How to wrap object being constructed with Proxy inside constructor?

How to wrap object being constructed with Proxy inside constructor? - javascript

I understand that Proxy can be used to alter object-level behaviors, such as bracket notation get and set. All the examples I can find show constructing an object and then wrapping it with a Proxy call. Is there a way to define a class Foo, using ES6 class constructor notation, such that the returned object from the constructor is already wrapped in Proxy, rather than the caller of the constructor having to also call Proxy separately?
Thanks in advance.

If I understand your question properly, what you want to do, is in the constructor return a new proxy like this:
class MyClass {
constructor() {
return new Proxy(this, {
// Proxy settings here
})
}
}
Here in this example, we create a new class, then call a few properties. The proxy will then just print out the properties that were called for simplicity.
class MyClass {
constructor() {
return new Proxy(this, {
get: (target, key) => {
console.log('I am the key: ' + key)
return Reflect.get(target, key)
}
})
}
}
let c = new MyClass
c.awesome
c.billy
c.superTroopers
if (c instanceof MyClass) {
console.log('I am an instance of MyClass')
} else {
console.log('I am not an instance of MyClass')
}

As far as I know: no, but you can set the prototype afterwards. Something like this:
class Thing {
constructor() {
// ...
}
}
Thing.prototype = new Proxy(Thing.prototype, {
get(target, name) {
// ...
}
});

Related

Is it possible to instantiate a class without a `new` operator? [duplicate]

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

Make JS class callable without side effects [duplicate]

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

Publish subscriber pattern using instance based decorators 'this' is always undefined

class Observable {
constructor() {
this.handlers = [];
}
publish(value) {
this.handlers.forEach(handler => {
handler(value);
});
}
subscribe(callback) {
this.handlers.push(callback);
}
}
const concreteObserver = new Observable();
function Subscribe(observable) {
return function functionDescriptor(target, propertyKey, descriptor) {
observable.subscribe(target[propertyKey]);
return descriptor;
}
}
class MyClass {
constructor(){
this.x = 5;
}
#Subscribe(concreteObserver)
subsribeToValue(value) {
console.log(this.x); // undefined
}
}
As you can see, the subscribe function is called each time, someone calls concreteObserver.publish() however, when you call observable.subscribe(target[propertyKey]); then 'this' becomes undefined.
I also tried overriding the descriptor getter, and calling that one, but i still get undefined. On classes i was able to wrap a function by calling target.prototype.functionName.
This works when i know what the function name will be called, but the function name for #Subscribe can be arbitrary, so i can't use it on a class level decorator unless i use Reflection to detect all the annotations of the class.
EDIT
Tried so far
observable.subscribe(target[propertyKey].bind(this));
which returns undefined, subscribe has the right context in this case.
observable.subscribe(data => descriptor.value.apply(this, data)); also has 'this' as undefined
descriptor.value = function(){
console.log(this); //undefined
}
descriptor.get = function(){
console.log(this); //undefined
}
The solution i came up with. Since it is only possible to get the instance of a class in the class decorator, then that is where this can be used properly, in the the subscribe function i tell what function i should subscribe to, then in the ClassDecorator i iterate through each method to determine if they have __subscribeFunction in their prototype and thus subscribe to the method while binding instance
class Observable {
constructor() {
this.handlers = [];
}
publish(value) {
this.handlers.forEach(handler => {
handler(value);
});
}
subscribe(callback) {
this.handlers.push(callback);
}
}
const concreteObserver = new Observable();
function ClassDecorator(target) {
const originalTarget = target;
const Override = function (...args) {
const instance = originalTarget.apply(this, args);
Object.values(instance.__proto__).forEach(method => {
const observableFunction = method.prototype.__subscribeFunction;
if (observableFunction) {
observableFunction.subscribe(method.bind(instance));
}
});
return instance;
};
Override.prototype = originalTarget.prototype;
customElements.define(elementName, target);
return Override;
}
function Subscribe(observable) {
return function functionDescriptor(target, propertyKey, descriptor) {
target[propertyKey].prototype.__subscribeFunction = observable;
}
}
#ClassDecorator
class MyClass {
constructor(){
this.x = 5;
}
#Subscribe(concreteObserver)
subsribeToValue(value) {
console.log(this.x); // 5
}
}

This doesn't work because the decorator is called when the class itself is constructed, but before any instance is created. Since there's no instance, there can't be a this – you only have access to the prototype, but class properties aren't on the prototype (unlike methods).
You can verify this using this example:
function Example() {
console.log("#Example initialized");
return function exampleDescriptior(target, propertyKey, descriptor) {
console.log("#Example called");
}
}
console.log("Before declaring class");
class Test {
#Example()
public test() {}
}
console.log("After declaring class");
console.log("Before creating instance");
const test = new Test();
console.log("After creating instance");
console.log("Before calling method");
test.test();
console.log("After calling method");
which yields the output
Before declaring class
#Example initialized
#Example called
After declaring class
Before creating instance
After creating instance
Before calling method
After calling method
That said, what you can do is write another decorator applied on, say, class level which proxies the constructor. If your #Subscribe annotation stores some meta-data on the prototype, the class decorator could then look for it and do the actual wiring. So getting something like
#AutoSubscribe()
class MyClass {
#Subscribe(observer)
subscribe(value) {
console.log(this.x);
}
}
to work should be possible. In fact, you could maybe even get rid of the second decorator by proxying the constructor from the #Subscribe decorator, but you'd still have to store metadata that you can look through during instantiation.

how to call method of abstract class in javascript

I have an abstract class that implements some methods on its prototype and I want to create an instance directly of this class without subclassing it.
I can instantiate that class by creating a Proxy and trapping construct and it seems to work. Properties of the new instance are set correctly but I have a hard time calling its methods.
function AbstractNumbers(...args) {
if (new.target === AbstractNumbers) {
throw new Error('Cannot instantiate abstract class');
}
this.numbers = args;
}
AbstractNumbers.prototype.showNumbers = function() { console.log(this.numbers); }
const AbstractNumbersProxy = new Proxy(AbstractNumbers, {
construct(target, args) {
// change 3rd argument to bypass new.target test
return Reflect.construct(target, args, function() {});
}
});
const n = new AbstractNumbersProxy(1, 2, 3);
// set prototype back to AbstractNumbers
Object.setPrototypeOf(n, AbstractNumbers);
// n.__proto__ shows the correct prototype
console.log(n.__proto__);
// property n.numbers is set correctly
console.log(n.numbers);
// calling its prototype method fail
n.showNumbers();
How can I properly instantiate that abstract class so that I am able to call its methods?

In
// set prototype back to AbstractNumbers
Object.setPrototypeOf(n, AbstractNumbers);
you've set the prototype back to the constructor function instead of its prototype property. Try
Object.setPrototypeOf(n, AbstractNumbers.prototype);
instead:
function AbstractNumbers(...args) {
if (new.target === AbstractNumbers) {
throw new Error('Cannot instantiate abstract class');
}
this.numbers = args;
}
AbstractNumbers.prototype.showNumbers = function() { console.log(this.numbers); }
const AbstractNumbersProxy = new Proxy(AbstractNumbers, {
construct(target, args) {
// change 3rd argument to bypass new.target test
return Reflect.construct(target, args, function() {});
}
});
const n = new AbstractNumbersProxy(1, 2, 3);
// set prototype back to AbstractNumbers
Object.setPrototypeOf(n, AbstractNumbers.prototype);
// n.__proto__ shows the correct prototype
console.log(n.__proto__);
// property n.numbers is set correctly
console.log(n.numbers);
// calling its prototype method fail
n.showNumbers();
Please do not ask me to investigate what you are doing.

How to extend object with ES6 class instance properties and methods

I am refactoring some code from an older ES5 codebase, where I was doing the following:
function ObjectCreatorFunction() {
this.someProperty= {};
}
/*
* Static method, to extend the object passed as a parameter with the
* ObjectCreatorFunction instance properties and methods
*/
ObjectCreatorFunction.extend = function extend(object) {
var key;
ObjectCreatorFunction.call(object);
for (key in ObjectCreatorFunction.prototype) {
if (ObjectCreatorFunction.prototype.hasOwnProperty(key)) {
if (!object.hasOwnProperty(key)) {
object[key] = ObjectCreatorFunction.prototype[key];
}
}
}
return object;
};
ObjectCreatorFunction.prototype.someMethod = function someMethod() {...}
//etc
I am trying to do the same with an ES6 rewrite, so I a have this
class ClassName{
constructor() {
this.someProperty= {};
}
static extend(object) {
let key;
ClassName.constructor.call(object);
for (key in ClassName.prototype) {
if (ClassName.prototype.hasOwnProperty(key)) {
if (!object.hasOwnProperty(key)) {
object[key] = ClassName.prototype[key];
}
}
}
return object;
}
someMethod() {...}
//etc
}
My problem is that the line
ClassName.constructor.call(object);
does not work as intended, i.e. The passed object does not get the instance properties of the class.
I have tried s few ways to rewrite this (even some unorthodox ones) to no avail.
How do I extend an object with a class' instance properties, using ES6?
DISCLAIMER:
My code is passed through a transpiling process, with babel and webpack. In case it has any effect to how classes work internally.

No, this does not work with class syntax. It's a bit more than just syntactic sugar. The prototypical inheritance stayed the same, but the initialisation of instances works differently now especially for inherited classes, and you cannot invoke a constructor without new to not create a new instance.
I would recommend to be explicit about your mixin, and give it an init method:
class Mixin {
constructor(methods) {
this.descriptors = Object.getOwnPropertyDescriptors(methods);
}
extend(object) {
for (const p in this.descriptors)) {
if (Object.prototype.hasOwnProperty.call(object, p)) {
if (process.env.NODE_ENV !== 'production') {
console.warn(`Object already has property "${p}"`);
}
} else {
Object.defineProperty(object, p, this.descriptors[p]);
}
}
}
}
// define a mixin:
const xy = new Mixin({
initXY() {
this.someProperty= {};
},
someMethod() { … }
});
// and use it:
class ClassName {
constructor() {
this.initXY();
}
}
xy.extend(ClassName.prototype);

Develop Reference

JavaScript is the programming language of the Web.

How to wrap object being constructed with Proxy inside constructor? - javascript

As far as I know: no, but you can set the prototype afterwards. Something like this: class Thing { constructor() { // ... } } Thing.prototype = new Proxy(Thing.prototype, { get(target, name) { // ... } });

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