JavaScript Dependency Injection

Question

I am new at JavaScript. I wonder how dependency injection is being implemented in JavaScript? I searched the internet but couldn\'t find anything.

Answer 1

For me yusufaytas answer was exactly what I needed! The only missing features were:

1. Getting a dependency with custom parameters.
2. Registering dependencies using callbacks.

I wanted to have the ability to do something like this:

Injector.register('someDependency', function () {
        return new ConcreteDependency();
});

function SomeViewModel(userId, someDependency) {
    this.userId = userId;
    this.someDependency = someDependency;
}

var myVm = Injector.get(SomeViewModel, { "userId": "1234" });

So I ended up with the following code:

var Injector = {

    factories = {},        
    singletons = {},

    register: function (key, factory) {
        this.factories[key] = factory;
    },

    registerSingle: function (key, instance) {
        this.singletons[key] = instance;
    },

    get: function (CTor, params) {            

        var dependencies = this.resolveDependencies(CTor, params);

        // a workaround to allow calling a constructor through .apply
        // see https://stackoverflow.com/questions/1606797/use-of-apply-with-new-operator-is-this-possible
        function MiddlemanCTor() {
            CTor.apply(this, dependencies);
        }

        MiddlemanCTor.prototype = CTor.prototype;

        return new MiddlemanCTor();
    },

    resolveDependencies: function(CTor, params) {
        params = params || {};
        var args = this.getArguments(CTor);

        var dependencies = [];
        for (var i = 0; i < args.length; i++) {
            var paramName = args[i];
            var factory = this.factories[paramName];

            // resolve dependency using:
            // 1. parameters supplied by caller
            // 2. registered factories
            // 3. registered singletons
            var dependency = params[paramName] ||
                (typeof factory === "function" ? factory() : undefined) ||
                this.singletons[paramName];

            dependencies.push(dependency);
        }
        return dependencies;
    }

    getArguments: func(func) {
        // Regex from require.js
        var FN_ARGS = /^function\s*[^\(]*\(\s*([^\)]*)\)/m;
        var args = func.toString().match(FN_ARGS)[1].split(',').map(function (str) {
            return str.trim();
        });
        return args;
    }
};

Update - 21.5.2018

I've been using this solution for a few years now. As I moved my code base to TypeScript the solution evolved with it to support both TypeScript and JavaScript. After quite a while that the code was running in production I recently (two days ago) published a library based on this solution. Feel free to check it out, open issues, etc.

peppermint-di