§ Dependency injection

Dependency injection is a technique in software development that allows you to decouple the implementation of a module from its dependencies. This means that a module does not need to know how to create its own dependencies, but instead they are passed to it when it is created or called. This allows for greater flexibility and reusability of code, as well as making it easier to test.

Here is an example of dependency injection in JavaScript:

class User {
  constructor(db) {
    this.db = db;
  }

  getUsername(id) {
    return this.db.query(`SELECT username FROM users WHERE id = ${id}`);
  }
}

const mySql = new MySql();
const user = new User(mySql);
console.log(user.getUsername(1));

In this example, the User class has a dependency on a db object. Instead of creating the db object inside the User class, it is passed to it when it is created. This allows the User class to be reused with different implementations of the db object, such as a different database or a mock object for testing.