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Dependency Inversion Principle

High-level modules should not depend on low-level modules.

 In simpler terms, this principle is about depending on abstractions, both high and low level modules, rather than concrete implementations. This allows for greater flexibility in terms of changing the implementation of a certain feature without affecting the rest of the system.

 For example:

class Database {

 constructor() {

  this.data = [];

 }

 add(item) {

  this.data.push(item);

 }

 remove(item) {

  this.data.splice(this.data.indexOf(item), 1);

 }

 find(item) {

  return this.data.indexOf(item);

 }

}

class Users {

 constructor(db) {

  this.db = db;

 }

 add(user) {

  this.db.add(user);

 }

 remove(user) {

  this.db.remove(user);

 }

 find(user) {

  return this.db.find(user);

 }

}

 In this example, the Users class depends on a specific implementation of a database, the Database class. But if we want to change the database implementation to, for example, use a different database system, we would have to modify the Users class, which would violate the Dependency Inversion Principle. To fix this, we can create an abstraction for the database, and make both the Users and Database classes depend on it:

class Database {

 add(item) {

  this.data.push(item);

 }

 remove(item) {

  this.data.splice(this.data.indexOf(item), 1);

 }

 find(item) {

  return this.data.indexOf(item);

 }

}

class Users {

 constructor(db) {

  this.db = db;

 }

 add(user) {

  this.db.add(user);

 }

 remove(user) {

  this.db.remove(user);

 }

 find(user) {

  return this.db.find(user);

 }

}

class InMemoryDatabase extends Database {

 constructor() {

  super();

  this.data = [];

 }

}

class MongoDbDatabase extends Database {

 constructor() {

 super(); //connect to mongodb

 }

}

 In this way, the Users class no longer depends on a specific implementation of the database, but rather on an abstraction of it, the Database class. This allows us to change the database implementation without affecting the Users class. It also makes the code more flexible, as it can be easily extended to support different types of databases.