bindr

Dependency injection is a software pattern that helps increase the testability of a given class by removing hard coded references to its dependencies.

In other words, take the following code...

function Car(make, model, color) {
    this.make = make;
    this.model = model;
    this.color = color;

    this.service = new Service();
}

In the preceding code, Car is now dependent upon Service. This makes it more difficult to unit test the Car in complete isolation. There's no way to create an instance of a Car without in turn creating an instance of a Service.

Dependency injection allows for a small bit of configuration that says, 'When a class needs a dependency named x, create an instance of y'.

So, you can now have code that would look like...

function Car(make, model, color, service) {
    this.make = make;
    this.model = model;
    this.color = color;
    this.service = service;

    bindr(this, arguments);
}

bindr.bind('service', Service);

You now accept your service as an argument in the constructor.

bindr comes in and allows you to switch a dependencies constructor out. In production code, the Car might be dependent upon Service, but when writing unit tests, bindr allows you to swap that dependency out for a FakeService.

var Car = require('Car'),
    bindr = require('bindr');

function FakeService() {}

bindr.bind('service', FakeService);

describe('when a car is created', function() {
    it('should be awesome', function() {
        var car = new Car();

        expect(car.service instanceof FakeService).to.be.ok();
    });
});

In the unit tests, you simply bind service to FakeService instead of Service and the dependency will automatically get passed in.

AMD

Another nice feature of bindr is its use in AMD projects with Require.js.

In a typical AMD module, you'd have code such as the following...

// car.js
define(['service'], function(Service) {
    function Car() {
        this.make = make;
        this.model = model;
        this.color = color;
        this.service = new Service();
    }

    return Car;
});

You can see that when you require car, you automtically end up requiring service as well. bindr allows you to remove that dependency from the car module and swap out what service the car uses.

// main.js
require.config({
    paths: {
        'bindr': '../../../dist/bindr'
    }
});

require(['bindr', 'car', 'service'], function(bindr, Car, Service) {
    bindr.bind('service', Service);

    var ford = new Car('Ford', 'Fusion', 'Maroon');

    document.getElementById('car').innerHTML = JSON.stringify(ford, null, 4);
});

// car.js
define(['bindr'], function (bindr) {
    function Car(make, model, color, service) {
        this.make = make;
        this.model = model;
        this.color = color;
        this.service = service;
        
        bindr(this, arguments);
    }

    return Car;
});

This way you can easily swap out the service dependency of the car.

Any feedback would be appreciated!