
});
}
obj.prototype.func = function() { ... }
var o = new obj;
o.test;
o.func();
Usually you want class methods. The answer by #Raynos on May 7, 2011 gets the job done, but it defines an instance method, not a class method.
The following illustrates a class definition with a the getter and setter being part of the class. This definition is a lot like the answer by #Raynos, but with two differences in the code: (1) The "defineProperties()" action has been moved out of the constructor. (2) The argument to "defineProperties()"as been changed from the instance object "this", to the constructor's prototype object.
function TheConstructor(side) {
this.side = side;
}
Object.defineProperties(TheConstructor.prototype, {
area: {
get: function() { return this.side * this.side; }
,set: function(val) { this.side = Math.sqrt(val); }
}
});
// Test code:
var anInstance = new TheConstructor(2);
console.log("initial Area:"+anInstance.area);
anInstance.area = 9;
console.log("modified Area:"+anInstance.area);
Which produces these results:
initial Area:4
modified Area:9
Although usually the distinction between class versus instance
definition is just a matter of style, there is a purpose to
good style, and there is a case where the distinction matters:
the memoized getter. The purpose for a memoized getter is
described here: Smart/self-overwriting/lazy getters
Define the getter at the class level when the memoized value is to
pertain to the entire class. For example, a configuration file
should be read only once; the resulting values should then apply
for the duration of the program. The following sample code
defines a memoized getter at the class level.
function configureMe() {
return 42;
}
Object.defineProperties(TheConstructor.prototype, {
memoizedConfigParam: {
get: function() {
delete TheConstructor.prototype.memoizedConfigParam;
return TheConstructor.prototype.memoizedConfigParam = configureMe();
}
,configurable: true
}
});
// Test code:
console.log("memoizedConfigParam:"+anInstance.memoizedConfigParam);
Produces:
memoizedConfigParam:42
As can be seen in the example, memoized getters have the
characteristic that the getter function deletes itself,
then replaces itself with a simple value that
(presumably) will never change.
Note that 'configurable' must be set to 'true'.
Define the getter at the instance level when the memoized value
depends upon the contents of instance. The definition moves
inside the constructor, and the object of attention is 'this'.
function TheConstructorI(side) {
this.side = side;
Object.defineProperties(this, {
memoizedCalculation: {
get: function() {
delete this.memoizedCalculation;
return this.memoizedCalculation = this.expensiveOperation();
}
,configurable: true
}
});
}
TheConstructorI.prototype.expensiveOperation = function() {
return this.side * this.side * this.side;
}
//Test code:
var instance2 = new TheConstructorI(2);
var instance3 = new TheConstructorI(3);
console.log("memoizedCalculation 2:"+instance2.memoizedCalculation);
console.log("memoizedCalculation 3:"+instance3.memoizedCalculation);
Produces:
memoizedCalculation 2:8
memoizedCalculation 3:27
If you want to guarantee (rather than presume) that the memoized
value will never be changed, the 'writable' attribute needs to
be changed. That makes the code a bit more complicated.
function TheConstructorJ(side) {
this.side = side;
Object.defineProperties(this, {
memoizedCalculation: {
get: function() {
delete this.memoizedCalculation;
Object.defineProperty( this, 'memoizedCalculation'
,{ value : this.expensiveOperation()
,writable : false
});
return this.memoizedCalculation;
}
,configurable: true
}
});
}
TheConstructorJ.prototype.expensiveOperation = function() {
return this.side * this.side * this.side;
}
//Test code:
var instanceJ = new TheConstructorJ(2);
console.log("memoizedCalculation:"+instanceJ.memoizedCalculation);
instanceJ.memoizedCalculation = 42; // results in error
Produces:
memoizedCalculation:8
>Uncaught TypeError: Cannot assign to read only property 'memoizedCalculation' of object '#<TheConstructorJ>'
The OP's original question, from March 7, 2011, presented basic
getter and setter syntax, noted that it worked on an object but
not on 'this', and asked how to define getters and setters within
a constructor. In addition to all the examples above, there is
also a "cheap-shot" way of doing it: create a new object within
the constructor, like the OP did, but then assign the object to
be a member within 'this'. So, the original code would look like
this:
var MyClass = function(value) {
var test = !!value; // 'test' has to be a boolean
this.data = {
get test() { return test },
set test(value) { test = !!value }
};
};
var instance = new MyClass(true);
// But now 'data' is part of the access path
instance.data.test = 0;
console.log(instance.data.test);
Produces:
false
Believe it or not, I have actually run into situations where
this "cheap-shot" is the best solution. Specifically, I used this
technique when I had records from several tables encapsulated within
a single class, and wanted to present a unified view as though
they were a single record called 'data'.
Have fun.
IAM_AL_X
Update for ES6 -- have a look at section 19.3.1 of Alex Rauschmayer's book Exploring ES6 http://exploringjs.com/es6/ch_maps-sets.html#sec_weakmaps-private-data which demonstrates how to use WeakMaps with getters and setters to hold private data. Combining with section 16.2.2.3 http://exploringjs.com/es6/ch_classes.html#leanpub-auto-getters-and-setters would result in something like
# module test_WeakMap_getter.js
var _MyClassProp = new WeakMap();
class MyClass {
get prop() {
return _MyClassProp.get( this );
}
set prop(value) {
_MyClassProp.set( this, value );
}
}
var mc = new MyClass();
mc.prop = 5 ;
console.log( 'My value is', mc.prop );
$ node --use_strict test_WeakMap_getter.js
My value is 5
function Obj(value){
this.value = !!value;
}
Obj.prototype = {
get test () {
return this.value;``
},
set test (value) {
this.value = !!this.value;
}
};
var obj = new Obj(true);
I know this might be extremely late but I figured out a different way to accomplish what you want and for the sake of people, like myself, googling for an answer to this here it is.
function Constructor(input){
this.input = input;
}
Object.__defineGetter__.call(Constructor.prototype, "value", function(){
return this.input * 2;
});
var test = new Constructor(5);
alert(test.value) // 10
I've tested this in chrome, safari, mobile safari, firefox and they all work (latest versions of course)
#Alex I see it as more option and more power, programming is art, #Nat share his finding with us, and for that I thank him. Maybe someone want to do it that way.
I'm sure the setter version is the same but just changing that g to a s.
i.g:
function Constructor(input){
this.input = input;
}
Object.__defineGetter__.call(Constructor.prototype, "value", function(){
return this.input * 2;
});
Object.__defineSetter__.call(Constructor.prototype, "bar", function(foo){
return this.input *= foo;
});
var test = new Constructor(5);
console.log(test.value); // 10
test.bar = 5;
console.log(test.input); //25
With that said, this feature is deprecated, advices to not to use in production coding.
I have been learning how to use classes in JavaScript, and something that always confused me was the how getters and setters work. I think I now finally understand them, is the below explanation correct?
They are no different to normal methods, and simply provide an alternative syntax.
A getter is simply an alternative to a method which cannot have a parameter, and means you don't have to use () to call, e.g.:
get myGetter() { return { msg: "hello" } };
...
classInstance.myGetter.msg; // "hello"
Is equivalent to:
myGetter() { return { msg: "hello" } };
...
classInstance.myGetter().msg; // "hello"
A setter is simply an alternative for a method that does take a parameter, e.g.:
set mySetter(value) { this.value = value };
...
classInstance.mySetter = "hello";
Is equivalent to:
mySetter(value) { this.value = value };
...
classInstance.mySetter("hello");
Functionally, that explanation is mostly correct, however they also have a more semantic meaning. Getters/setters are very useful for updating things that depend on a value or calculating a value, but they shouldn't be used for triggering actions. For example, this is a wrong usage of a getter:
const alerter = new Alerter;
// [...]
alerter.alert = "Hi there!"; // Alerts "Hi there!"
This is a good one:
const player = new Player;
// [...]
player.health--; // Also updates the health bar
It's also worth noting that, while in most circumstances, they behave like methods, they aren't methods at all! They are part of properties.
In JS, properties can have data descriptors and accessor descriptors. Data descriptors are "normal" properties. They have a value and you can get/set it.
const obj = {
prop: 1;
};
console.log(obj.prop); // Get; logs 1
obj.prop = 2; // Set
Accessor descriptors don't hold a value, and allow for setting what happens when the property is get and set.
const obj = {};
Object.defineProperty(obj, "prop", {
get() {
console.log("Getter was called");
return 1;
},
set(v) {
console.log("Setter was called with the value %o.", v)
}
});
/* Alternative syntax:
class Example {
get prop() {
console.log("Getter was called");
return 1;
}
set prop(v) {
console.log("Setter was called with the value %o.", v)
}
}
const obj = new Example;
*/
console.log(obj.prop); // Get; logs 1
obj.prop = 2; // Set
That code logs:
Getter was called
1
Setter was called with the value 2.
There is a huge difference between getters/setters and normal properties, in their most simple form you could think of them as an alternative syntax. however getters/setters provide more convenient solutions for certain use cases - though eventually getters/setters and methods are properties, getters/setters has accessor descriptors while methods has data descriptors.
I'm gonna list some few use cases on top of my head
getters/setters enable you to trigger custom functionality when reading/setting a property without having to create two different methods
let xThatShouldBeHidden = 1;
const object = {
get x() {
return xThatShouldBeHidden
},
set x(newX) {
if (newX === 0) {
throw new Error('You can not set x to 0')
}
xThatShouldBeHidden = newX
}
}
Triggering custom functionality is a cool feature, it enables you to do optimizations while still abstracting that behind simple syntax.
Imagine you you have array of items that has values, and later you want to get the weight of the item (value / sum of all values of items)
const items = [{val: 2}, {val:4}]
one way to do it would be which required you to loop twice even if eventually the weight was read from only one item
const totalSum = items.reduce((acc,cur), acc + cur.val,0));
const itemsWithWeights = items.map(item => ({...item, weight: item.val / totalSum});
now with getters we do it in one loop plus number of actual reads
const getItemsWithWeightsGetter = () => {
let totalSum;
return items.map(item => ({
...item,
get weight() {
if (totalSum === undefined) {
totalSum = items.reduce((acc, cur) => acc + cur.val, 0);
}
return item.val / totalSum;
},
}));
};
const itemsWithWeightsGetter = getItemsWithWeightsGetter();
another use case is the example i just shared above, when you provide just a getter that makes the value read only, making code throws when you try to set the value - in strict mode only
The difference is, you can have a getter/setter pair with the same name. For example, when working with DOM, there is the innerHTML getter/setter pair.
const element = document.querySelector("div")
console.log(element.innerHTML) // Outputs HTML as string
element.innerHTML = "Hello!" // Sets the HTML of element to "Hello!"
Currently in ES5 many of us are using the following pattern in frameworks to create classes and class variables, which is comfy:
// ES 5
FrameWork.Class({
variable: 'string',
variable2: true,
init: function(){
},
addItem: function(){
}
});
In ES6 you can create classes natively, but there is no option to have class variables:
// ES6
class MyClass {
const MY_CONST = 'string'; // <-- this is not possible in ES6
constructor(){
this.MY_CONST;
}
}
Sadly, the above won't work, as classes only can contain methods.
I understand that I can this.myVar = true in constructor…but I don't want to 'junk' my constructor, especially when I have 20-30+ params for a bigger class.
I was thinking of many ways to handle this issue, but haven't yet found any good ones. (For example: create a ClassConfig handler, and pass a parameter object, which is declared separately from the class. Then the handler would attach to the class. I was thinking about WeakMaps also to integrate, somehow.)
What kind of ideas would you have to handle this situation?
2018 update:
There is now a stage 3 proposal - I am looking forward to make this answer obsolete in a few months.
In the meantime anyone using TypeScript or babel can use the syntax:
varName = value
Inside a class declaration/expression body and it will define a variable. Hopefully in a few months/weeks I'll be able to post an update.
Update: Chrome 74 now ships with this syntax working.
The notes in the ES wiki for the proposal in ES6 (maximally minimal classes) note:
There is (intentionally) no direct declarative way to define either prototype data properties (other than methods) class properties, or instance property
Class properties and prototype data properties need be created outside the declaration.
Properties specified in a class definition are assigned the same attributes as if they appeared in an object literal.
This means that what you're asking for was considered, and explicitly decided against.
but... why?
Good question. The good people of TC39 want class declarations to declare and define the capabilities of a class. Not its members. An ES6 class declaration defines its contract for its user.
Remember, a class definition defines prototype methods - defining variables on the prototype is generally not something you do.
You can, of course use:
constructor(){
this.foo = bar
}
In the constructor like you suggested. Also see the summary of the consensus.
ES7 and beyond
A new proposal for ES7 is being worked on that allows more concise instance variables through class declarations and expressions - https://esdiscuss.org/topic/es7-property-initializers
Just to add to Benjamin's answer — class variables are possible, but you wouldn't use prototype to set them.
For a true class variable you'd want to do something like the following:
class MyClass {}
MyClass.foo = 'bar';
From within a class method that variable can be accessed as this.constructor.foo (or MyClass.foo).
These class properties would not usually be accessible from to the class instance. i.e. MyClass.foo gives 'bar' but new MyClass().foo is undefined
If you want to also have access to your class variable from an instance, you'll have to additionally define a getter:
class MyClass {
get foo() {
return this.constructor.foo;
}
}
MyClass.foo = 'bar';
I've only tested this with Traceur, but I believe it will work the same in a standard implementation.
JavaScript doesn't really have classes. Even with ES6 we're looking at an object- or prototype-based language rather than a class-based language. In any function X () {}, X.prototype.constructor points back to X.
When the new operator is used on X, a new object is created inheriting X.prototype. Any undefined properties in that new object (including constructor) are looked up from there. We can think of this as generating object and class properties.
Babel supports class variables in ESNext, check this example:
class Foo {
bar = 2
static iha = 'string'
}
const foo = new Foo();
console.log(foo.bar, foo.iha, Foo.bar, Foo.iha);
// 2, undefined, undefined, 'string'
In your example:
class MyClass {
const MY_CONST = 'string';
constructor(){
this.MY_CONST;
}
}
Because of MY_CONST is primitive https://developer.mozilla.org/en-US/docs/Glossary/Primitive we can just do:
class MyClass {
static get MY_CONST() {
return 'string';
}
get MY_CONST() {
return this.constructor.MY_CONST;
}
constructor() {
alert(this.MY_CONST === this.constructor.MY_CONST);
}
}
alert(MyClass.MY_CONST);
new MyClass
// alert: string ; true
But if MY_CONST is reference type like static get MY_CONST() {return ['string'];} alert output is string, false. In such case delete operator can do the trick:
class MyClass {
static get MY_CONST() {
delete MyClass.MY_CONST;
return MyClass.MY_CONST = 'string';
}
get MY_CONST() {
return this.constructor.MY_CONST;
}
constructor() {
alert(this.MY_CONST === this.constructor.MY_CONST);
}
}
alert(MyClass.MY_CONST);
new MyClass
// alert: string ; true
And finally for class variable not const:
class MyClass {
static get MY_CONST() {
delete MyClass.MY_CONST;
return MyClass.MY_CONST = 'string';
}
static set U_YIN_YANG(value) {
delete MyClass.MY_CONST;
MyClass.MY_CONST = value;
}
get MY_CONST() {
return this.constructor.MY_CONST;
}
set MY_CONST(value) {
this.constructor.MY_CONST = value;
}
constructor() {
alert(this.MY_CONST === this.constructor.MY_CONST);
}
}
alert(MyClass.MY_CONST);
new MyClass
// alert: string, true
MyClass.MY_CONST = ['string, 42']
alert(MyClass.MY_CONST);
new MyClass
// alert: string, 42 ; true
Since your issue is mostly stylistic (not wanting to fill up the constructor with a bunch of declarations) it can be solved stylistically as well.
The way I view it, many class based languages have the constructor be a function named after the class name itself. Stylistically we could use that that to make an ES6 class that stylistically still makes sense but does not group the typical actions taking place in the constructor with all the property declarations we're doing. We simply use the actual JS constructor as the "declaration area", then make a class named function that we otherwise treat as the "other constructor stuff" area, calling it at the end of the true constructor.
"use strict";
class MyClass
{
// only declare your properties and then call this.ClassName(); from here
constructor(){
this.prop1 = 'blah 1';
this.prop2 = 'blah 2';
this.prop3 = 'blah 3';
this.MyClass();
}
// all sorts of other "constructor" stuff, no longer jumbled with declarations
MyClass() {
doWhatever();
}
}
Both will be called as the new instance is constructed.
Sorta like having 2 constructors where you separate out the declarations and the other constructor actions you want to take, and stylistically makes it not too hard to understand that's what is going on too.
I find it's a nice style to use when dealing with a lot of declarations and/or a lot of actions needing to happen on instantiation and wanting to keep the two ideas distinct from each other.
NOTE: I very purposefully do not use the typical idiomatic ideas of "initializing" (like an init() or initialize() method) because those are often used differently. There is a sort of presumed difference between the idea of constructing and initializing. Working with constructors people know that they're called automatically as part of instantiation. Seeing an init method many people are going to assume without a second glance that they need to be doing something along the form of var mc = MyClass(); mc.init();, because that's how you typically initialize. I'm not trying to add an initialization process for the user of the class, I'm trying to add to the construction process of the class itself.
While some people may do a double-take for a moment, that's actually the bit of the point: it communicates to them that the intent is part of construction, even if that makes them do a bit of a double take and go "that's not how ES6 constructors work" and take a second looking at the actual constructor to go "oh, they call it at the bottom, I see", that's far better than NOT communicating that intent (or incorrectly communicating it) and probably getting a lot of people using it wrong, trying to initialize it from the outside and junk. That's very much intentional to the pattern I suggest.
For those that don't want to follow that pattern, the exact opposite can work too. Farm the declarations out to another function at the beginning. Maybe name it "properties" or "publicProperties" or something. Then put the rest of the stuff in the normal constructor.
"use strict";
class MyClass
{
properties() {
this.prop1 = 'blah 1';
this.prop2 = 'blah 2';
this.prop3 = 'blah 3';
}
constructor() {
this.properties();
doWhatever();
}
}
Note that this second method may look cleaner but it also has an inherent problem where properties gets overridden as one class using this method extends another. You'd have to give more unique names to properties to avoid that. My first method does not have this problem because its fake half of the constructor is uniquely named after the class.
As Benjamin said in his answer, TC39 explicitly decided not to include this feature at least for ES2015. However, the consensus seems to be that they will add it in ES2016.
The syntax hasn't been decided yet, but there's a preliminary proposal for ES2016 that will allow you to declare static properties on a class.
Thanks to the magic of babel, you can use this today. Enable the class properties transform according to these instructions and you're good to go. Here's an example of the syntax:
class foo {
static myProp = 'bar'
someFunction() {
console.log(this.myProp)
}
}
This proposal is in a very early state, so be prepared to tweak your syntax as time goes on.
What about the oldschool way?
class MyClass {
constructor(count){
this.countVar = 1 + count;
}
}
MyClass.prototype.foo = "foo";
MyClass.prototype.countVar = 0;
// ...
var o1 = new MyClass(2); o2 = new MyClass(3);
o1.foo = "newFoo";
console.log( o1.foo,o2.foo);
console.log( o1.countVar,o2.countVar);
In constructor you mention only those vars which have to be computed.
I like prototype inheritance for this feature -- it can help to save a lot of memory(in case if there are a lot of never-assigned vars).
[Long thread, not sure if its already listed as an option...].
A simple alternative for contsants only, would be defining the const outside of class.
This will be accessible only from the module itself, unless accompanied with a getter.
This way prototype isn't littered and you get the const.
// will be accessible only from the module itself
const MY_CONST = 'string';
class MyClass {
// optional, if external access is desired
static get MY_CONST(){return MY_CONST;}
// access example
static someMethod(){
console.log(MY_CONST);
}
}
ES7 class member syntax:
ES7 has a solution for 'junking' your constructor function. Here is an example:
class Car {
wheels = 4;
weight = 100;
}
const car = new Car();
console.log(car.wheels, car.weight);
The above example would look the following in ES6:
class Car {
constructor() {
this.wheels = 4;
this.weight = 100;
}
}
const car = new Car();
console.log(car.wheels, car.weight);
Be aware when using this that this syntax might not be supported by all browsers and might have to be transpiled an earlier version of JS.
Bonus: an object factory:
function generateCar(wheels, weight) {
class Car {
constructor() {}
wheels = wheels;
weight = weight;
}
return new Car();
}
const car1 = generateCar(4, 50);
const car2 = generateCar(6, 100);
console.log(car1.wheels, car1.weight);
console.log(car2.wheels, car2.weight);
You can mimic es6 classes behaviour... and use your class variables :)
Look mum... no classes!
// Helper
const $constructor = Symbol();
const $extends = (parent, child) =>
Object.assign(Object.create(parent), child);
const $new = (object, ...args) => {
let instance = Object.create(object);
instance[$constructor].call(instance, ...args);
return instance;
}
const $super = (parent, context, ...args) => {
parent[$constructor].call(context, ...args)
}
// class
var Foo = {
classVariable: true,
// constructor
[$constructor](who){
this.me = who;
this.species = 'fufel';
},
// methods
identify(){
return 'I am ' + this.me;
}
}
// class extends Foo
var Bar = $extends(Foo, {
// constructor
[$constructor](who){
$super(Foo, this, who);
this.subtype = 'barashek';
},
// methods
speak(){
console.log('Hello, ' + this.identify());
},
bark(num){
console.log('Woof');
}
});
var a1 = $new(Foo, 'a1');
var b1 = $new(Bar, 'b1');
console.log(a1, b1);
console.log('b1.classVariable', b1.classVariable);
I put it on GitHub
Still you can't declare any classes like in another programming languages. But you can create as many class variables. But problem is scope of class object. So According to me, Best way OOP Programming in ES6 Javascript:-
class foo{
constructor(){
//decalre your all variables
this.MY_CONST = 3.14;
this.x = 5;
this.y = 7;
// or call another method to declare more variables outside from constructor.
// now create method level object reference and public level property
this.MySelf = this;
// you can also use var modifier rather than property but that is not working good
let self = this.MySelf;
//code .........
}
set MySelf(v){
this.mySelf = v;
}
get MySelf(v){
return this.mySelf;
}
myMethod(cd){
// now use as object reference it in any method of class
let self = this.MySelf;
// now use self as object reference in code
}
}
If its only the cluttering what gives the problem in the constructor why not implement a initialize method that intializes the variables. This is a normal thing to do when the constructor gets to full with unnecessary stuff. Even in typed program languages like C# its normal convention to add an Initialize method to handle that.
Just define a getter.
class MyClass
{
get MY_CONST () { return 'string'; }
constructor ()
{
console.log ("MyClass MY_CONST:", this.MY_CONST);
}
}
var obj = new MyClass();
The way I solved this, which is another option (if you have jQuery available), was to Define the fields in an old-school object and then extend the class with that object. I also didn't want to pepper the constructor with assignments, this appeared to be a neat solution.
function MyClassFields(){
this.createdAt = new Date();
}
MyClassFields.prototype = {
id : '',
type : '',
title : '',
createdAt : null,
};
class MyClass {
constructor() {
$.extend(this,new MyClassFields());
}
};
-- Update Following Bergi's comment.
No JQuery Version:
class SavedSearch {
constructor() {
Object.assign(this,{
id : '',
type : '',
title : '',
createdAt: new Date(),
});
}
}
You still do end up with 'fat' constructor, but at least its all in one class and assigned in one hit.
EDIT #2:
I've now gone full circle and am now assigning values in the constructor, e.g.
class SavedSearch {
constructor() {
this.id = '';
this.type = '';
this.title = '';
this.createdAt = new Date();
}
}
Why? Simple really, using the above plus some JSdoc comments, PHPStorm was able to perform code completion on the properties. Assigning all the vars in one hit was nice, but the inability to code complete the properties, imo, isn't worth the (almost certainly minuscule) performance benefit.
Well, you can declare variables inside the Constructor.
class Foo {
constructor() {
var name = "foo"
this.method = function() {
return name
}
}
}
var foo = new Foo()
foo.method()
Recent browsers as of 2021 (not IE, see MDN browser chart) implement Public class fields which seems to be what you're looking for:
class MyClass {
static foo = 3;
}
console.log(MyClass.foo);
However apparently it's not possible to make this a const: Declaring static constants in ES6 classes?
A static getter looks pretty close:
class MyClass {
static get CONST() {
return 3;
}
}
MyClass.CONST = 4; // property unaffected
console.log(MyClass.CONST);
This is a bit hackish combo of static and get works for me
class ConstantThingy{
static get NO_REENTER__INIT() {
if(ConstantThingy._NO_REENTER__INIT== null){
ConstantThingy._NO_REENTER__INIT = new ConstantThingy(false,true);
}
return ConstantThingy._NO_REENTER__INIT;
}
}
elsewhere used
var conf = ConstantThingy.NO_REENTER__INIT;
if(conf.init)...
Currently in ES5 many of us are using the following pattern in frameworks to create classes and class variables, which is comfy:
// ES 5
FrameWork.Class({
variable: 'string',
variable2: true,
init: function(){
},
addItem: function(){
}
});
In ES6 you can create classes natively, but there is no option to have class variables:
// ES6
class MyClass {
const MY_CONST = 'string'; // <-- this is not possible in ES6
constructor(){
this.MY_CONST;
}
}
Sadly, the above won't work, as classes only can contain methods.
I understand that I can this.myVar = true in constructor…but I don't want to 'junk' my constructor, especially when I have 20-30+ params for a bigger class.
I was thinking of many ways to handle this issue, but haven't yet found any good ones. (For example: create a ClassConfig handler, and pass a parameter object, which is declared separately from the class. Then the handler would attach to the class. I was thinking about WeakMaps also to integrate, somehow.)
What kind of ideas would you have to handle this situation?
2018 update:
There is now a stage 3 proposal - I am looking forward to make this answer obsolete in a few months.
In the meantime anyone using TypeScript or babel can use the syntax:
varName = value
Inside a class declaration/expression body and it will define a variable. Hopefully in a few months/weeks I'll be able to post an update.
Update: Chrome 74 now ships with this syntax working.
The notes in the ES wiki for the proposal in ES6 (maximally minimal classes) note:
There is (intentionally) no direct declarative way to define either prototype data properties (other than methods) class properties, or instance property
Class properties and prototype data properties need be created outside the declaration.
Properties specified in a class definition are assigned the same attributes as if they appeared in an object literal.
This means that what you're asking for was considered, and explicitly decided against.
but... why?
Good question. The good people of TC39 want class declarations to declare and define the capabilities of a class. Not its members. An ES6 class declaration defines its contract for its user.
Remember, a class definition defines prototype methods - defining variables on the prototype is generally not something you do.
You can, of course use:
constructor(){
this.foo = bar
}
In the constructor like you suggested. Also see the summary of the consensus.
ES7 and beyond
A new proposal for ES7 is being worked on that allows more concise instance variables through class declarations and expressions - https://esdiscuss.org/topic/es7-property-initializers
Just to add to Benjamin's answer — class variables are possible, but you wouldn't use prototype to set them.
For a true class variable you'd want to do something like the following:
class MyClass {}
MyClass.foo = 'bar';
From within a class method that variable can be accessed as this.constructor.foo (or MyClass.foo).
These class properties would not usually be accessible from to the class instance. i.e. MyClass.foo gives 'bar' but new MyClass().foo is undefined
If you want to also have access to your class variable from an instance, you'll have to additionally define a getter:
class MyClass {
get foo() {
return this.constructor.foo;
}
}
MyClass.foo = 'bar';
I've only tested this with Traceur, but I believe it will work the same in a standard implementation.
JavaScript doesn't really have classes. Even with ES6 we're looking at an object- or prototype-based language rather than a class-based language. In any function X () {}, X.prototype.constructor points back to X.
When the new operator is used on X, a new object is created inheriting X.prototype. Any undefined properties in that new object (including constructor) are looked up from there. We can think of this as generating object and class properties.
Babel supports class variables in ESNext, check this example:
class Foo {
bar = 2
static iha = 'string'
}
const foo = new Foo();
console.log(foo.bar, foo.iha, Foo.bar, Foo.iha);
// 2, undefined, undefined, 'string'
In your example:
class MyClass {
const MY_CONST = 'string';
constructor(){
this.MY_CONST;
}
}
Because of MY_CONST is primitive https://developer.mozilla.org/en-US/docs/Glossary/Primitive we can just do:
class MyClass {
static get MY_CONST() {
return 'string';
}
get MY_CONST() {
return this.constructor.MY_CONST;
}
constructor() {
alert(this.MY_CONST === this.constructor.MY_CONST);
}
}
alert(MyClass.MY_CONST);
new MyClass
// alert: string ; true
But if MY_CONST is reference type like static get MY_CONST() {return ['string'];} alert output is string, false. In such case delete operator can do the trick:
class MyClass {
static get MY_CONST() {
delete MyClass.MY_CONST;
return MyClass.MY_CONST = 'string';
}
get MY_CONST() {
return this.constructor.MY_CONST;
}
constructor() {
alert(this.MY_CONST === this.constructor.MY_CONST);
}
}
alert(MyClass.MY_CONST);
new MyClass
// alert: string ; true
And finally for class variable not const:
class MyClass {
static get MY_CONST() {
delete MyClass.MY_CONST;
return MyClass.MY_CONST = 'string';
}
static set U_YIN_YANG(value) {
delete MyClass.MY_CONST;
MyClass.MY_CONST = value;
}
get MY_CONST() {
return this.constructor.MY_CONST;
}
set MY_CONST(value) {
this.constructor.MY_CONST = value;
}
constructor() {
alert(this.MY_CONST === this.constructor.MY_CONST);
}
}
alert(MyClass.MY_CONST);
new MyClass
// alert: string, true
MyClass.MY_CONST = ['string, 42']
alert(MyClass.MY_CONST);
new MyClass
// alert: string, 42 ; true
Since your issue is mostly stylistic (not wanting to fill up the constructor with a bunch of declarations) it can be solved stylistically as well.
The way I view it, many class based languages have the constructor be a function named after the class name itself. Stylistically we could use that that to make an ES6 class that stylistically still makes sense but does not group the typical actions taking place in the constructor with all the property declarations we're doing. We simply use the actual JS constructor as the "declaration area", then make a class named function that we otherwise treat as the "other constructor stuff" area, calling it at the end of the true constructor.
"use strict";
class MyClass
{
// only declare your properties and then call this.ClassName(); from here
constructor(){
this.prop1 = 'blah 1';
this.prop2 = 'blah 2';
this.prop3 = 'blah 3';
this.MyClass();
}
// all sorts of other "constructor" stuff, no longer jumbled with declarations
MyClass() {
doWhatever();
}
}
Both will be called as the new instance is constructed.
Sorta like having 2 constructors where you separate out the declarations and the other constructor actions you want to take, and stylistically makes it not too hard to understand that's what is going on too.
I find it's a nice style to use when dealing with a lot of declarations and/or a lot of actions needing to happen on instantiation and wanting to keep the two ideas distinct from each other.
NOTE: I very purposefully do not use the typical idiomatic ideas of "initializing" (like an init() or initialize() method) because those are often used differently. There is a sort of presumed difference between the idea of constructing and initializing. Working with constructors people know that they're called automatically as part of instantiation. Seeing an init method many people are going to assume without a second glance that they need to be doing something along the form of var mc = MyClass(); mc.init();, because that's how you typically initialize. I'm not trying to add an initialization process for the user of the class, I'm trying to add to the construction process of the class itself.
While some people may do a double-take for a moment, that's actually the bit of the point: it communicates to them that the intent is part of construction, even if that makes them do a bit of a double take and go "that's not how ES6 constructors work" and take a second looking at the actual constructor to go "oh, they call it at the bottom, I see", that's far better than NOT communicating that intent (or incorrectly communicating it) and probably getting a lot of people using it wrong, trying to initialize it from the outside and junk. That's very much intentional to the pattern I suggest.
For those that don't want to follow that pattern, the exact opposite can work too. Farm the declarations out to another function at the beginning. Maybe name it "properties" or "publicProperties" or something. Then put the rest of the stuff in the normal constructor.
"use strict";
class MyClass
{
properties() {
this.prop1 = 'blah 1';
this.prop2 = 'blah 2';
this.prop3 = 'blah 3';
}
constructor() {
this.properties();
doWhatever();
}
}
Note that this second method may look cleaner but it also has an inherent problem where properties gets overridden as one class using this method extends another. You'd have to give more unique names to properties to avoid that. My first method does not have this problem because its fake half of the constructor is uniquely named after the class.
As Benjamin said in his answer, TC39 explicitly decided not to include this feature at least for ES2015. However, the consensus seems to be that they will add it in ES2016.
The syntax hasn't been decided yet, but there's a preliminary proposal for ES2016 that will allow you to declare static properties on a class.
Thanks to the magic of babel, you can use this today. Enable the class properties transform according to these instructions and you're good to go. Here's an example of the syntax:
class foo {
static myProp = 'bar'
someFunction() {
console.log(this.myProp)
}
}
This proposal is in a very early state, so be prepared to tweak your syntax as time goes on.
What about the oldschool way?
class MyClass {
constructor(count){
this.countVar = 1 + count;
}
}
MyClass.prototype.foo = "foo";
MyClass.prototype.countVar = 0;
// ...
var o1 = new MyClass(2); o2 = new MyClass(3);
o1.foo = "newFoo";
console.log( o1.foo,o2.foo);
console.log( o1.countVar,o2.countVar);
In constructor you mention only those vars which have to be computed.
I like prototype inheritance for this feature -- it can help to save a lot of memory(in case if there are a lot of never-assigned vars).
[Long thread, not sure if its already listed as an option...].
A simple alternative for contsants only, would be defining the const outside of class.
This will be accessible only from the module itself, unless accompanied with a getter.
This way prototype isn't littered and you get the const.
// will be accessible only from the module itself
const MY_CONST = 'string';
class MyClass {
// optional, if external access is desired
static get MY_CONST(){return MY_CONST;}
// access example
static someMethod(){
console.log(MY_CONST);
}
}
ES7 class member syntax:
ES7 has a solution for 'junking' your constructor function. Here is an example:
class Car {
wheels = 4;
weight = 100;
}
const car = new Car();
console.log(car.wheels, car.weight);
The above example would look the following in ES6:
class Car {
constructor() {
this.wheels = 4;
this.weight = 100;
}
}
const car = new Car();
console.log(car.wheels, car.weight);
Be aware when using this that this syntax might not be supported by all browsers and might have to be transpiled an earlier version of JS.
Bonus: an object factory:
function generateCar(wheels, weight) {
class Car {
constructor() {}
wheels = wheels;
weight = weight;
}
return new Car();
}
const car1 = generateCar(4, 50);
const car2 = generateCar(6, 100);
console.log(car1.wheels, car1.weight);
console.log(car2.wheels, car2.weight);
You can mimic es6 classes behaviour... and use your class variables :)
Look mum... no classes!
// Helper
const $constructor = Symbol();
const $extends = (parent, child) =>
Object.assign(Object.create(parent), child);
const $new = (object, ...args) => {
let instance = Object.create(object);
instance[$constructor].call(instance, ...args);
return instance;
}
const $super = (parent, context, ...args) => {
parent[$constructor].call(context, ...args)
}
// class
var Foo = {
classVariable: true,
// constructor
[$constructor](who){
this.me = who;
this.species = 'fufel';
},
// methods
identify(){
return 'I am ' + this.me;
}
}
// class extends Foo
var Bar = $extends(Foo, {
// constructor
[$constructor](who){
$super(Foo, this, who);
this.subtype = 'barashek';
},
// methods
speak(){
console.log('Hello, ' + this.identify());
},
bark(num){
console.log('Woof');
}
});
var a1 = $new(Foo, 'a1');
var b1 = $new(Bar, 'b1');
console.log(a1, b1);
console.log('b1.classVariable', b1.classVariable);
I put it on GitHub
Still you can't declare any classes like in another programming languages. But you can create as many class variables. But problem is scope of class object. So According to me, Best way OOP Programming in ES6 Javascript:-
class foo{
constructor(){
//decalre your all variables
this.MY_CONST = 3.14;
this.x = 5;
this.y = 7;
// or call another method to declare more variables outside from constructor.
// now create method level object reference and public level property
this.MySelf = this;
// you can also use var modifier rather than property but that is not working good
let self = this.MySelf;
//code .........
}
set MySelf(v){
this.mySelf = v;
}
get MySelf(v){
return this.mySelf;
}
myMethod(cd){
// now use as object reference it in any method of class
let self = this.MySelf;
// now use self as object reference in code
}
}
If its only the cluttering what gives the problem in the constructor why not implement a initialize method that intializes the variables. This is a normal thing to do when the constructor gets to full with unnecessary stuff. Even in typed program languages like C# its normal convention to add an Initialize method to handle that.
Just define a getter.
class MyClass
{
get MY_CONST () { return 'string'; }
constructor ()
{
console.log ("MyClass MY_CONST:", this.MY_CONST);
}
}
var obj = new MyClass();
The way I solved this, which is another option (if you have jQuery available), was to Define the fields in an old-school object and then extend the class with that object. I also didn't want to pepper the constructor with assignments, this appeared to be a neat solution.
function MyClassFields(){
this.createdAt = new Date();
}
MyClassFields.prototype = {
id : '',
type : '',
title : '',
createdAt : null,
};
class MyClass {
constructor() {
$.extend(this,new MyClassFields());
}
};
-- Update Following Bergi's comment.
No JQuery Version:
class SavedSearch {
constructor() {
Object.assign(this,{
id : '',
type : '',
title : '',
createdAt: new Date(),
});
}
}
You still do end up with 'fat' constructor, but at least its all in one class and assigned in one hit.
EDIT #2:
I've now gone full circle and am now assigning values in the constructor, e.g.
class SavedSearch {
constructor() {
this.id = '';
this.type = '';
this.title = '';
this.createdAt = new Date();
}
}
Why? Simple really, using the above plus some JSdoc comments, PHPStorm was able to perform code completion on the properties. Assigning all the vars in one hit was nice, but the inability to code complete the properties, imo, isn't worth the (almost certainly minuscule) performance benefit.
Well, you can declare variables inside the Constructor.
class Foo {
constructor() {
var name = "foo"
this.method = function() {
return name
}
}
}
var foo = new Foo()
foo.method()
Recent browsers as of 2021 (not IE, see MDN browser chart) implement Public class fields which seems to be what you're looking for:
class MyClass {
static foo = 3;
}
console.log(MyClass.foo);
However apparently it's not possible to make this a const: Declaring static constants in ES6 classes?
A static getter looks pretty close:
class MyClass {
static get CONST() {
return 3;
}
}
MyClass.CONST = 4; // property unaffected
console.log(MyClass.CONST);
This is a bit hackish combo of static and get works for me
class ConstantThingy{
static get NO_REENTER__INIT() {
if(ConstantThingy._NO_REENTER__INIT== null){
ConstantThingy._NO_REENTER__INIT = new ConstantThingy(false,true);
}
return ConstantThingy._NO_REENTER__INIT;
}
}
elsewhere used
var conf = ConstantThingy.NO_REENTER__INIT;
if(conf.init)...
I recently read about the fact that there is a possibility of defining getters/setters in JavaScript. It seems extremely helpful - the setter is a kind of 'helper' which can parse the value to be set first, before actually setting it.
For example, I currently have this code:
var obj = function(value) {
var test = !!value; // 'test' has to be a boolean
return {
get test() { return test },
set test(value) { test = !!value }
};
};
var instance = new obj(true);
This code always converts value to a boolean. So if you code instance.test = 0, then instance.test === false.
However, for this to work you have to actually return an object, which means that the new instance is not of type obj but just is a plain object. This means that changing the prototype of obj has no effect on instances. For example, this does not work - instance.func is undefined:
obj.prototype.func = function() { console.log(this.value); };
because instance is not of type obj. To get the prototype functions work, I guess I should not return a plain object, but rather not return anything so that instance would just be of type obj, like a regular constructor works.
The problem then is how to implement getters/setters? I can only find articles describing how to add these to an object, not as being part of the constructor of a custom type.
So how do I implement getters/setters in the constructor so as to be able to both use getters/setters and extending the prototype?
You can't do that.
You can set setter/getters for properties of objects though. I advice you use ES5 Object.defineProperties though. of course this only works in modern browsers.
var obj = function() {
...
Object.defineProperties(this, {
"test": {
"get": function() { ... },
"set": function() { ... }
}
});
}
obj.prototype.func = function() { ... }
var o = new obj;
o.test;
o.func();
Usually you want class methods. The answer by #Raynos on May 7, 2011 gets the job done, but it defines an instance method, not a class method.
The following illustrates a class definition with a the getter and setter being part of the class. This definition is a lot like the answer by #Raynos, but with two differences in the code: (1) The "defineProperties()" action has been moved out of the constructor. (2) The argument to "defineProperties()"as been changed from the instance object "this", to the constructor's prototype object.
function TheConstructor(side) {
this.side = side;
}
Object.defineProperties(TheConstructor.prototype, {
area: {
get: function() { return this.side * this.side; }
,set: function(val) { this.side = Math.sqrt(val); }
}
});
// Test code:
var anInstance = new TheConstructor(2);
console.log("initial Area:"+anInstance.area);
anInstance.area = 9;
console.log("modified Area:"+anInstance.area);
Which produces these results:
initial Area:4
modified Area:9
Although usually the distinction between class versus instance
definition is just a matter of style, there is a purpose to
good style, and there is a case where the distinction matters:
the memoized getter. The purpose for a memoized getter is
described here: Smart/self-overwriting/lazy getters
Define the getter at the class level when the memoized value is to
pertain to the entire class. For example, a configuration file
should be read only once; the resulting values should then apply
for the duration of the program. The following sample code
defines a memoized getter at the class level.
function configureMe() {
return 42;
}
Object.defineProperties(TheConstructor.prototype, {
memoizedConfigParam: {
get: function() {
delete TheConstructor.prototype.memoizedConfigParam;
return TheConstructor.prototype.memoizedConfigParam = configureMe();
}
,configurable: true
}
});
// Test code:
console.log("memoizedConfigParam:"+anInstance.memoizedConfigParam);
Produces:
memoizedConfigParam:42
As can be seen in the example, memoized getters have the
characteristic that the getter function deletes itself,
then replaces itself with a simple value that
(presumably) will never change.
Note that 'configurable' must be set to 'true'.
Define the getter at the instance level when the memoized value
depends upon the contents of instance. The definition moves
inside the constructor, and the object of attention is 'this'.
function TheConstructorI(side) {
this.side = side;
Object.defineProperties(this, {
memoizedCalculation: {
get: function() {
delete this.memoizedCalculation;
return this.memoizedCalculation = this.expensiveOperation();
}
,configurable: true
}
});
}
TheConstructorI.prototype.expensiveOperation = function() {
return this.side * this.side * this.side;
}
//Test code:
var instance2 = new TheConstructorI(2);
var instance3 = new TheConstructorI(3);
console.log("memoizedCalculation 2:"+instance2.memoizedCalculation);
console.log("memoizedCalculation 3:"+instance3.memoizedCalculation);
Produces:
memoizedCalculation 2:8
memoizedCalculation 3:27
If you want to guarantee (rather than presume) that the memoized
value will never be changed, the 'writable' attribute needs to
be changed. That makes the code a bit more complicated.
function TheConstructorJ(side) {
this.side = side;
Object.defineProperties(this, {
memoizedCalculation: {
get: function() {
delete this.memoizedCalculation;
Object.defineProperty( this, 'memoizedCalculation'
,{ value : this.expensiveOperation()
,writable : false
});
return this.memoizedCalculation;
}
,configurable: true
}
});
}
TheConstructorJ.prototype.expensiveOperation = function() {
return this.side * this.side * this.side;
}
//Test code:
var instanceJ = new TheConstructorJ(2);
console.log("memoizedCalculation:"+instanceJ.memoizedCalculation);
instanceJ.memoizedCalculation = 42; // results in error
Produces:
memoizedCalculation:8
>Uncaught TypeError: Cannot assign to read only property 'memoizedCalculation' of object '#<TheConstructorJ>'
The OP's original question, from March 7, 2011, presented basic
getter and setter syntax, noted that it worked on an object but
not on 'this', and asked how to define getters and setters within
a constructor. In addition to all the examples above, there is
also a "cheap-shot" way of doing it: create a new object within
the constructor, like the OP did, but then assign the object to
be a member within 'this'. So, the original code would look like
this:
var MyClass = function(value) {
var test = !!value; // 'test' has to be a boolean
this.data = {
get test() { return test },
set test(value) { test = !!value }
};
};
var instance = new MyClass(true);
// But now 'data' is part of the access path
instance.data.test = 0;
console.log(instance.data.test);
Produces:
false
Believe it or not, I have actually run into situations where
this "cheap-shot" is the best solution. Specifically, I used this
technique when I had records from several tables encapsulated within
a single class, and wanted to present a unified view as though
they were a single record called 'data'.
Have fun.
IAM_AL_X
Update for ES6 -- have a look at section 19.3.1 of Alex Rauschmayer's book Exploring ES6 http://exploringjs.com/es6/ch_maps-sets.html#sec_weakmaps-private-data which demonstrates how to use WeakMaps with getters and setters to hold private data. Combining with section 16.2.2.3 http://exploringjs.com/es6/ch_classes.html#leanpub-auto-getters-and-setters would result in something like
# module test_WeakMap_getter.js
var _MyClassProp = new WeakMap();
class MyClass {
get prop() {
return _MyClassProp.get( this );
}
set prop(value) {
_MyClassProp.set( this, value );
}
}
var mc = new MyClass();
mc.prop = 5 ;
console.log( 'My value is', mc.prop );
$ node --use_strict test_WeakMap_getter.js
My value is 5
function Obj(value){
this.value = !!value;
}
Obj.prototype = {
get test () {
return this.value;``
},
set test (value) {
this.value = !!this.value;
}
};
var obj = new Obj(true);
I know this might be extremely late but I figured out a different way to accomplish what you want and for the sake of people, like myself, googling for an answer to this here it is.
function Constructor(input){
this.input = input;
}
Object.__defineGetter__.call(Constructor.prototype, "value", function(){
return this.input * 2;
});
var test = new Constructor(5);
alert(test.value) // 10
I've tested this in chrome, safari, mobile safari, firefox and they all work (latest versions of course)
#Alex I see it as more option and more power, programming is art, #Nat share his finding with us, and for that I thank him. Maybe someone want to do it that way.
I'm sure the setter version is the same but just changing that g to a s.
i.g:
function Constructor(input){
this.input = input;
}
Object.__defineGetter__.call(Constructor.prototype, "value", function(){
return this.input * 2;
});
Object.__defineSetter__.call(Constructor.prototype, "bar", function(foo){
return this.input *= foo;
});
var test = new Constructor(5);
console.log(test.value); // 10
test.bar = 5;
console.log(test.input); //25
With that said, this feature is deprecated, advices to not to use in production coding.