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Discussion on: Dependency Injection in JavaScript

binaryforgeltd profile image
Bart Karalus • Edited

To put my two penny worth into this, I think one underrated benefit from this approach is that the code becomes more testable out of the box. If you think about component mocking, it is suddenly super easy to mock an object and inject it like nothing happened. :) But that is in general, not only JavaScript related.

jillesvangurp profile image
Jilles van Gurp

This is in fact the reason a lot of Javascript code is notoriously hard to test. You basically need a complete browser to test even the most trivial stuff. Testability and good design go hand in hand. Anything that can be injected, can trivially be mocked. Once everything not core to the unit you are testing is injectable, the thing is highly testable.

The approach above is what is known as DYI dependency injection in the Java world. It's totally valid as DI is just a design pattern. But not very common because it does result in a lot of boiler plate code and it's a bit of a PITA to manually wire things up (not to mention error prone).

In the Java world, frameworks such as Spring provide a lot of DI infrastructure that has evolved to require less and less boiler plate. Combined with modern languages such as Kotlin, there is now a lot less need for stating the obvious than ever before.

Simply having a class with a constructor that takes arguments and a single annotation tells Spring "This is a bean, please construct an instance and pass it to places where it is needed as a dependency. It needs other beans to work as well. The names of those beans are the constructor arguments". If you then define those other beans in the same way, Spring takes care of all the plumbing, new calls to constructors of all the beans it has identified, configuration injection (with different profiles, default values, override mechanisms, etc), bean graph validation (no cycles, everything needs to resolve, etc) and then fires the whole thing up.

With stuff like Spring Boot, there is almost no code involved at all and you get a lot of things self initializing using sane defaults simply because the library is on the class path. I've coached a few frontend engineers dealing with some Spring Boot code. I wouldn't go as far as to say that they like it but they were all able to get productive pretty quickly.

Something like Spring is missing in the Javascript world and given the sorry state a lot of js projects end up in, it is actually needed. I see why code such as above is necessary. It's clearly better than the traditional spaghetti code approach (which, lets face it, is the common alternative). But it still looks like a lot of boiler plate to me to write, maintaint, test, and debug. And you don't even get typesafety with that. This just begs some decent automation and a bit more systematic approach to enforcing sane architecture. You get bits and pieces of that if you use react or similar frameworks but it requires a lot of discipline and expertise to stay on top of things.