Related
I am looking for a way to use something similar to Function.apply for classes so that the this property of an executed constructor is an external object.
With a function I can simply use apply
function Bar() {
this.value = 'value'
}
const proxy = {}
Bar.apply(proxy, [])
console.log(proxy) // { value: 'value' }
However this does not, of course, work with classes
class Foo {
constructor() {
this.value = 'value'
}
}
const proxy = {}
Foo.apply(proxy, [])
console.log(proxy)
Resulting in
Foo.apply(proxy, [])
^
TypeError: Class constructor Foo cannot be invoked without 'new'
Is it possible to bind the this context of a class constructor to another object?
I don't have any legacy clients so I am able to use Reflect.construct (though I am not sure if it can solve the problem)
EDIT:
Alternatively, I can work with replacing this after construction. Is that possible?
const foo = new Foo()
foo.bind(proxy)
If you have a look at [[Construct]] of function objects (which is the internal method which will be executed when you use new or Reflect.construct), then you'll find this step in the specification:
Let thisArgument be ? OrdinaryCreateFromConstructor(newTarget, "%Object.prototype%").
As there is no way to change this behavior, you cannot change what thisArgument is, it is always a regular object and cannot be a proxy. However, if you use Reflect.construct, you can influence the newTarget, and pass something else in than the regular constructor:
class Constructed {} // this is the function object that will be [[Construct]]ed
class Trapped {} // the "thisArgument" will inherit Trapped.prototype
Reflect.construct(Constructed, [], Trapped);
By injecting a proxy into Trapped.prototype you can have some reflection onto this inside a class constructor. An example can be found here.
You could create a Proxy that traps the construct and returns a proxified instance to trap the mutations:
class Foo {
constructor() {
this.value = 'value';
}
}
const ProxifiedFoo = new Proxy(Foo, {
construct: (target, args) => {
const instance = new target(...args);
return new Proxy(instance, {
set: (_, prop, value) => console.log(`Trying to set ${prop} to ${value}`)
});
}
});
const foo = new ProxifiedFoo;
console.log('Current value:', foo.value);
foo.value = 'otherValue';
I know this will work:
function Foo() {};
Foo.prototype.talk = function () {
alert('hello~\n');
};
var a = new Foo;
a.talk(); // 'hello~\n'
But if I want to call
Foo.talk() // this will not work
Foo.prototype.talk() // this works correctly
I find some methods to make Foo.talk work,
Foo.__proto__ = Foo.prototype
Foo.talk = Foo.prototype.talk
Are there other ways to do this? I don’t know whether it is right to do so. Do you use class methods or static methods in your JavaScript code?
First off, remember that JavaScript is primarily a prototypal language, rather than a class-based language1. Foo isn't a class, it's a function, which is an object. You can instantiate an object from that function using the new keyword which will allow you to create something similar to a class in a standard OOP language.
I'd suggest ignoring __proto__ most of the time because it has poor cross browser support, and instead focus on learning about how prototype works.
If you have an instance of an object created from a function2 and you access one of its members (methods, attributes, properties, constants etc) in any way, the access will flow down the prototype hierarchy until it either (a) finds the member, or (b) doesn't find another prototype.
The hierarchy starts on the object that was called, and then searches its prototype object. If the prototype object has a prototype, it repeats, if no prototype exists, undefined is returned.
For example:
foo = {bar: 'baz'};
console.log(foo.bar); // logs "baz"
foo = {};
console.log(foo.bar); // logs undefined
function Foo(){}
Foo.prototype = {bar: 'baz'};
f = new Foo();
console.log(f.bar);
// logs "baz" because the object f doesn't have an attribute "bar"
// so it checks the prototype
f.bar = 'buzz';
console.log( f.bar ); // logs "buzz" because f has an attribute "bar" set
It looks to me like you've at least somewhat understood these "basic" parts already, but I need to make them explicit just to be sure.
In JavaScript, everything is an object3.
everything is an object.
function Foo(){} doesn't just define a new function, it defines a new function object that can be accessed using Foo.
This is why you can access Foo's prototype with Foo.prototype.
What you can also do is set more functions on Foo:
Foo.talk = function () {
alert('hello world!');
};
This new function can be accessed using:
Foo.talk();
I hope by now you're noticing a similarity between functions on a function object and a static method.
Think of f = new Foo(); as creating a class instance, Foo.prototype.bar = function(){...} as defining a shared method for the class, and Foo.baz = function(){...} as defining a public static method for the class.
ECMAScript 2015 introduced a variety of syntactic sugar for these sorts of declarations to make them simpler to implement while also being easier to read. The previous example can therefore be written as:
class Foo {
bar() {...}
static baz() {...}
}
which allows bar to be called as:
const f = new Foo()
f.bar()
and baz to be called as:
Foo.baz()
1: class was a "Future Reserved Word" in the ECMAScript 5 specification, but ES6 introduces the ability to define classes using the class keyword.
2: essentially a class instance created by a constructor, but there are many nuanced differences that I don't want to mislead you
3: primitive values—which include undefined, null, booleans, numbers, and strings—aren't technically objects because they're low-level language implementations. Booleans, numbers, and strings still interact with the prototype chain as though they were objects, so for the purposes of this answer, it's easier to consider them "objects" even though they're not quite.
You can achieve it as below:
function Foo() {};
Foo.talk = function() { alert('I am talking.'); };
You can now invoke "talk" function as below:
Foo.talk();
You can do this because in JavaScript, functions are objects as well.
Call a static method from an instance:
function Clazz() {};
Clazz.staticMethod = function() {
alert('STATIC!!!');
};
Clazz.prototype.func = function() {
this.constructor.staticMethod();
}
var obj = new Clazz();
obj.func(); // <- Alert's "STATIC!!!"
Simple Javascript Class Project: https://github.com/reduardo7/sjsClass
Here is a good example to demonstrate how Javascript works with static/instance variables and methods.
function Animal(name) {
Animal.count = Animal.count+1||1;// static variables, use function name "Animal"
this.name = name; //instance variable, using "this"
}
Animal.showCount = function () {//static method
alert(Animal.count)
}
Animal.prototype.showName=function(){//instance method
alert(this.name);
}
var mouse = new Animal("Mickey");
var elephant = new Animal("Haddoop");
Animal.showCount(); // static method, count=2
mouse.showName();//instance method, alert "Mickey"
mouse.showCount();//Error!! mouse.showCount is not a function, which is different from Java
In additions, now it is possible to do with class and static
'use strict'
class Foo {
static talk() {
console.log('talk')
};
speak() {
console.log('speak')
};
};
will give
var a = new Foo();
Foo.talk(); // 'talk'
a.talk(); // err 'is not a function'
a.speak(); // 'speak'
Foo.speak(); // err 'is not a function'
I use namespaces:
var Foo = {
element: document.getElementById("id-here"),
Talk: function(message) {
alert("talking..." + message);
},
ChangeElement: function() {
this.element.style.color = "red";
}
};
And to use it:
Foo.Talk("Testing");
Or
Foo.ChangeElement();
ES6 supports now class & static keywords like a charm :
class Foo {
constructor() {}
talk() {
console.log("i am not static");
}
static saying() {
console.log(this.speech);
}
static get speech() {
return "i am static method";
}
}
If you have to write static methods in ES5 I found a great tutorial for that:
//Constructor
var Person = function (name, age){
//private properties
var priv = {};
//Public properties
this.name = name;
this.age = age;
//Public methods
this.sayHi = function(){
alert('hello');
}
}
// A static method; this method only
// exists on the class and doesn't exist
// on child objects
Person.sayName = function() {
alert("I am a Person object ;)");
};
see #https://abdulapopoola.com/2013/03/30/static-and-instance-methods-in-javascript/
Just additional notes. Using class ES6, When we create static methods..the Javacsript engine set the descriptor attribute a lil bit different from the old-school "static" method
function Car() {
}
Car.brand = function() {
console.log('Honda');
}
console.log(
Object.getOwnPropertyDescriptors(Car)
);
it sets internal attribute (descriptor property) for brand() to
..
brand: [object Object] {
configurable: true,
enumerable: true,
value: ..
writable: true
}
..
compared to
class Car2 {
static brand() {
console.log('Honda');
}
}
console.log(
Object.getOwnPropertyDescriptors(Car2)
);
that sets internal attribute for brand() to
..
brand: [object Object] {
configurable: true,
enumerable: false,
value:..
writable: true
}
..
see that enumerable is set to false for static method in ES6.
it means you cant use the for-in loop to check the object
for (let prop in Car) {
console.log(prop); // brand
}
for (let prop in Car2) {
console.log(prop); // nothing here
}
static method in ES6 is treated like other's class private property (name, length, constructor) except that static method is still writable thus the descriptor writable is set to true { writable: true }. it also means that we can override it
Car2.brand = function() {
console.log('Toyota');
};
console.log(
Car2.brand() // is now changed to toyota
);
When you try to call Foo.talk, the JS tries to search a function talk through __proto__ and, of course, it can't be found.
Foo.__proto__ is Function.prototype.
Static method calls are made directly on the class and are not callable on instances of the class. Static methods are often used to
create utility function
Pretty clear description
Taken Directly from mozilla.org
Foo needs to be bound to your class
Then when you create a new instance you can call myNewInstance.foo()
If you import your class you can call a static method
When i faced such a situation, i have done something like this:
Logger = {
info: function (message, tag) {
var fullMessage = '';
fullMessage = this._getFormatedMessage(message, tag);
if (loggerEnabled) {
console.log(fullMessage);
}
},
warning: function (message, tag) {
var fullMessage = '';
fullMessage = this._getFormatedMessage(message, tag);
if (loggerEnabled) {
console.warn(fullMessage);`enter code here`
}
},
_getFormatedMessage: function () {}
};
so now i can call the info method as
Logger.info("my Msg", "Tag");
In your case, if you want to Foo.talk():
function Foo() {};
// But use Foo.talk would be inefficient
Foo.talk = function () {
alert('hello~\n');
};
Foo.talk(); // 'hello~\n'
But it's an inefficient way to implement, using prototype is better.
Another way, My way is defined as static class:
var Foo = new function() {
this.talk = function () {
alert('hello~\n');
};
};
Foo.talk(); // 'hello~\n'
Above static class doesn't need to use prototype because it will be only constructed once as static usage.
https://github.com/yidas/js-design-patterns/tree/master/class
Javascript has no actual classes rather it uses a system of prototypal inheritance in which objects 'inherit' from other objects via their prototype chain. This is best explained via code itself:
function Foo() {};
// creates a new function object
Foo.prototype.talk = function () {
console.log('hello~\n');
};
// put a new function (object) on the prototype (object) of the Foo function object
var a = new Foo;
// When foo is created using the new keyword it automatically has a reference
// to the prototype property of the Foo function
// We can show this with the following code
console.log(Object.getPrototypeOf(a) === Foo.prototype);
a.talk(); // 'hello~\n'
// When the talk method is invoked it will first look on the object a for the talk method,
// when this is not present it will look on the prototype of a (i.e. Foo.prototype)
// When you want to call
// Foo.talk();
// this will not work because you haven't put the talk() property on the Foo
// function object. Rather it is located on the prototype property of Foo.
// We could make it work like this:
Foo.sayhi = function () {
console.log('hello there');
};
Foo.sayhi();
// This works now. However it will not be present on the prototype chain
// of objects we create out of Foo
There are tree ways methods and properties are implemented on function or class objects, and on they instances.
On the class (or function) itself : Foo.method() or Foo.prop. Those are static methods or properties
On its prototype : Foo.prototype.method() or Foo.prototype.prop. When created, the instances will inherit those object via the prototype witch is {method:function(){...}, prop:...}. So the foo object will receive, as prototype, a copy of the Foo.prototype object.
On the instance itself : the method or property is added to the object itself. foo={method:function(){...}, prop:...}
The this keyword will represent and act differently according to the context. In a static method, it will represent the class itself (witch is after all an instance of Function : class Foo {} is quite equivalent to let Foo = new Function({})
With ECMAScript 2015, that seems well implemented today, it is clearer to see the difference between class (static) methods and properties, instance methods and properties and own methods ans properties. You can thus create three method or properties having the same name, but being different because they apply to different objects, the this keyword, in methods, will apply to, respectively, the class object itself and the instance object, by the prototype or by its own.
class Foo {
constructor(){super();}
static prop = "I am static" // see 1.
static method(str) {alert("static method"+str+" :"+this.prop)} // see 1.
prop="I am of an instance"; // see 2.
method(str) {alert("instance method"+str+" : "+this.prop)} // see 2.
}
var foo= new Foo();
foo.prop = "I am of own"; // see 3.
foo.func = function(str){alert("own method" + str + this.prop)} // see 3.
Given class
class Test {
test() {
console.log('test called');
}
}
And some object toExtend = {}
How can I extend this object so it will have test method?
Object.assign ( as well as _.extend, _.assign, $.extend) do not copy methods. What is preferable way to do that?
Note that toExtend is passed from outside
UPD:
toExtend is instance of another class and has it's own prototype's methods
Object Literals
For object literals, which start with no protoype of their own (Object.getPrototypeOf(toExtend) === Object.protoype)), you can simply use Object.setPrototypeOf to extend the object:
class Test {
test() {
console.log('test called');
}
}
const toExtend = {};
// Set the prototype, so you "inherit" methods:
Object.setPrototypeOf(toExtend, Test.prototype);
toExtend.test();
In older runtimes, you would have to manually assign the prototype:
function Test() {
// noop ctor
}
Test.prototype.test = function() {
console.log('test called');
};
var toExtend = {};
// Set the prototype, so you "inherit" methods:
toExtend.__proto__ = Test.prototype;
toExtend.test();
Class Instances
For instances of an existing class, things are significantly more complex. They do have a prototype of their own, potentially with properties that must be copied, so you need to walk through those:
class Foo {
test() {
console.log('test');
}
}
class Bar {
toast() {
console.log('toast');
}
}
function dynamicExtend(target, base) {
const baseProto = Object.getPrototypeOf(target);
if (baseProto == Object.prototype) {
// simple case: no existing prototype
Object.setPrototypeOf(target, base.prototype);
} else {
// complex case: existing prototype
const proxyClass = class extends base {};
const proxyProto = proxyClass.prototype;
// assign the target properties
Object.getOwnPropertyNames(baseProto).forEach(n => {
const desc = Object.getOwnPropertyDescriptor(baseProto, n);
Object.defineProperty(proxyProto, n, desc);
});
Object.setPrototypeOf(target, proxyProto);
}
}
const targets = [{},
new Bar()
];
targets.forEach(t => {
dynamicExtend(t, Foo);
t.test();
if (t.toast) {
t.toast();
}
});
Note that, thanks to the proxy class, this does break instanceof style inheritance checks.
__proto__
As #PatrickRoberts noted in the comments, __proto__ is deprecated, so you should prefer setPrototypeOf whenever possible.
Given a simple class
class Foo {
constructor(x) {
if (!(this instanceof Foo)) return new Foo(x);
this.x = x;
}
hello() {
return `hello ${this.x}`;
}
}
Is it possible to call the class constructor without the new keyword?
Usage should allow
(new Foo("world")).hello(); // "hello world"
Or
Foo("world").hello(); // "hello world"
But the latter fails with
Cannot call a class as a function
Classes have a "class body" that is a constructor.
If you use an internal constructor() function, that function would be the same class body as well, and would be what is called when the class is called, hence a class is always a constructor.
Constructors require the use of the new operator to create a new instance, as such invoking a class without the new operator results in an error, as it's required for the class constructor to create a new instance.
The error message is also quite specific, and correct
TypeError: Class constructors cannot be invoked without 'new'
You could:
either use a regular function instead of a class1.
Always call the class with new.
Call the class inside a wrapping regular function, always using new, that way you get the benefits of classes, but the wrapping function can still be called with and without the new operator2.
1)
function Foo(x) {
if (!(this instanceof Foo)) return new Foo(x);
this.x = x;
this.hello = function() {
return this.x;
}
}
2)
class Foo {
constructor(x) {
this.x = x;
}
hello() {
return `hello ${this.x}`;
}
}
var _old = Foo;
Foo = function(...args) { return new _old(...args) };
As others have pointed out, ES2015 spec strictly states that such call should throw TypeError, but at the same time, it provides feature that can be used to achieve exactly the desired result, namely Proxies.
Proxies allows us to virtualize over a concept of an object. For instance, they can be used to change some behaviour of particular object without affecting anything else.
In your specific use case, class Foo is Function object which can be called -- this normally means that body of this function will be executed. But this can be changed with Proxy:
const _Foo = new Proxy(Foo, {
// target = Foo
apply (target, thisArg, argumentsList) {
return new target(...argumentsList);
}
});
_Foo("world").hello();
const f = _Foo("world");
f instanceof Foo; // true
f instanceof _Foo; // true
(Note that _Foo is now the class you want to expose, so identifiers should probably be the other way round)
If run by browsers that support Proxies, calling _Foo(...) will now execute apply trap function instead of the original constructor.
At the same time, this "new" _Foo class is indistinguishable from original Foo (apart from being able to call it as a normal function). Similarly, there is no difference by which you can tell object created with Foo and _Foo.
The biggest downside of this is that it cannot be transpiled or polyfilled, but still it's viable solution for having Scala-like class applied in JS in the future.
Here's a pattern I've come across that really helps me. It doesn't use a class, but it doesn't require the use of new either. Win/Win.
const Foo = x => ({
x,
hello: () => `hello ${x}`,
increment: () => Foo(x + 1),
add: ({x: y}) => Foo(x + y)
})
console.log(Foo(1).x) // 1
console.log(Foo(1).hello()) // hello 1
console.log(Foo(1).increment().hello()) // hello 2
console.log(Foo(1).add(Foo(2)).hello()) // hello 3
i just made this npm module for you ;)
https://www.npmjs.com/package/classy-decorator
import classy from "classy-decorator";
#classy()
class IamClassy {
constructor() {
console.log("IamClassy Instance!");
}
}
console.log(new IamClassy() instanceof IamClassy()); // true
console.log(IamClassy() instanceof IamClassy()); // true
No, this is not possible. Constructors that are created using the class keyword can only be constructed with new, if they are [[call]]ed without they always throw a TypeError1 (and there's not even a way to detect this from the outside).
1: I'm not sure whether transpilers get this right
You can use a normal function as a workaround, though:
class Foo {
constructor(x) {
this.x = x;
}
hello() {
return `hello ${this.x}`;
}
}
{
const _Foo = Foo;
Foo = function(...args) {
return new _Foo(...args);
};
Foo.prototype = _Foo.prototype;
}
Disclaimer: instanceof and extending Foo.prototype work as normal, Foo.length does not, .constructor and static methods do not but can be fixed by adding Foo.prototype.constructor = Foo; and Object.setPrototypeOf(Foo, _Foo) if required.
For subclassing Foo (not _Foo) with class Bar extends Foo …, you should use return Reflect.construct(_Foo, args, new.target) instead of the new _Foo call. Subclassing in ES5 style (with Foo.call(this, …)) is not possible.
class MyClass {
constructor(param) {
// ...
}
static create(param) {
return new MyClass(param);
}
doSomething() {
// ...
}
}
MyClass.create('Hello World').doSomething();
Is that what you want?
If you need some logic when creating a new instance of MyClass, it could be helpful to implement a "CreationStrategy", to outsorce the logic (for example complex builder logic with validation)
Edit: As discussed in the comments It does not make sense to create some sort of Builder Pattern with a separate class in JavaScript. Removed related example.
Here's a where you can use a 'scope safe constructor'
Observe this code:
function Student(name) {
if(this instanceof Student) {
this.name = name;
} else {
return new Student(name);
}
}
Now you can create a Student object without using new as follows:
var stud1 = Student('Kia');
Dug up this one in the draft
Constructors defined using class definition syntax throw when called as functions
So I guess that's not possible with classes.
Call class constructor manually can be usefull when refactoring code (having parts of the code in ES6, other parts beeing function & prototype definition)
I ended up with a small, yet usefull boilerplate, slicing the constructor into another function. Period.
class Foo {
constructor() {
//as i will not be able to call the constructor, just move everything to initialize
this.initialize.apply(this, arguments)
}
initialize() {
this.stuff = {};
//whatever you want
}
}
function Bar () {
Foo.prototype.initialize.call(this);
}
Bar.prototype.stuff = function() {}
I had problems extending classes converted with the transformation function mentioned in some other answers. The issue seems to be that node (as of v9.4.0) doesn't properly support the argument spread operator ((...args) =>).
This function based on the transpiled output of the classy-decorator (mentioned in another answer) works for me and doesn't require support for decorators or the argument spread operator.
// function that calls `new` for you on class constructors, simply call
// YourClass = bindNew(YourClass)
function bindNew(Class) {
function _Class() {
for (
var len = arguments.length, rest = Array(len), key = 0;
key < len;
key++
) {
rest[key] = arguments[key];
}
return new (Function.prototype.bind.apply(Class, [null].concat(rest)))();
}
_Class.prototype = Class.prototype;
return _Class;
}
Usage:
class X {}
X = bindNew(X);
// or
const Y = bindNew(class Y {});
const x = new X();
const x2 = X(); // woohoo
x instanceof X; // true
x2 instanceof X; // true
class Z extends X {} // works too
As a bonus, TypeScript (with "es5" output) seems to be fine with the old instanceof trick (well, it won't typecheck if used without new but it works anyhow):
class X {
constructor() {
if (!(this instanceof X)) {
return new X();
}
}
}
because it compiles it down to:
var X = /** #class */ (function () {
function X() {
if (!(this instanceof X)) {
return new X();
}
}
return X;
}());
Alright I have another answer here, and I think this one is pretty innovative.
Basically, the problem with doing something similar to Naomik's answer is that you create functions each and every time you chain methods together.
EDIT: This solution shares the same problem, however, this answer is being left up for educational purposes.
So here I'm offering a way to merely bind new values to your methods--which are basically just independent functions. This offer the additional benefit of being able to import functions from different modules into the newly constructed object.
Okay, so here it goes.
const assoc = (prop, value, obj) =>
Object.assign({},obj,{[prop]: value})
const reducer = ( $values, accumulate, [key,val] ) => assoc( key, val.bind( undefined,...$values ), accumulate )
const bindValuesToMethods = ( $methods, ...$values ) =>
Object.entries( $methods ).reduce( reducer.bind( undefined, ...$values), {} )
const prepareInstance = (instanceMethods, staticMethods = ({}) ) => Object.assign(
bindValuesToMethods.bind( undefined, instanceMethods ),
staticMethods
)
// Let's make our class-like function
const RightInstanceMethods = ({
chain: (x,f) => f(x),
map: (x,f) => Right(f(x)),
fold: (x,l,r) => r(x),
inspect: (x) => `Right(${x})`
})
const RightStaticMethods = ({
of: x => Right(x)
})
const Right = prepareInstance(RightInstanceMethods,RightStaticMethods)
Now you can do
Right(4)
.map(x=>x+1)
.map(x=>x*2)
.inspect()
You can also do
Right.of(4)
.map(x=>x+1)
.map(x=>x*2)
.inspect()
You also have the added benefit of being able to export from modules as such
export const Right = prepareInstance(RightInstanceMethods,RightStaticMethods)
While you don't get ClassInstance.constructor you do have FunctorInstance.name (note, you may need to polyfill Function.name and/or not use an arrow function for export for browser compatibility with Function.name purposes)
export function Right(...args){
return prepareInstance(RightInstanceMethods,RightStaticMethods)(...args)
}
PS - New name suggestions for prepareInstance welcomed, see Gist.
https://gist.github.com/babakness/56da19ba85e0eaa43ae5577bc0064456
As pointed out by you and others
Foo("world").hello();
fails with an error because it is an error,
according to rules of ES6 syntax.
Others pointed out that
(new Foo("world")).hello();
works but is clunky because
It needs the 'new' AND
It needs the extra parenthesis.
I agree it is clunky. So I'm often using
this solution instead:
In your class Foo, create a static method
named 'new':
static new (...args)
{ return new this (...args);
}
Use it like this:
Foo.new("world").hello();
This way I hide the "clunkiness" inside
this static method 'new()'.
Note that this method new() is generic,
it will work as is also
when inherited to sub-classes. If you need
to customize it in a subclass you can first call:
super.new(...args)
and then add any other stuff you need in the
method in a subclass, before returning its result.
A recapped working "one-line" solution for ES6: explained
The answer posted above by Bergi is basically correct.
TLDR; skip to the end 😎 for the one-liner solution
Bergi's answer may seem a unclear when reading it. So, here is a more expanded code-sample that illustrates TWO new ES6 features to achieve the desired goals.
Together, they let a single function C (below) provide the dual-role of a factory and new-able fn; which constructs a B inst that derives from a A.
The B constructor utilizes super handling to invoke the A constructor with initialization arguments. In our final #3 - #4 examples constructed by C.
The A constructor demonstrates the semantics of the new.target psuedo-var to discover new was actually invoked with B.
First, we will make use of ES6 new.target psuedo-var that gives us the RHS of a new RHS() expression.
Technically, we could have gotten new.target as this?.__proto__?.constructor; they are equivalent.
Second, we will make use of ES6 Reflect.construct. Which is crucial to working around the ES6 class constructor invocation constraints; if we are bound and determined to not use new RHS(...).
Test the following and see for yourself its output (also provided in #1-4 below).
class A {
constructor(...a) {
const descendentType = new.target;
console.log(`A's constructor seeing 'new' invoked on ${descendentType?.name} with args: %o`,a);
}
}
class B extends A {
constructor(...a) {
super(...a);
}
}
// C is our DUAL mode Factory
function C(...a) {
console.log(`C's new.target => ${new.target?.name}`);
const inst = new.target ? Reflect.construct(B, a) : new B(...a);
console.log(`C has constructed a ${inst.__proto__.constructor.name} inst`);
return inst;
}
Which we can then invoke it in the following ways:
new A('NEW-A()')
output => "A's constructor seeing 'new' invoked on A with args: ['NEW-A()']"
new B('NEW-B()')
output => "A's constructor seeing 'new' invoked on B with args: ['NEW-B()']"
new C('NEW-C()')
output => "C's new.target => C"
output => "A's constructor seeing 'new' invoked on B with args: ['NEW-C()']"
output => "C has constructed a B inst"
C('PLAIN-C()')
output => "C's new.target => undefined"
output => "A's constructor seeing 'new' invoked on B with args: ['PLAIN-C()']"
output => "C has constructed a B inst"
Where #3 and #4 achieve the originally desired goals.
The simplified `C` looks like:
function C(...a) {return Reflect.construct(B, a);}
OR - if 3rd arg of Reflect.construct not utilized for init.
function C(...a) {return new B(...a);}
Beware: C must be a function not a class for this to both be allowed, and to work returning an alternate this on a new C() invocation, etc.
Also to circumvent strict mode rules for arguments.callee requires using a closure (smalltalk-block. Illustrated below:
class B extends A {
// embedding within a class and generically referencing it requires =>
static C = (() => {
const $class = this; return function(...a) {
return Reflect.construct($class, a);}})();
// Read more on `Reflect.construct` 3rd argument to see more capabilities
// for why it does MORE than just `new $class(...a)` would do.
}
exports.C = B.C;
⛐⚠️⛐ You could do some awful things like fiddle the __proto__ on the resulting inst and change out its constructor and name. Which would make it look and feel like a real subclass C of B depending on how far you want to go to manipulate the object-model. The subtleties abound in what happens with getters/setters, super and # privates. But for much of that you can STAY ES6 CLEAN and get clever with using extends and providing a template superclass flattened mixin tree; which I do a lot of in efekt for supporting tiny-but-complete µhtml reactive custom-elements parts and related PWA app models and responsive dynamic just-in-time versioned code bundling from EdgeS ESS backend servers. As in ... const M = $class => class extends $class {...}.
My motivations...
I posted this to help explain the semantics and a working ES6 solution, which is what I use to support subclassing Promise to provide FutureValue with better workflow handling capabilities in my github efekt library (EdgeS Front End Kit library).
In 2022, with ES6 onwards you can do it with the static method that can be called before the instance of the class is created, to create a instance of the class.
So the code should look something like this:
class Foo {
constructor(x) {
this.x = x;
}
//static class
static Init(x) {
return new Foo(x)
}
sayHello() {
return `hello ${this.x}`;
}
}
//so if i call
Foo.Init('world').sayHello();
//it prints: hello world
But if you are doing all this to make a chain of method you can also look at the following construct:
function MyName(name) {
if (this instanceof MyName) {
this.name = name,
this.prepend = function(n) {
this.name = `${n} ${this.name}`;
return this;
}
,
this.append = function(n) {
this.name = `${this.name} ${n} `;
return this;
}
,
this.show = function() {
return this.name;
}
} else {
return new MyName(name);
}
}
//Call
MyName('vinod').prepend('dev').append('hacks').show();
//prints: dev vinod hacks
The method above returns this at the end of each method which makes the object, properties & method avaialble.
The good part is these methods can be used again & again to create a sentence as
MyName('vinod').prepend('dev').append('hacks')
.prepend("the").append('javascript').append('for Stackoverflow').show();
I have used it as a stringBuilder or to generate xml dynamically.
Calling the class constructor without the new keyword is not possible.
The error message is quite specific.
See a blog post on 2ality and the spec:
However, you can only invoke a class via new, not via a function call (Sect. 9.2.2 in the spec):
> Point()
TypeError: Classes can’t be function-called
I'm adding this as a follow up to a comment by naomik and utilizing on the method illustrated by Tim and Bergi. I'm also going to suggest an of function to use as a general case.
To do this in a functional way AND utilize the efficiency of prototypes (not re-create all method each time a new instance is created), one could use this pattern
const Foo = function(x){ this._value = x ... }
Foo.of = function(x){ return new Foo(x) }
Foo.prototype = {
increment(){ return Foo.of(this._value + 1) },
...
}
Please note that this is consistent with fantasy-land JS specs
https://github.com/fantasyland/fantasy-land#of-method
I personally feel that it is cleaner to use the ES6 class syntax
class Foo {
static of(x) { new Foo(x)}
constructor(x) { this._value = x }
increment() { Foo.of(this._value+1) }
}
Now one could wrap this in a closure as such
class Foo {
static of(x) { new _Foo(x)}
constructor(x) { this._value = x }
increment() { Foo.of(this._value+1) }
}
function FooOf (x) {
return Foo.of(x)
}
Or rename FooOf and Foo as desired, ie the class could be FooClass and the function just Foo, etc.
This is better than place the class in the function because creating new instances doesn't burden us with creating new classes as well.
Yet another way is to create a an of function
const of = (classObj,...args) => (
classObj.of
? classObj.of(value)
: new classObj(args)
)
And then do something like of(Foo,5).increment()
Still finding interesting ways to use instanceof without relying on new or class keywords. In this example program, we compute the 100,000th fibonacci number in less than one second. The result is over 20,000 digits long -
const fib = x =>
Loop // <- no `new`
( (n, a, b) =>
n <= 0n
? String(a) // <- no `new`
: Recur(n - 1n, b, a + b) // <- no `new`
, BigInt(x) // <- no `new`
, 0n
, 1n
)
function Loop (f, ...init)
{ let r = f(...init)
while (r instanceof Recur) // <- instanceof works
r = f(...r)
return r
}
function Recur (...v)
{ return Object.create // <- not a class, but works
( Recur.prototype // <- set prototype
, { constructor: { value: Recur } // <- set constructor
, [Symbol.iterator]: { value: _ => v.values() } // <- whatever you want
}
)
}
document.body.textContent = fib(100000)
body { overflow-wrap: anywhere; }
I don't know why I haven't thought of this before -
function atom (T, v)
{ return Object.assign
( Object.create
( T.prototype
, { constructor: { value: T } }
)
, v
)
}
function pair (car, cdr)
{ return atom(pair, { car, cdr }) }
const p =
pair(1, 2)
console.log(p)
console.log(p instanceof pair)
Output -
{
"car": 1,
"cdr": 2
}
true
I wrote a small helper function which solves this problem. It effectively converts an ES6 class into an older ES5 constructor function which isn't subject to the same ruleset. This way you can create constructors which don't need new. You can also overload constructors in a similar way to the builtin Number, String etc.
function callableConstructor(c, f) {
function ret(...args) {
if(new.target) {
return new c(...args)
}
return f(...args)
}
ret.prototype = c.prototype
ret.prototype.constructor = ret
return ret
}
Test it below:
function callableConstructor(c, f) {
function ret(...args) {
if(new.target) {
return new c(...args)
}
return f(...args)
}
ret.prototype = c.prototype
ret.prototype.constructor = ret
return ret
}
// Usage
class Foo {
constructor(a, b) {
this.a = a
this.b = 2 * b
}
f() {
return this.a + this.b
}
}
Foo = callableConstructor(Foo, (...args) => new Foo(...args))
let foo = new Foo(2, 3)
console.log(foo) // Foo { a: 2, b: 6 }
console.log(foo.f()) // 8
console.log(foo instanceof Foo) // true
foo = Foo(2, 3)
console.log(foo) // Foo { a: 2, b: 6 }
console.log(foo.f()) // 8
console.log(foo instanceof Foo) // true
I came at this issue because I encountered the no-new "do not use new for side effects" eslint rule - which turns out it's a bad practice to use new for an object that is immediately discarded.
I still wanted to use the class syntax because I like it, but I agree that a regular class with new keyword for something that does not produce an object can be confusing.
The solution for me was simple. Define an unexported class in a module and export a function that instatinates it.
class SideEffects {
constructor() {
}
// ...
}
export function addSideEffects() {
// eslint-disable-next-line no-new
new SideEffects();
}
Yes, we are still using the new keyword, but it's used internally in the module and it's obvious from reading the module file that it's not a regular class - and the exported function also makes it clear that it does not create an object.
This might be a little contrived, but it works
function Foo(x){
"use strict"
class Bar {
constructor(x) {
if (!(this instanceof Bar)) return new Bar(x);
this.x = x;
}
hello() {
return `hello ${this.x}`;
}
}
return new Bar(x)
}
Foo("world").hello()
You can't use a class without the new constructor, in my case I didn't want to use the new constructor any time I wanted to use my class, so what you can do is to wrap your class as follows (in my case it's a Dates utils library):
const defaultOptions = {
defaultFormatOptions: 'dd/MM/yyyy'
}
class DatesClass {
constructor(date = new Date(), options) {
this.date = date
this.options = { ...defaultOptions, ...options }
}
get value() {
return this.date
}
add() {}
...
}
export default (date, options) => new DateClass(date, options)
// then you can use it as follow
import dates from 'path/to/yourClass/from/above'
dates(new Date()).add({ unit: 'day', qty: 2}).value
I know this will work:
function Foo() {};
Foo.prototype.talk = function () {
alert('hello~\n');
};
var a = new Foo;
a.talk(); // 'hello~\n'
But if I want to call
Foo.talk() // this will not work
Foo.prototype.talk() // this works correctly
I find some methods to make Foo.talk work,
Foo.__proto__ = Foo.prototype
Foo.talk = Foo.prototype.talk
Are there other ways to do this? I don’t know whether it is right to do so. Do you use class methods or static methods in your JavaScript code?
First off, remember that JavaScript is primarily a prototypal language, rather than a class-based language1. Foo isn't a class, it's a function, which is an object. You can instantiate an object from that function using the new keyword which will allow you to create something similar to a class in a standard OOP language.
I'd suggest ignoring __proto__ most of the time because it has poor cross browser support, and instead focus on learning about how prototype works.
If you have an instance of an object created from a function2 and you access one of its members (methods, attributes, properties, constants etc) in any way, the access will flow down the prototype hierarchy until it either (a) finds the member, or (b) doesn't find another prototype.
The hierarchy starts on the object that was called, and then searches its prototype object. If the prototype object has a prototype, it repeats, if no prototype exists, undefined is returned.
For example:
foo = {bar: 'baz'};
console.log(foo.bar); // logs "baz"
foo = {};
console.log(foo.bar); // logs undefined
function Foo(){}
Foo.prototype = {bar: 'baz'};
f = new Foo();
console.log(f.bar);
// logs "baz" because the object f doesn't have an attribute "bar"
// so it checks the prototype
f.bar = 'buzz';
console.log( f.bar ); // logs "buzz" because f has an attribute "bar" set
It looks to me like you've at least somewhat understood these "basic" parts already, but I need to make them explicit just to be sure.
In JavaScript, everything is an object3.
everything is an object.
function Foo(){} doesn't just define a new function, it defines a new function object that can be accessed using Foo.
This is why you can access Foo's prototype with Foo.prototype.
What you can also do is set more functions on Foo:
Foo.talk = function () {
alert('hello world!');
};
This new function can be accessed using:
Foo.talk();
I hope by now you're noticing a similarity between functions on a function object and a static method.
Think of f = new Foo(); as creating a class instance, Foo.prototype.bar = function(){...} as defining a shared method for the class, and Foo.baz = function(){...} as defining a public static method for the class.
ECMAScript 2015 introduced a variety of syntactic sugar for these sorts of declarations to make them simpler to implement while also being easier to read. The previous example can therefore be written as:
class Foo {
bar() {...}
static baz() {...}
}
which allows bar to be called as:
const f = new Foo()
f.bar()
and baz to be called as:
Foo.baz()
1: class was a "Future Reserved Word" in the ECMAScript 5 specification, but ES6 introduces the ability to define classes using the class keyword.
2: essentially a class instance created by a constructor, but there are many nuanced differences that I don't want to mislead you
3: primitive values—which include undefined, null, booleans, numbers, and strings—aren't technically objects because they're low-level language implementations. Booleans, numbers, and strings still interact with the prototype chain as though they were objects, so for the purposes of this answer, it's easier to consider them "objects" even though they're not quite.
You can achieve it as below:
function Foo() {};
Foo.talk = function() { alert('I am talking.'); };
You can now invoke "talk" function as below:
Foo.talk();
You can do this because in JavaScript, functions are objects as well.
Call a static method from an instance:
function Clazz() {};
Clazz.staticMethod = function() {
alert('STATIC!!!');
};
Clazz.prototype.func = function() {
this.constructor.staticMethod();
}
var obj = new Clazz();
obj.func(); // <- Alert's "STATIC!!!"
Simple Javascript Class Project: https://github.com/reduardo7/sjsClass
Here is a good example to demonstrate how Javascript works with static/instance variables and methods.
function Animal(name) {
Animal.count = Animal.count+1||1;// static variables, use function name "Animal"
this.name = name; //instance variable, using "this"
}
Animal.showCount = function () {//static method
alert(Animal.count)
}
Animal.prototype.showName=function(){//instance method
alert(this.name);
}
var mouse = new Animal("Mickey");
var elephant = new Animal("Haddoop");
Animal.showCount(); // static method, count=2
mouse.showName();//instance method, alert "Mickey"
mouse.showCount();//Error!! mouse.showCount is not a function, which is different from Java
In additions, now it is possible to do with class and static
'use strict'
class Foo {
static talk() {
console.log('talk')
};
speak() {
console.log('speak')
};
};
will give
var a = new Foo();
Foo.talk(); // 'talk'
a.talk(); // err 'is not a function'
a.speak(); // 'speak'
Foo.speak(); // err 'is not a function'
I use namespaces:
var Foo = {
element: document.getElementById("id-here"),
Talk: function(message) {
alert("talking..." + message);
},
ChangeElement: function() {
this.element.style.color = "red";
}
};
And to use it:
Foo.Talk("Testing");
Or
Foo.ChangeElement();
ES6 supports now class & static keywords like a charm :
class Foo {
constructor() {}
talk() {
console.log("i am not static");
}
static saying() {
console.log(this.speech);
}
static get speech() {
return "i am static method";
}
}
If you have to write static methods in ES5 I found a great tutorial for that:
//Constructor
var Person = function (name, age){
//private properties
var priv = {};
//Public properties
this.name = name;
this.age = age;
//Public methods
this.sayHi = function(){
alert('hello');
}
}
// A static method; this method only
// exists on the class and doesn't exist
// on child objects
Person.sayName = function() {
alert("I am a Person object ;)");
};
see #https://abdulapopoola.com/2013/03/30/static-and-instance-methods-in-javascript/
Just additional notes. Using class ES6, When we create static methods..the Javacsript engine set the descriptor attribute a lil bit different from the old-school "static" method
function Car() {
}
Car.brand = function() {
console.log('Honda');
}
console.log(
Object.getOwnPropertyDescriptors(Car)
);
it sets internal attribute (descriptor property) for brand() to
..
brand: [object Object] {
configurable: true,
enumerable: true,
value: ..
writable: true
}
..
compared to
class Car2 {
static brand() {
console.log('Honda');
}
}
console.log(
Object.getOwnPropertyDescriptors(Car2)
);
that sets internal attribute for brand() to
..
brand: [object Object] {
configurable: true,
enumerable: false,
value:..
writable: true
}
..
see that enumerable is set to false for static method in ES6.
it means you cant use the for-in loop to check the object
for (let prop in Car) {
console.log(prop); // brand
}
for (let prop in Car2) {
console.log(prop); // nothing here
}
static method in ES6 is treated like other's class private property (name, length, constructor) except that static method is still writable thus the descriptor writable is set to true { writable: true }. it also means that we can override it
Car2.brand = function() {
console.log('Toyota');
};
console.log(
Car2.brand() // is now changed to toyota
);
When you try to call Foo.talk, the JS tries to search a function talk through __proto__ and, of course, it can't be found.
Foo.__proto__ is Function.prototype.
Static method calls are made directly on the class and are not callable on instances of the class. Static methods are often used to
create utility function
Pretty clear description
Taken Directly from mozilla.org
Foo needs to be bound to your class
Then when you create a new instance you can call myNewInstance.foo()
If you import your class you can call a static method
When i faced such a situation, i have done something like this:
Logger = {
info: function (message, tag) {
var fullMessage = '';
fullMessage = this._getFormatedMessage(message, tag);
if (loggerEnabled) {
console.log(fullMessage);
}
},
warning: function (message, tag) {
var fullMessage = '';
fullMessage = this._getFormatedMessage(message, tag);
if (loggerEnabled) {
console.warn(fullMessage);`enter code here`
}
},
_getFormatedMessage: function () {}
};
so now i can call the info method as
Logger.info("my Msg", "Tag");
In your case, if you want to Foo.talk():
function Foo() {};
// But use Foo.talk would be inefficient
Foo.talk = function () {
alert('hello~\n');
};
Foo.talk(); // 'hello~\n'
But it's an inefficient way to implement, using prototype is better.
Another way, My way is defined as static class:
var Foo = new function() {
this.talk = function () {
alert('hello~\n');
};
};
Foo.talk(); // 'hello~\n'
Above static class doesn't need to use prototype because it will be only constructed once as static usage.
https://github.com/yidas/js-design-patterns/tree/master/class
Javascript has no actual classes rather it uses a system of prototypal inheritance in which objects 'inherit' from other objects via their prototype chain. This is best explained via code itself:
function Foo() {};
// creates a new function object
Foo.prototype.talk = function () {
console.log('hello~\n');
};
// put a new function (object) on the prototype (object) of the Foo function object
var a = new Foo;
// When foo is created using the new keyword it automatically has a reference
// to the prototype property of the Foo function
// We can show this with the following code
console.log(Object.getPrototypeOf(a) === Foo.prototype);
a.talk(); // 'hello~\n'
// When the talk method is invoked it will first look on the object a for the talk method,
// when this is not present it will look on the prototype of a (i.e. Foo.prototype)
// When you want to call
// Foo.talk();
// this will not work because you haven't put the talk() property on the Foo
// function object. Rather it is located on the prototype property of Foo.
// We could make it work like this:
Foo.sayhi = function () {
console.log('hello there');
};
Foo.sayhi();
// This works now. However it will not be present on the prototype chain
// of objects we create out of Foo
There are tree ways methods and properties are implemented on function or class objects, and on they instances.
On the class (or function) itself : Foo.method() or Foo.prop. Those are static methods or properties
On its prototype : Foo.prototype.method() or Foo.prototype.prop. When created, the instances will inherit those object via the prototype witch is {method:function(){...}, prop:...}. So the foo object will receive, as prototype, a copy of the Foo.prototype object.
On the instance itself : the method or property is added to the object itself. foo={method:function(){...}, prop:...}
The this keyword will represent and act differently according to the context. In a static method, it will represent the class itself (witch is after all an instance of Function : class Foo {} is quite equivalent to let Foo = new Function({})
With ECMAScript 2015, that seems well implemented today, it is clearer to see the difference between class (static) methods and properties, instance methods and properties and own methods ans properties. You can thus create three method or properties having the same name, but being different because they apply to different objects, the this keyword, in methods, will apply to, respectively, the class object itself and the instance object, by the prototype or by its own.
class Foo {
constructor(){super();}
static prop = "I am static" // see 1.
static method(str) {alert("static method"+str+" :"+this.prop)} // see 1.
prop="I am of an instance"; // see 2.
method(str) {alert("instance method"+str+" : "+this.prop)} // see 2.
}
var foo= new Foo();
foo.prop = "I am of own"; // see 3.
foo.func = function(str){alert("own method" + str + this.prop)} // see 3.