How to dynamically select module with modern nodeJS?

Question

I am using node --experimental-modules test.mjs (NodeJs v11.9.0).

There are many options to implement the same class, and I need to swith by terminal,

switch (process.argv[2]) {
  case 'a':
    import MyClass from './MyClass-v1.mjs'; break;
  case 'b':
    import MyClass from './MyClass-v2.mjs'; break;
  default:
    import MyClass from './MyClass-v3.mjs';
}

ERROR on import MyClass: Unexpected identifier

PS: working fine when using isolated import MyClass from './MyClass-v3.mjs';.

NOTE: const MyClass = require('./MyClass-v3.mjs') not works with modern Javascript. (ReferenceError: require is not defined). The file have only a class definition,

export default class MyClass { ... }

PS: there are nothing like C_preprocessor for NodeJs? An old pre-compiler (with some argv access) will be fine here.

Hi. Probably you need to use Factory pattern en.wikipedia.org/wiki/Factory_method_pattern — Vlad Kosko
– Vlad Kosko, Commented Mar 3, 2019 at 11:35
Hi @VladKosko, make sense... What the best way to implement it with Node? No easy way to dynamic loading? My code is also a kind of proxy pattern. — Peter Krauss
– Peter Krauss, Commented Mar 3, 2019 at 11:39
The import statement is only allowed at the top level of modules. To import anywhere else use require. — Dan D.
– Dan D., Commented Mar 3, 2019 at 13:48
You will need a dynamic import for this, you cannot put an import declaration inside a switch case. Not sure though whether node already supports import(). — Bergi
– Bergi, Commented Mar 3, 2019 at 14:52
@PeterKrauss You cannot do synchronous dynamic imports with ES6 modules. Given that this code seems to be located in your main.js, doing it asynchronously should not be a problem. — Bergi
– Bergi, Commented Mar 3, 2019 at 16:12

Markus · Accepted Answer · 2019-03-03 16:12:26Z

2

import ClassV1 from './MyClass-v1.mjs';
import ClassV2 from './MyClass-v2.mjs'; 
import ClassV3 from './MyClass-v3.mjs';

const classes = {
  a: ClassV1,
  b: ClassV2
}

const Class = classes[process.argv[2]] || ClassV2;

You could also write an index file with all the classes and do an import * as classes from 'classes.mjs';.

answered Mar 3, 2019 at 16:12

Markus

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2 Comments

Peter Krauss Over a year ago

Hi, is a valid solution because it works (!), but it is in the opposite of the "import only necessary" philosophy... An old pre-compiler solution will be better.

Markus Over a year ago

You can not do a precompiler solution if you don't know at compile time which class you want. So all modules have to be available at runtime. But if you want a precompiler, this is probably the closest you get: webpack.js.org/contribute/writing-a-loader Use can use codeshift for the transformation: github.com/facebook/jscodeshift

Peter Krauss · Accepted Answer · 2019-03-05 16:59:45Z

Modern Javascript (ES6+) implementations are under construction, some syntax are working with NodeJS (node --experimental-modules) and good browsers, other not.

This is the only way to do without cost of "load all" (see @Markus solution), but with the cost of non-global import, needs an ugly block to define scope and assyncronous operation... First step is to understand the basic impor block syntax:

import('./MyClass-v3.mjs').then(({default: MyClass}) => {
    // can use MyClass only here in this block
    let x = new MyClass();
    console.log("DEBUG=",x);
});

Them, putting all together, the argv selection and the optional imports.

Solution

const classes = {
  a: './MyClass-v1.mjs'
  ,b: './MyClass-v3.mjs'
  ,c: './MyClass-v3.mjs'
};
import(classes[process.argv[2]] || classes.c ).then(({default: MyClass}) => {
    let x = new MyClass();
    console.log("DEBUG=",x);
});

PS: some of the magic (no rational) is to use ({default: MyClass}) instead (MyClass).

Thanks

To @Bergi (!), for explain this solution (see question comments).

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