Inject It

Inject it provides a simple and easy way to handle your Node.js application level dependencies.

Installation

npm install -S inject-it

Configuring the injector

The inject-it porpuse is to work like a simple flat map of your application dependencies, so, imagine that it's just a hash with extra convenience, where you define your dependencies by name. While the name of the dependencies are all strings, the values of dependencies can be whatever you want, instances, classes...

What we recommend is for you to have a file just for your injector configuration, so you can easly reuse it over your application, here is a sample one:

config/injector.js

var injector = require('inject-it')()

injector.value('injector', injector); // I found that having the injector itself as a dependency is very helpful

injector.value('User', require('../app/models/user'));
injector.value('SomeLib', require('../lib/some_lib'));

module.exports = injector;

Using the injector

Once you have your injector configured, it's time to use it, let's start with the simplest way of reading dependencies, that's by using the get method:

var injector = require('./config/injector');

function doSomething() {
  var User = injector.get('User');

  var user = new User();
  user.sayHello();
}

doSomething();

Simple enough, right? And a notice, if you try to get a dependency that is not defined, an error with be raised.

The previous version works, but there is a more convenient way, that is to use the function argument names in order to fetch the dependencies, see the next example:

var injector = require('./config/injector');

function doSomething(User) {
  var user = new User();
  user.sayHello();
}

// this will read 'User' from the argument name and inject it automatically
injector.call(doSomething);

The cool thing about this is that you can just list your dependencies on the parameters (without even caring about the order) and have them propertly injected at runtime.

If you need to construct an object (calling new instead of simple calling the function) you can use the construct method:

var injector = require('./config/injector');

function MyClass(User) {
  this.User = User;
}

MyClass.prototype.sayHello = function() {
  var user = new this.User();
  user.sayHello();
};

injector.construct(MyClass).sayHello();