ES6 类变量替代方案

Question

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?

Answer 1

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

Answer 2

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. ie 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.

Answer 3

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'

Answer 4

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

Answer 5

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(); 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.

Answer 6

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).

Answer 7

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.

Answer 8

[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);
    }
}

Answer 9

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);

Answer 10

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

Answer 11

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.

Answer 12

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, eg

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.

Answer 13

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()

Answer 14

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
   }
}

Answer 15

Just define a getter .

 class MyClass { get MY_CONST () { return 'string'; } constructor () { console.log ("MyClass MY_CONST:", this.MY_CONST); } } var obj = new MyClass();

Answer 16

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);

Answer 17

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)...

Answer 18

Can you avoid the whole issue by using strong literals and a little library of templates logic run contained in a larger closure?

ignoring the closure for now

const myDynamicInputs=(items)=>\backtick -${ items.map((item, I, array)=>'${do tons of junk}').join('')}';

http://codepen.io/jfrazz/pen/BQJPBZ/

THIS is the simplest example I can offer from repository, The first 400 lines are a data library+ some basic utility functions. Plus a handful of utility constants.

After the boiler plate, which we are turning into a data uri--downloaded by app users--we have array templates, that have to be lifted and redeployed, but which can be composed to be anything from inputs, dropdowns, or 52 pages of questions and data.

That is this second example: Eat an object, get inputs of various types, all using const as the base variable of library, being constructed.

http://codepen.io/jfrazz/pen/rWprVR/

Not exactly what you asked, but a clear showing that constant can be pretty dynamic.