Functional Dependency Injection in JavaScript

README.md

Dependency Inversion Principle (DIP) is a design pattern popular in Object Oriented Programming but it's not alien to functional programming. Parameterizing dependencies is actually critical to maintaining functional purity--one of the most important aspects of FP.

While special dependency injection frameworks are sometimes used, a similar effect can be achieved using partial application.

index.js

// Import an arbitrary dependency to be used with a module
const someDependency = require('someDependency')

// Import our module that has a dependency
// We can apply the dependency immediately so we don't have to keep a reference to the factory.
const myMod = require('myModule')(someDependency)

//Use it
myMod.bar('do something')

myModule.js

// Rather than exporting the module directly you can export a function that takes your dependency and exports what you want.
// The dependency is available to functions defined this way via closure.
module.exports = (someDep) => ({
  foo: () => someDep.SPECIAL_CONSTANT + 2,
  bar: (str) => someDep.useTheThing(str)
})