Develop Reference

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

javascript and memoized getters

This article describe getters. It has a section "
Smart / self-overwriting / lazy getters"
And it's unclear for me, are getters 'memoized' by default or should I implement this feature by myself
e.g.
class Foo() {
get boo() {
this._boo = this._boo || new Boo();
return this._boo;
}
}
or can I just write:
class Foo() {
get boo() {
return new Boo();
}
}
to have the same result?

The most interesting bit of that article was Smart / self-overwriting / lazy getters, which offers this technique:
class Foo {
get boo() {
delete this.boo;
return this.boo = new Boo();
}
}
With this your Foo objects don't go through the hassle of creating their boo properties until you ask for it. Then it's created once and further requests for it simply return the same object. This makes sense if new Boo() is in someway resource-intensive to create and reasonably often is not needed.
Theoretically, you could extend this to allow you to delete the current version and recreate it on next access. But that's a lot more code, and is probably a fairly rare need.
Update
A comment from vrugtehagel correctly pointed out that the above technique, while fine for plain objects, does not work for classes.
Here's a variant which does work:
class Boo {
static counter = 0
constructor () {
this.x = ++Boo.counter
console .log (`creating Boo(${this.x})`)
}
}
class Foo {
get boo () {
Object .defineProperty (
this,
"boo",
{ value: new Boo(), writable: false}
)
return this .boo;
}
}
const f = new Foo()
console .log (f.boo)
console .log (f.boo) // no 'creating Boo' log, Boo constructor only created once

No, there is no language-level support for memoized getters in JavaScript. In your second example, a new object would be created every time boo was accessed.

You're free to add memoization, e.g.
Non memoized,
class NonMemoized {
constructor(prefix) {
this.prefix = prefix;
}
get myFunc() {
return this.prefix + Math.random().toString();
}
}
let nonMemoized = new NonMemoized('new number each time ');
console.log(nonMemoized.myFunc);
console.log(nonMemoized.myFunc);
Memoized, nice for when u want to create an object once and always return the same object (but don't want to create in the constructor because maybe it's not necessary all the time or some other reason)
class MemoizedManually {
constructor(prefix) {
this.prefix = prefix;
}
get myFunc() {
return this._myFunc_ = this._myFunc_ || this.prefix + Math.random().toString();
}
}
let memoizedManually = new MemoizedManually('same number ');
console.log(memoizedManually.myFunc);
console.log(memoizedManually.myFunc);
Lastly, if you have a bunch of functions you want to memoize but don't want to repeat that this.x = this.x || something computation in each function (which you really shoudln't repeat, as it's not really the job of myFunc to memoize itself:
class Memoized {
constructor(prefix) {
this.prefix = prefix;
}
get myFunc() {
return this.prefix + Math.random().toString();
}
}
const memoizeGetter = (clazz, functionName) => {
let func = Object.getOwnPropertyDescriptor(clazz.prototype, functionName);
let cacheKey = `_${functionName}-cache_`;
Object.defineProperty(clazz.prototype, functionName, {
get: function () {
return this[cacheKey] = this[cacheKey] || func.get.call(this);
}
});
};
memoizeGetter(Memoized, 'myFunc');
let memoized = new Memoized('also same number ');
console.log(memoized.myFunc);
console.log(memoized.myFunc);
Nice thing about getters is they don't take arguments, so you don't have to worry about ...args, but do need to worry about binding this

consider this code:
class Person {
static get SHORT() { return 0; }//rvalue
}
versus
class Person {}
Person.SHORT = 0;//lvalue
Although both return the same result, the latter is actually faster (because it avoids the function call overhead); though the js engine can make optimizations that nullify one over the other.

You can do it all on one line:
class Foo() {
get boo() {
return this._boo = this._boo || new Boo();
}
}
This is easy to remember and isn't convoluted. Got to consider the maintenance factor, and try to keep things simple.

Split a Javascript class (ES6) over multiple files?

I have a Javascript class (in ES6) that is getting quite long. To organize it better I'd like to split it over 2 or 3 different files. How can I do that?
Currently it looks like this in a single file:
class foo extends bar {
constructor(a, b) {} // Put in file 1
methodA(a, b) {} // Put in file 1
methodB(a, b) {} // Put in file 2
methodC(a, b) {} // Put in file 2
}
Thanks!

When you create a class
class Foo extends Bar {
constructor(a, b) {
}
}
you can later add methods to this class by assigning to its prototype:
// methodA(a, b) in class Foo
Foo.prototype.methodA = function(a, b) {
// do whatever...
}
You can also add static methods similarly by assigning directly to the class:
// static staticMethod(a, b) in class Foo
Foo.staticMethod = function(a, b) {
// do whatever...
}
You can put these functions in different files, as long as they run after the class has been declared.
However, the constructor must always be part of the class declaration (you cannot move that to another file). Also, you need to make sure that the files where the class methods are defined are run before they are used.

Here's my solution. It:
uses regular modern classes and .bind()ing, no prototype. (EDIT: Actually, see the comments for more on this, it may not be desirable.)
works with modules. (I'll show an alternative option if you don't use modules.)
supports easy conversion from existing code.
yields no concern for function order (if you do it right).
yields easy to read code.
is low maintenance.
unfortunately does not play well with static functions in the same class, you'll need to split those off.
First, place this in a globals file or as the first <script> tag etc.:
BindToClass(functionsObject, thisClass) {
for (let [ functionKey, functionValue ] of Object.entries(functionsObject)) {
thisClass[functionKey] = functionValue.bind(thisClass);
}
}
This loops through an object and assigns and binds each function, in that object, by its name, to the class. It .bind()'s it for the this context, so it's like it was in the class to begin with.
Then extract your function(s) from your class into a separate file like:
//Use this if you're using NodeJS/Webpack. If you're using regular modules,
//use `export` or `export default` instead of `module.exports`.
//If you're not using modules at all, you'll need to map this to some global
//variable or singleton class/object.
module.exports = {
myFunction: function() {
//...
},
myOtherFunction: function() {
//...
}
};
Finally, require the separate file and call BindToClass like this in the constructor() {} function of the class, before any other code that might rely upon these split off functions:
//If not using modules, use your global variable or singleton class/object instead.
let splitFunctions = require('./SplitFunctions');
class MySplitClass {
constructor() {
BindToClass(splitFunctions, this);
}
}
Then the rest of your code remains the same as it would if those functions were in the class to begin with:
let msc = new MySplitClass();
msc.myFunction();
msc.myOtherFunction();
Likewise, since nothing happens until the functions are actually called, as long as BindToClass() is called first, there's no need to worry about function order. Each function, inside and outside of the class file, can still access any property or function within the class, as usual.

I choose to have all privte variables/functions in an object called private, and pass it as the first argument to the external functions.
this way they have access to the local variables/functions.
note that they have implicit access to 'this' as well
file: person.js
const { PersonGetAge, PersonSetAge } = require('./person_age_functions.js');
exports.Person = function () {
// use privates to store all private variables and functions
let privates={ }
// delegate getAge to PersonGetAge in an external file
// pass this,privates,args
this.getAge=function(...args) {
return PersonGetAge.apply(this,[privates].concat(args));
}
// delegate setAge to PersonSetAge in an external file
// pass this,privates,args
this.setAge=function(...args) {
return PersonSetAge.apply(this,[privates].concat(args));
}
}
file: person_age_functions.js
exports.PersonGetAge =function(privates)
{
// note: can use 'this' if requires
return privates.age;
}
exports.PersonSetAge =function(privates,age)
{
// note: can use 'this' if requires
privates.age=age;
}
file: main.js
const { Person } = require('./person.js');
let me = new Person();
me.setAge(17);
console.log(`I'm ${me.getAge()} years old`);
output:
I'm 17 years old
note that in order not to duplicate code on person.js, one can assign all functions in a loop.
e.g.
person.js option 2
const { PersonGetAge, PersonSetAge } = require('./person_age_functions.js');
exports.Person = function () {
// use privates to store all private variables and functions
let privates={ }
{
// assign all external functions
let funcMappings={
getAge:PersonGetAge,
setAge:PersonSetAge
};
for (const local of Object.keys(funcMappings))
{
this[local]=function(...args) {
return funcMappings[local].apply(this,[privates].concat(args));
}
}
}
}

You can add mixins to YourClass like this:
class YourClass {
ownProp = 'prop'
}
class Extension {
extendedMethod() {
return `extended ${this.ownProp}`
}
}
addMixins(YourClass, Extension /*, Extension2, Extension3 */)
console.log('Extended method:', (new YourClass()).extendedMethod())
function addMixins() {
var cls, mixin, arg
cls = arguments[0].prototype
for(arg = 1; arg < arguments.length; ++ arg) {
mixin = arguments[arg].prototype
Object.getOwnPropertyNames(mixin).forEach(prop => {
if (prop == 'constructor') return
if (Object.getOwnPropertyNames(cls).includes(prop))
throw(`Class ${cls.constructor.name} already has field ${prop}, can't mixin ${mixin.constructor.name}`)
cls[prop] = mixin[prop]
})
}
}

TypeScript Solution
foo-methods.ts
import { MyClass } from './class.js'
export function foo(this: MyClass) {
return 'foo'
}
bar-methods.ts
import { MyClass } from './class.js'
export function bar(this: MyClass) {
return 'bar'
}
class.ts
import * as barMethods from './bar-methods.js'
import * as fooMethods from './foo-methods.js'
const myClassMethods = { ...barMethods, ...fooMethods }
class _MyClass {
baz: string
constructor(baz: string) {
this.baz = baz
Object.assign(this, myClassMethods);
}
}
export type MyClass = InstanceType<typeof _MyClass> &
typeof myClassMethods;
export const MyClass = _MyClass as unknown as {
new (
...args: ConstructorParameters<typeof _MyClass>
): MyClass;
};

My solution is similar to the one by Erez (declare methods in files and then assign methods to this in the constructor), but
it uses class syntax instead of declaring constructor as a function
no option for truly private fields - but this was not a concern for this question anyway
it does not have the layer with the .apply() call - functions are inserted into the instance directly
one method per file: this is what works for me, but the solution can be modified
results in more concise class declaration
1. Assign methods in constructor
C.js
class C {
constructor() {
this.x = 1;
this.addToX = require('./addToX');
this.incX = require('./incX');
}
}
addToX.js
function addToX(val) {
this.x += val;
return this.x;
}
module.exports = addToX;
incX.js
function incX() {
return this.addToX(1);
}
module.exports = incX;
2. Same, but with instance fields syntax
Note that this syntax is a Stage 3 proposal as of now.
But it works in Node.js 14 - the platform I care about.
C.js
class C {
x = 1;
addToX = require('./addToX');
incX = require('./incX');
}
Test
const c = new C();
console.log('c.incX()', c.incX());
console.log('c.incX()', c.incX());

How to define private constructors in javascript?

I have defined pure objects in JS which expose certain static methods which should be used to construct them instead of the constructor. How can I make a constructor for my class private in Javascript?
var Score = (function () {
// The private constructor
var Score = function (score, hasPassed) {
this.score = score;
this.hasPassed = hasPassed;
};
// The preferred smart constructor
Score.mkNewScore = function (score) {
return new Score(score, score >= 33);
};
return Score;
})();
Update: The solution should still allow me to test for x instanceof Score. Otherwise, the solution by #user2864740 of exposing only the static constructor works.

One can use a variable (initializing) inside a closure which can throw an error if the constructor was called directly instead of via a class method:
var Score = (function () {
var initializing = false;
var Score = function (score, hasPassed) {
if (!initializing) {
throw new Error('The constructor is private, please use mkNewScore.');
}
initializing = false;
this.score = score;
this.hasPassed = hasPassed;
};
Score.mkNewScore = function (score) {
intializing = true;
return new Score(score, score >= 33);
};
return Score;
})();

Is there a solution which will allow me to say x instanceof Score?
Yes. Conceptually, #user2864740 is right, but for instanceof to work we need to expose (return) a function instead of a plain object. If that function has the same .prototype as our internal, private constructor, the instanceof operator does what is expected:
var Score = (function () {
// the module API
function PublicScore() {
throw new Error('The constructor is private, please use Score.makeNewScore.');
}
// The private constructor
var Score = function (score, hasPassed) {
this.score = score;
this.hasPassed = hasPassed;
};
// Now use either
Score.prototype = PublicScore.prototype; // to make .constructor == PublicScore,
PublicScore.prototype = Score.prototype; // to leak the hidden constructor
PublicScore.prototype = Score.prototype = {…} // to inherit .constructor == Object, or
PublicScore.prototype = Score.prototype = {constructor:null,…} // for total confusion :-)
// The preferred smart constructor
PublicScore.mkNewScore = function (score) {
return new Score(score, score >= 33);
};
return PublicScore;
}());
> Score.mkNewScore(50) instanceof Score
true
> new Score
Error (…)

Simply don't expose the constructor function. The core issue with the original code is the "static method" is defined as a property of the constructor (which is used as a "class") as opposed a property of the module.
Consider:
return {
mkNewScore: Score.mkNewScore
// .. and other static/module functions
};
The constructor can still be accessed via .constructor, but .. meh. At this point, might as well just let a "clever user" have access.
return {
mkNewScore: function (score) {
var s = new Score(score, score >= 33);
/* Shadow [prototype]. Without sealing the object this can
be trivially thwarted with `del s.constructor` .. meh.
See Bergi's comment for an alternative. */
s.constructor = undefined;
return s;
}
};

In order to create a private constructor in JS, I like to create a private key that is only accessible in the class (function) file and provide a static factory function as the only allowed way to construct said class:
// in PrivateConstructorClass.js
// Use a Symbol as this will always be unique.
// If you don't have Symbol in your runtime,
// use a random string that nobody can reliably guess,
// such as the current time plus some other random values.
const PRIVATE_CONSTRUCTOR_KEY = Symbol()
class PrivateConstructorClass {
constructor(arg1, arg2, argN, constructorKey) {
if (constructorKey !== PRIVATE_CONSTRUCTOR_KEY) {
throw new Error('You must use the PrivateConstructorClass.create() to construct an instance.')
}
this.arg1 = arg1
this.arg2 = arg2
this.argN = argN
}
static create(arg1, arg2, argN) {
return new PrivateConstructorClass(arg1, arg2, argN, PRIVATE_CONSTRUCTOR_KEY)
}
}
// From Another JS File:
try {
const myFailedInstanceA = new PrivateConstructorClass('foo', 123, {
size: 'n'
})
} catch (err) {
console.error('Failed:', err.message)
}
const myFactoryInstance = PrivateConstructorClass.create('foo', 123, {
size: 'n'
})
console.log('Success:', myFactoryInstance)

Another possible simple approach is to use predicate function instead of instanceof. For typescript it can be a type guard and type synonym instead of a class can be exported:
// class is private
class _Score {
constructor() {}
}
export type Score = _Score
export function isScore(s): s is Score {
return s instanceof _Score
}

So to be fair the simplest answer is usually the best. An object literal is always a single instance. Not much reason for anything more complex other than, perhaps allocation of memory on demand.
That being said, here is a classical implementation of a singleton using ES6.
The instance "field" is "private". This really means we hide the instance as a property of the constructor. Somewhere not Constructor.prototype, which will be available to the instance through prototipical inheritance.
The constructor is "private". We really are just throwing an error when the caller is not the static getInstance method.
Also of note. It’s important to understand what the keyword this means in different contexts.
In the constructor, this points to the instance being created.
In the static getInstance method, this points to the left of the dot, Universe constructor function which, is an object like most things in JS and can hold properties.
class Universe {
constructor() {
if (!((new Error).stack.indexOf("Universe.getInstance") > -1)) {
throw new Error("Constructor is private. Use static method getInstance.");
}
this.constructor.instance = this;
this.size = 1;
}
static getInstance() {
if (this.instance) {
return this.instance;
}
return new this;
}
expand() {
this.size *= 2;
return this.size;
}
}
//console.log(Universe.getInstance())
//console.log(Universe.getInstance().expand())
//console.log(Universe.getInstance())
//console.log(new Universe())
const getInstance= () => { console.log('hi');
console.log("From singleton: ", Universe.getInstance()); return new Universe() };
console.log(getInstance())

reusable javascript objects, prototypes and scope

MyGlobalObject;
function TheFunctionICanUseRightAwaySingleForAllInstansesAndWithoutInstanse() {
function() {
alert('NO CONSTRUCTOR WAS CALLED');
}
};
The Long-named function must be callable from MyGlobalObject, which in turn must be available as a global (to window) variable in all times after script was loaded. It should support extensibility in accordance with latest standards.
I'm at architectural dilemma of how to built JS base for an application (almost 100% JS).
We need an object i.e. window.MyObject (like a module, like jQuery) so
It can be created with
VAR1
var MyGlobalObjConstructor = function(){
this.GlobalFunctionInObject = function(){
alert('called with MyGlobalObj.GlobalFunctionInObject()');
}
};
window.MyGlobalObj = new MyGlobalObjConstructor();
Is MyGlobalObj extensible? Can I create child objects, which will inherit current state of MyGlobalObj (extended functions/properties MyGlobalObj.NewFunc e.g.)? What is the main difference between using prototype (VAR3)?
By GlobaldFunction I mean single instance for all initialized/instantiated (possibly instantializable) instances..
Or with
VAR2
var MyGlobalObj = {
GlobalFunctionInObject: function...
GlobalFunctionInObject2: function...
};
MyGlobalObj.GlobalFunctionInObject();
// here I lose all hierarchy elements, no prototype,
// can I use GlobalFunctionInObject2 in GlobalFunctionInObject?
Or with
VAR3
var MyGlobalConstuctor = function(){} // already 'well-formed' object
MyGlobalConstuctor.prototype.GlobalFunctionInObject = function...
};
var MyGlobalObj = new MyGlobalConstuctor();
// so I'm sceptical to NEW, because I have ALREADY wrote my functions
// which I expect to be in memory, single instance of each of them,
// so creating MyObject2,3,4 with NEW MyGC() makes no sense to me.
// DO I REALLY HAVE TO USE "MyGlobalConstuctor.prototype." FOR EACH FUNCTION?!!!!
What's the difference defining MyGlobalObj as a function and as an object (result of func or VAR2)?
OR VAR4?
I see in Chrome Debugger both prototype and __proto__ special fields. I've read that that's OK, but why are they not saved in a single prototype?
So, what is the correct/optimal way to implement window.MyObject, so one could MyObject.MyFunction(); What are the differences (pro/contra) of variants 1 2 and 3?

Variation 1 - Mixin
function SomeType() {
var priv = "I'm private";
this.publ = "I'm public";
this.action = function() {
return priv + this.publ;
};
}
var obj = new SomeType();
With this method you are creating a new object every time you call new SomeType(), creating all its methods and adding all this method to the new object. Every time you create an object.
Pros
It looks like classical inheritance so it's easy to understand to Java-C#-C++-etc people.
It can have private variables per instance since you have one function closure per each object you create
It allows multiple inheritance, also known as Twitter-mixins or functional mixins
obj instanceof SomeType will return true
Cons
It consumes more memory as more objects you create because with each object you are creating a new closure and creating each of it's methods again.
Private properties are private, not protected, subtypes can't access them
No easy way to know if a object has some Type as superclass.
Inheritance
function SubType() {
SomeType.call(this);
this.newMethod = function() {
// can't access priv
return this.publ;
};
}
var child = new SubType();
child instanceof SomeType will return false there is no other way to know if child has SomeType methods than look if it has them one by one.
Variation 2 - Object literal with prototyping
var obj = {
publ: "I'm public",
_convention: "I'm public too, but please don't touch me!",
someMethod: function() {
return this.publ + this._convention;
}
};
In this case you are creating a single object. If you are going to need only one instance of this type it can be the best solution.
Pros
It's quick and easy to understand.
Performant
Cons
No privacy, every property is public.
Inheritance
You can inherit a object prototyping it.
var child = Object.create(obj);
child.otherMethod = function() {
return this._convention + this.publ;
};
If you are on a old browser you will need to garantee Object.create works:
if (!Object.create) {
Object.create = function(obj) {
function tmp() { }
tmp.prototype = obj;
return new tmp;
};
}
To know if a object is a prototype of another you can use
obj.isPrototypeOf(child); // true
Variation 3 - Constructor pattern
UPDATE: This is the pattern ES6 classes are sugar syntax of. If you use ES6 classes you are following this pattern under the hood.
class SomeType {
constructor() {
// REALLY important to declare every non-function property here
this.publ = "I'm public";
this._convention = "I'm public too, but please don't touch me!";
}
someMethod() {
return this.publ + this._convention;
}
}
class SubType extends SomeType {
constructor() {
super(/* parent constructor parameters here */);
this.otherValue = 'Hi';
}
otherMethod() {
return this._convention + this.publ + this.otherValue;
}
}
function SomeType() {
// REALLY important to declare every non-function property here
this.publ = "I'm public";
this._convention = "I'm public too, but please don't touch me!";
}
SomeType.prototype.someMethod = function() {
return this.publ + this._convention;
};
var obj = new SomeType();
You can re-assign the prototype insteadd of adding each method if you are not inheriting and remember to re-assign the constructor property:
SomeType.prototype = {
constructor: SomeType,
someMethod = function() {
return this.publ + this._convention;
}
};
Or use _.extend or $.extend if you have underscore or jquery in your page
_.extend(SomeType.prototype, {
someMethod = function() {
return this.publ + this._convention;
}
};
The new keyword under the hood simply does this:
function doNew(Constructor) {
var instance = Object.create(Constructor.prototype);
instance.constructor();
return instance;
}
var obj = doNew(SomeType);
What you have is a function than has no methods; it just has a prototype property with a list of functions, the new operator means to create a new object and use this function's prototype (Object.create) and constructor property as initializer.
Pros
Performant
Prototype chain will allow you to know if a object inherits from some type
Cons
Two-step inheritance
Inheritance
function SubType() {
// Step 1, exactly as Variation 1
// This inherits the non-function properties
SomeType.call(this);
this.otherValue = 'Hi';
}
// Step 2, this inherits the methods
SubType.prototype = Object.create(SomeType.prototype);
SubType.prototype.otherMethod = function() {
return this._convention + this.publ + this.otherValue;
};
var child = new SubType();
You may think it looks like a super-set of Variation 2... and you'll be right. It's like variation 2 but with a initializer function (the constructor);
child instanceof SubType and child instanceof SomeType will return both true
Curiosity: Under the hood instanceof operator does is
function isInstanceOf(obj, Type) {
return Type.prototype.isPrototypeOf(obj);
}
Variation 4 - Overwrite __proto__
When you do Object.create(obj) under the hood it does
function fakeCreate(obj) {
var child = {};
child.__proto__ = obj;
return child;
}
var child = fakeCreate(obj);
The __proto__ property modifies directly the object's hidden [Prototype] property. As this can break JavaScript behaviour, it's not standard. And the standard way is preferred (Object.create).
Pros
Quick and performant
Cons
Non-standard
Dangerous; you can't have a hashmap since the __proto__ key can change the object's prototype
Inheritance
var child = { __proto__: obj };
obj.isPrototypeOf(child); // true
Comment questions
1. var1: what happens in SomeType.call(this)? Is 'call' special function?
Oh, yes, functions are objects so they have methods, I will mention three: .call(), .apply() and .bind()
When you use .call() on a function, you can pass one extra argument, the context, the value of this inside the function, for example:
var obj = {
test: function(arg1, arg2) {
console.log(this);
console.log(arg1);
console.log(arg2);
}
};
// These two ways to invoke the function are equivalent
obj.test('hi', 'lol');
// If we call fn('hi', 'lol') it will receive "window" as "this" so we have to use call.
var fn = obj.test;
fn.call(obj, 'hi', 'lol');
So when we do SomeType.call(this) we are passing the object this to function SomeCall, as you remember this function will add methods to object this.
2. var3: With your "REALLY define properties" do you mean if I use them in functions? Is it a convention? Because getting this.newProperty without it being defined at the same level with other member functions is not a problem.
I mean any property your object will have that is not a function must be defined on the constructor, not on the prototype, otherwise you will face one of the more confusing JS problems. You can see it here, but it's outside of the focus of this question.
3. Var3: what happens if I don't re-assign constructor?
Actually you might not see the difference and this is what makes it a dangerous bug. Every function's prototype object has a constructor property so you can access the constructor from an instance.
function A() { }
// When you create a function automatically, JS does this:
// A.prototype = { constructor: A };
A.prototype.someMethod = function() {
console.log(this.constructor === A); // true
this.constructor.staticMethod();
return new this.constructor();
};
A.staticMethod = function() { };
It's not a best practice because not everybody knows about it, but sometimes it helps. But if you reassign the prototype...
A.prototype = {
someMethod = function() {
console.log(this.constructor === A); // false
console.log(this.constructor === Object); // true
this.constructor.staticMethod();
return new this.constructor();
}
};
A.prototype is a new object, a instance of Object than prototypes Object.prototype and Object.prototype.constructor is Object. Confusing, right? :P
So if you overwrite the prototype and don't reset the "constructor" property, it will refer to Object instead of A, and if you try to use the "constructor" property to access some static method you may get crazy.

I usually settle with returning an object with functions as properties:
var newCat = function (name) {
return {name: name, purr: function () {alert(name + ' purrs')}};
};
var myCat = newCat('Felix');
myCat.name; // 'Felix'
myCat.purr(); // alert fires
You can have inheritance by calling the newCat function and extend the object you get:
var newLion = function (name) {
var lion = newCat(name);
lion.roar = function () {
alert(name + ' roar loudly');
}
return lion;
}
If you want a global cats object:
var cats = (function () {
var newCat = function (name) {
return {
name: name,
purr: function () {
alert(name + ' is purring')
}
};
};
return {
newCat: newCat
};
}());
Now you can call:
var mySecondCat = cats.newCat('Alice');