Create class declaration programmatically in javascript?

Question

Eventually what I need is to build a class declaration at runtime providing dynamic parameters to a property annotation:

import {Type} from "external-module";

export default class TypeWrapper {

    @Type(() => '{this part of the class declaration should be changable at runtime}')
    data

}

I have a feeling that this should be possible to achieve but can't figure out a proper way yet.

As a proof of concept I was trying to do something like below:

let MyClass = eval('(class MyClass{})')
let myClass = new MyClass()

That works, however MyClass needs to define some imports:

        let MyClass = eval('import {Type} from "external-module"' +
            '(class MyClass{})')

That one fails with "Cannot use import statement outside a module" which is quite expected.

Another approach I tried is to load a module from string:

        var moduleData = '' +
            'import module from "./module/path/file.js"\n' +
            '\n' +
            'export default class MyClass {\n' +
            '}\n' +
            '\n';
        var b64moduleData = "data:text/javascript;base64," + btoa(moduleData);


        let MyClass = await import(b64moduleData)

But it fails with "Cannot find module", suggesting it assumes b64moduleData is a path rather than module data itself.

Anyone has any suggestions?

"let MyClass = eval('class MyClass{}') let myClass = new MyClass": that works? — Andy
– Andy, Commented Oct 31, 2021 at 12:16
better not use eval but return a class from a function like shown below. — The Fool
– The Fool, Commented Oct 31, 2021 at 12:21

hayavuk · Accepted Answer · 2021-10-31 12:20:00Z

2

Normally, for this type of thing I would use a class factory:

import { Type } from 'some-module';

function createClass(parameters) {
  return class {
    ....
  }
}

const MyClass = createClass(...);

I'm unsure if this fits your particular use case based on the details provided.

EDIT: As a side-note, as far as I know, you cannot construct modules from a string, which is what your code is doing, and what the compiler is complaining about.

answered Oct 31, 2021 at 12:20

hayavuk

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1 Comment

vir us Over a year ago

Perfect, that's exactly what I needed! Didn't know that syntax, thanks man!

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