Dependency injection in Javascript without objects

By David Fekke
January 13th, 2016

After programming in an object oriented way for 15 years, I am started to make the transition of programming in a more functional way with languages like Javascript and Swift. Javascript has always treated functions as first class citizens, and their is a movement in the Javascript community from developers such as Eric Elliot to never use the new keyword again.

I recently upgraded an express site from 3 to version 4. As part of that process I decided to refactor the code that called external services so they would be more testable and more loosely coupled. In languages such as Java and C# this can be achieved by using dependency injection.

In express it is actually very easy to inject functionality into a route. This can be done by either using middleware or injecting another function into a route.

var routes = require('./routes/index');

var exposeService = function(req, resp, next){
    req.service = require('./myservice');

app.use('/', exposeService, routes);

For one of the routes I needed to be able to inject two services that could be used by the route. In an earlier version of the combined service I created an object that had two properties that held references to other objects that had functions for returning the data I needed in my route. Here is how I intially wrote the service as an object.

"use strict";

// Creating function object
var Service = function Service(meetupdata, twitterdata) {
	this.Meetup = meetupdata;
	this.Twitter = twitterdata;

// Prototype function for getting the next meeting
Service.prototype.getNextMeetup = function getNextMeetup(cb) {

// Prototype function for getting the tweets
Service.prototype.getTweets = function getTweets(cb) {

// factory function for creating a new version of the object
function create(meetupdata, twitterdata) {
	return new Service(meetupdata, twitterdata); 

module.exports = create;

While this worked, it turns out there is a much simpler and more elegant way of creating this service.

"use strict";

function Service(meetupDataFN, twitterDataFN) {
	return {
        getNextMeetup: meetupDataFN,
        getTweets: twitterDataFN

module.exports = Service;

In the current version of my service I am returning an object with two functions. Not only is this approach cleaner, it is also more functional. I am also just passing in functions instead of whole objects.

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