Inject dependencies into ES2015 module

Question

Is it possible to inject dependencies into ES2015 modules like in other programming languages like C# or Java? If I import a module I create a hard dependency to it and can not change it later at runtime. For example I have following JavaScript code:

import Animal from './dog';

class Person {
  feedAnimal() {
    new Animal().feed();
  }
}

I am importing the dog module. But what if I want to change it to a cat? At the moment I have to modify line 1 by hand but in some situations I want it configurable from the outside so that under some conditions there should be a cat and under some other conditions it should be a cat. All that things that can be done with classical dependency injection.

I know there are some DI frameworks out there like Scatter, Electrolyte, Wire and so on but unfortunately most of them require some special syntax and are not made for ES2015 modules.

Sweven

How would you want dependency injection to work? You could do class Person { constructor(Animal) { this.Animal = Animal; } feedAnimal() { new this.Animal().feed() } }, but it sounds like you expect something more advanced. – Lyndel

I am not sure if require-inject supports that, but you might want to take a look. – Chadbourne

@Lyndel thank you but I want to change internal module dependencies like I would write an unit test. – Sweven

@Chadbourne thank you but the thing is that I do not require anything. Requiering is a CommonJS related thing. I use the shiny new ES2015 import feature. – Sweven

You could probably hijack the module loader one way or the other. Not that I recommend doing that. – Cubital

Answer 1

I moved over to SystemJS. With SystemJS you can make dynamic imports like System.import('foo').then(() => console.log('Loaded));

Another advantage is that System will be the new ECMAScript standard module loader system.

Sweven

Answer 2

You cannot dynamically define dependencies. See this question and its accepted answer:

Question: ES6 variable import name in node.js?

Answer: Not with the import statement. import and export are defined in such a way that they are statically analyzable, so they cannot depend on runtime information.

Signpost

Hmm ok. Too bad. Do you know any workaround or something like that for this? – Sweven

A module loader like SystemJS would allow what you want. – Signpost

I ask myself how do the all write unit tests if I can't mock any dependencies in my module to test? – Sweven

Mocking a dependency means replacing or altering the actual dependency without affecting the code that uses it, not altering the code that uses a dependency to use a different dependency. I hope that makes sense, please ask if you need clarification. – Signpost

I know that. Because of this I created the question above ;-) – Sweven

@sdgluck: I think OP is asking exactly for this kind of dependency-injection, from the outside, which does not need dynamic/variable import names. It just needs a mocking module name resolver. – Malonylurea

Answer 3

You can use inject-loader to achieve this if you are bundling with Webpack.

Hopefully this helps someone who stumbles upon this old post.

Mehalek