Learn TypeScript 3 by Building Web Applications

Learning about injectors and the singleton design pattern

Injectors are elements that Angular uses to actually instantiate, retrieve, and inject dependencies where they are needed. Injectors instantiate dependencies once and store the created instances in a DI container. Each time a dependency is needed, the injector first checks whether it already has an instance of it in its container. If it does, then it returns that instance instead of creating a new one.

By creating a single instance of each injectable element, injectors apply the singleton design pattern. Singleton is a pattern that you should definitely be aware of.

If we simplify this, we can consider these containers as simple maps storing token-instance pairs. Each pair is composed ...