Javascript Es6 Class Static Variable !!

javascript - How do I use a static variable in ES6 class? - Stack Overflow

I'm trying to use a static variable in es6. I'd like to declare a static variable count in Animal class and increase it. However, I couldn't declare a static variable through static count = 0;, so I I'm trying to use a static variable in es6. I'd like to declare a static variable count in Animal class and increase it. However, I couldn't declare a static variable through static count = 0;, so I tried another way like this:

I expected console.log(Animal.getCount()); to be 1, but it doesn't work. How do I declare a static variable and modify it by calling a method?

Your class has no static variables (if by static variable you mean static property). getCount returns NaN (after you call increaseCount) because Animal has no count property initially. Then increaseCount does undefined + 1 which is NaN. Instances created by new Animal have a count property initially, but Animal itself does not until you call increaseCount. this within a static method refers to the Animal class (constructor function) itself (if you call it via Animal.methodName( )).

With the static class fields proposal (currently at Stage 3), you could do that declaratively with static count = 0; in Animal. Live Example (Stack Snippets' Babel configuration seems to support it):

Side note: Using this within a static method to refer to the class (constructor function) is a bit tricky if there are subclasses, because for instance, if you had:

As mentioned in other answers, this.count refers to instance property in constructor. In order for static property to be initialized, Animal.count should be set.

An alternative in ES6 is to use initial values, in this case Animal.count initial value doesn't need to be set explicitly, e.g.:

Static-only class is considered antipattern in JavaScript because a state or other traits that are specific to classes aren't used. In case there should be only one instance, plain object should be used (unless there are other concerns that could benefit from class):

To set a static variable, set it on the object Animal itself. As of now in Javascript you cannot directly declare static properties inside classes like you can declare static methods.

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javascript - How do I make a "public static field" in an ES6 class

There is a Stage 3 ECMAScript proposal called "Static Class Features" by Shu-yu Guo and Daniel Ehrenberg that aims to solve this problem. Along with the Stage 3 "Class Fields" proposal, you can do something like this: I'm making a Javascript class and I'd like to have a public static field like in Java. This is the relevant code:

It looks like ES6 modules don't allow this. Is there a way to get the desired behavior or do I have to write a getter?

So from there you can follow to 14.5.14 — Runtime Semantics: ClassDefinitionEvaluation — to double check if it really does what it looks like it does. Specifically, step 20:

So PropertyMethodDefinition is called with "F" (constructor, function object) as an argument, which in its turn creates an accessor method on that object.

The features were added in the "Static Class Features" and "Class Fields" proposals by Daniel Ehrenberg et al. Google Chrome (and new Edge) started supporting both proposals in version 72, equivalent to Node.js 12+. Firefox supports public instance fields since version 69 and static instance fields since version 75. Safari supports both since version 14.1. See more info at caniuse.com.

For older browsers that don't yet support these features, you can use Babel to transpile class fields. This requires @babel/plugin-proposal-class-properties to be enabled (enabled by default in @babel/plugin-env starting from v7.14.0).

Compared to @kangax's solution of declaring a getter, this solution can also be more performant, since here the property is accessed directly instead of through calling a function.

Edit (June 2021): Both proposals are accepted by TC39, the ECMAScript language committee, and Safari shipped this feature in version 14.1!

In current drafts of ECMAScript 6 (as of February 2015), all class properties must be methods, not values (note in ECMAScript a "property" is similar in concept to an OOP field, except the field value must be a Function object, not any other value such as a Number or Object).

To get full advantage of static variable I followed this approach. To be more specific, we can use it to use private variable or having only public getter, or having both getter or setter. In the last case it's same as one of the solution posted above.

@kangax 's answer does not imitate the whole static behaviour of traditional OOP language's, because you cannot access the static property by it's instance like const agent = new Agent; agent.CIRCLE; // Undefined

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javascript - ES6 class variable alternatives - Stack Overflow

ES6 class variable alternatives. Currently in ES5 many of us are using the following pattern in frameworks to create classes and class variables, which is comfy: In ES6 you can create classes natively, but there is no option to have class variables: Sadly, the above won't work, as classes only can contain methods. Currently in ES5 many of us are using the following pattern in frameworks to create classes and class variables, which is comfy:

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.)

There is now a stage 3 proposal - I am looking forward to make this answer obsolete in a few months.

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.

There is (intentionally) no direct declarative way to define either prototype data properties (other than methods) class properties, or instance property

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:

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

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:

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.

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:

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.

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.

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.

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).

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:

[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.

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.

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.

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.

You still do end up with 'fat' constructor, but at least its all in one class and assigned in one hit.

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.

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:-

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javascript - Member variables in ES6 classes - Stack Overflow

A bit of background: I'm currently toying with the Google closure compiler performing advanced compilation, using ES6 as input. For that to work, I need a place to put my type annotations for member variables, and I used to place them using syntax like /** @type string */ MyClass.prototype.arg; which is a semantic no-op in ECMAScript but provides the type information to the closure compiler

Declaring static constants in ES6 classes? - Stack Overflow

429. Here's a few things you could do: Export a const from the module. Depending on your use case, you could just: export const constant1 = 33; And import that from the module where necessary. Or, building on your static method idea, you could declare a static get accessor: const constant1 = 33, constant2 = 2; class Example { static get

ES6 static properties.. With the introduction of static keyword… | by

ES6 static properties. Deepak. Feb 24, 2018 · 3 min read. With the introduction of static keyword in ES6, it is possible to define static methods inside the class definition itself. In order to

static - JavaScript | MDN

The following example demonstrates several things: How a static member (method or property) is defined on a class. That a class with a static member can be sub-classed.

JavaScript Class static Keyword - W3Schools

Definition and Usage. The static keyword defines static methods for classes.. Static methods are called directly on the class (Car from the example above) - without creating an instance/object (mycar) of the class.

Static Properties in JavaScript Classes with Inheritance | www

Static Properties With Inheritance. The key idea is that a JavaScript class is just another object, so you can distinguish between own properties and inherited properties. This pattern is neat with classes, but it also works with pre-ES6 JavaScript inheritance. This is important because Mongoose still uses pre-ES6 style inheritance.

Call ES5 class method from static method - Stack Overflow

JavaScript is not based on classes like Java, it is based on prototypes. With ES5 syntax, there are two ways to create what you call a "class": An object literal. A constructor function. When you use an object literal, your class looks like this: var myClass = myAttribute: "foo", myMethod: function () return "bar"; ; javascript es6 class static variable

javascript es6 class static variable

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