Test the existence of a dynamically chosen class

Question

I have data coming from an external source that I want to process. In order to do that, the objects I'm receiving are tagged with their original class name. Now I want to take that tag name and use it to populate a model in my own application. I'm stuck at the step where I check for that class having an equivalent in my codebase. Its going to look something like this:

this.objects.forEach((object) => {

if (typeof object.class_tag !== 'undefined') { //the problem line
//create class instance
}

});

In php I'd simply call class_exists to achieve this

<?php
if (class_exists($object->class_tag)) {}

What is the correct approach here?

How important is the type safety in your app? For example, does it *have* to be an instance of X class? Just thinking anonymous objects would be a lot easier here. — James, Jul 27 '18 at 09:24
@James there is a requirement to treat data differently on a per model basis. That would be difficult in plain objects — tracer tong, Jul 27 '18 at 09:35

score 1 · Answer 1 · answered Jul 27 '18 at 09:13

1

I don't see the clear way to do this in a just one line.

One of the possible approaches is the way you register your existing classes. For example if you use some kind of a namespace later on you can simply check the class for existance in the namespace. Sample code:

class A {}
const a = "A"
const namespace = { A };

if (namespace[a]) {
  // class exists, you can create object
  const instance = new namespace[a]();
}

Probably, much better approach would be to make some service, that will registerClass, checkClass and createInstance for you. So your logic is wrapped in one place.

answered Jul 27 '18 at 09:13

Lazyexpert

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For me, a service is the way to go here - there's no idiomatic way of doing this and I would say explicitness would be better to avoid any unexpected behaviour. – James Jul 27 '18 at 10:39
@James I would say, there is some architectural problem in the app, if TS has to do something like this. – Lazyexpert Jul 27 '18 at 11:43

score 0 · Accepted Answer · answered Jul 27 '18 at 09:38

I found a way of doing it

(credit to https://stackoverflow.com/a/34656123/746549)

let logger = require('../javascripts/serverlog');
let util = require('util');

let Extension = require('../models/object/Extension');

const classes = {Extension: Extension};

/**
 * Utility to emulate class exists / dynamic class naming
 * @param className
 * @returns {*}
 */
module.exports.dynamicClass = (className) => {

    logger.debug(classes);
    logger.debug(className);
    if (classes[className]) {
        return classes[className];
    } else {
        return false;
    }

};

Usage:

let ClassOrFalse = dynamicClass.dynamicClass(object._class_tag);

Test the existence of a dynamically chosen class

2 Answers2