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Dependency Inversion Principle (DIP)

What is this?​

  • This principle say:

Depend on abstractions, not implementations

Why?​

  • We need to create specific interfaces instead a generic interface
  • With specific interfaces, the classes do not require useless information

Examples (JS)​

Bad example​

class InventoryRequester {
  constructor() {
    this.REQ_METHODS = ["HTTP"];
  }

  requestItem(item) {
    // ...
  }
}

class InventoryTracker {
  constructor(items) {
    this.items = items;

    // We have created a dependency on a specific request implementation.
    this.requester = new InventoryRequester();
  }

  requestItems() {
    this.items.forEach(item => {
      this.requester.requestItem(item);
    });
  }
}

const inventoryTracker = new InventoryTracker(["apples", "bananas"]);
inventoryTracker.requestItems();

Good example​

class InventoryTracker {
  constructor(items, requester) {
    this.items = items;
    this.requester = requester;
  }

  requestItems() {
    this.items.forEach(item => {
      this.requester.requestItem(item);
    });
  }
}

class InventoryRequesterV1 {
  constructor() {
    this.REQ_METHODS = ["HTTP"];
  }

  requestItem(item) {
    // ...
  }
}

class InventoryRequesterV2 {
  constructor() {
    this.REQ_METHODS = ["WS"];
  }

  requestItem(item) {
    // ...
  }
}

/* Constructing our dependencies externally and injecting them, we can easily substitute our request module for another */
const inventoryTracker = new InventoryTracker(
  ["apples", "bananas"],
  new InventoryRequesterV2()
);
inventoryTracker.requestItems();