JavaScript Dependency Injection

Let's learn it doing a super simple real world example :)

The example class I am going to talk about here is a Printer which needs a driver to print something. I have demonstrated the advantages of dependency injection design pattern in 4 steps to arrive at the best solution in the end.

Case 1: no dependency injection used:

class Printer {
   constructor() {
      this.lcd = '';
   }

   /* umm! Not so flexible! */
   print(text) {
     this.lcd = 'printing...';
     console.log(`This printer prints ${text}!`);
   }
}
    
// Usage:
var printer = new Printer();
printer.print('hello');

Usage is simple, it is easy to make a new printer this way but this printer is not flexible.

Case 2: abstract the functionalities inside the print method into a new class called Driver:

class Printer {
  constructor() {
    this.lcd = '';
    this.driver = new Driver();
  }

  print(text) {
    this.lcd = 'printing...';
    this.driver.driverPrint(text);
  }
}

class Driver {
  driverPrint(text) {
    console.log(`I will print the ${text}`);
  }
}

// Usage:
var printer = new Printer();
printer.print('hello');

So our Printer class is now more modular, clean and easy to understand but It is not flexible yet again. Any time you use new keyword you are actually hard-coding something. In this case you are constructing a driver inside your Printer which in real world is an example of a printer that comes with a built-in driver that can never change!

Case 3: inject an already made driver into your printer

A better version is to inject a driver at the time we construct a printer meaning you can make any type of printer, color or black & white, because this time the driver is being made in isolation and outside the Printer class and then given (INJECTED!) into the Printer…

class Printer {
  constructor(driver) {
    this.lcd = '';
    this.driver = driver;
  }

  print(text) {
    this.lcd = 'printing...';
    this.driver.driverPrint(text);
  }
}

class BWDriver {
  driverPrint(text) {
    console.log(`I will print the ${text} in Black and White.`);
  }
}

class ColorDriver {
  driverPrint(text) {
    console.log(`I will print the ${text} in color.`);
  }
}

// Usage:
var bwDriver = new BWDriver();
var printer = new Printer(bwDriver);
printer.print('hello'); // I will print the hello in Black and White.

Usage is now different, as a user, in order to have a printer you need to first construct (make) a driver (of your choice!) and then pass this driver to your printer. It may seem that end user now needs to know a bit more about the system, however this structure gives them more flexibility. Users can pass ANY driver as long as valid! for example let's say we have a BWDriver (black & white) type of driver; user can create a new driver of this type and use that to make a new printer that prints black and white.

So far so good! But what you think we can do better and what you think has still some room to address here?! I am sure you can see it too!

We are creating a new printer each time we need our printer to print with a different driver! That is because we are passing our driver of choice to the Printer class at the construction time; if user wants to use another driver they need to create a new Printer with that driver. For example, if now I want to do a color print I need to do:

var cDriver = new ColorDriver();
var printer = new Printer(cDriver); // Yes! This line here is the problem!
printer.print('hello'); // I will print the hello in color.

Case 4: provide a setter function to set the driver of your printer at ANY TIME!

class Printer {
  constructor() {
    this.lcd = '';
  }

  setDriver(driver) {
    this.driver = driver;
  }

  print(text) {
    this.lcd = 'printing...';
    this.driver.driverPrint(text);
  }
}

class BWDriver {
  driverPrint(text) {
    console.log(`I will print the ${text} in Black and White.`);
  }
}

class ColorDriver {
  driverPrint(text) {
    console.log(`I will print the ${text} in color.`);
  }
}

// Usage:
var bwDriver = new BWDriver();
var cDriver = new ColorDriver();
var printer = new Printer(); // I am happy to see this line only ONCE!

printer.setDriver(bwDriver);
printer.print('hello'); // I will print the hello in Black and White.

printer.setDriver(cDriver);
printer.print('hello'); // I will print the hello in color.

Dependency Injection is not a really difficult concept to understand. The term may be a bit overloaded but once you have realised its purpose you will find yourself using it most of the time.

var Injector = {
   dependencies: {},
   add : function(qualifier, obj){
      this.dependencies[qualifier] = obj; 
   },
   get : function(func){
      var obj = new func;
      var dependencies = this.resolveDependencies(func);
      func.apply(obj, dependencies);
      return obj;
   },
   resolveDependencies : function(func) {
      var args = this.getArguments(func);
      var dependencies = [];
      for ( var i = 0; i < args.length; i++) {
         dependencies.push(this.dependencies[args[i]]);
      }
      return dependencies;
   },
   getArguments : function(func) {
      //This regex is from require.js
      var FN_ARGS = /^function\s*[^\(]*\(\s*([^\)]*)\)/m;
      var args = func.toString().match(FN_ARGS)[1].split(',');
      return args;
   }
};

The first thing we need a configuration to provide necessary dependencies with qualifiers. To do that, we define a dependency set as dependencies in the Injector class. We use dependency set as our container which will take care of our object instances mapped to qualifiers. In order to add new instance with a qualifier to dependency set, we define an add method. Following that, we define get method to retrieve our instance. In this method, we first find the arguments array and then map those arguments to dependencies. After that, we just construct the object with our dependencies and return it. For more information and examples, please see the post on my blog.