JavaScript New Object by Variable

Question

I'm trying to implement polymorphism in Javascript/node.js. I'm trying to do something, but I'm not sure what it's called so I'm not even sure what I'm searching for.

I have a number of different ES6 Classes that are interchangeable and have the same interface. I'm trying to make a system that is extensible and pluginable, so the classes available won't be known at design time.

I want to have a variable which defines the name of the class that I want to create an instance of. So effectively I want to have the following:

class Foo { }
class Bar { }

var classToLoad = "Foo";

var myFoo = new classToLoad;   // I want this to be equivalent to new Foo;

var classToLoad = "Bar";
var myBar = new classToLoad;   // I want this to be equivalent to new Bar;

Can anyone tell me what the name for this is and how to properly do it?

You can put them in an object cls = { Foo: class Foo {}, Bar: class Bar {} } and do property access myFoo = new cls[classToLoad]() — elclanrs
– elclanrs, Commented May 22, 2018 at 21:31
Is it possible to do that if I don't know what classes may be available at design time? This is meant to be a pluginable system. — David Findlay
– David Findlay, Commented May 22, 2018 at 21:32
Plug them in by extending the main object. Then make sure that property exists before calling it — charlietfl
– charlietfl, Commented May 22, 2018 at 21:33
Imagine var x=1, y=2; var iNeed="y"; console.log(iNeed); you want it to print "2"? — Ivan Kuckir
– Ivan Kuckir, Commented May 22, 2018 at 21:35
It seems like the sensible thing to do would be to require plugins to register themselves before you can use them. The registration would associate a string to a class. This would also allow you to provide sensible errors if a user tried to use a non-existent plugin. — Mark
– Mark, Commented May 22, 2018 at 21:46

Estus Flask · Accepted Answer · 2018-05-23 04:49:09Z

3

There should be a container to identify classes by their names. Notice that there may be more than one class of same name, and function/class names are minified in client-side scripts.

class Foo { }
class Bar { }

const container = { Foo, Bar };

let classToLoad = "Foo";

let myFoo = new container[classToLoad]();

answered May 23, 2018 at 4:49

Estus Flask

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Arber · Accepted Answer · 2018-05-22 21:39:38Z

0

This could help:

var Foo = class Foo { }
var Bar = class Bar { }

var classToLoad = "Foo";

var myFoo = new window[classToLoad]();   // I want this to be equivalent to new Foo;

var classToLoad = "Bar";
var myBar = new window[classToLoad]();   // I want this to be equivalent to new Bar;

answered May 22, 2018 at 21:39

Arber

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10 Comments

Ivan Kuckir Over a year ago

You need to have Foo = class Foo { } without "var".

David Findlay Over a year ago

This may work, but I'm not sure it's really achieving what I'm trying to do, which is a create a node.js application to which users can install plugins.

Barmar Over a year ago

@IvanKuckir If this code is not inside a function, the variables will be global.

Barmar Over a year ago

@DavidFindlay Put the name-to-class mapping in an object, and provide a plug-in registration function that adds to the object.

a better oliver Over a year ago

@estus const, let and class don't add a property to the global object even when they are evaluated in global scope.

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