WEBVTT

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Now let's understand what is the dependency injection or the shorthand will be the DI.

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If you click on fundamentals and click on Custom provider, now you can see dependency injection is

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a an inversion of control techniques.

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Okay.

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This is the techniques we will delegate the in instance of dependency into the something called IOC

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container.

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Now let me summarize what this means.

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The DI or dependency injection.

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It will manage the object creation.

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What is mean?

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Now let's go back to the example right here.

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Now right here you can see, in order to.

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In order to create a person, we must create the car and pass it into the parameter.

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Right.

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It means we as a developer we will responsible for creating the object by hand, right?

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And with dependency injection, we will leave the managing object creation for dependency

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injection or the framework.

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Okay.

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We will leave that for framework and framework in this case will be the NestJS.

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So I will leave that for Nest.js.

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It's mean.

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Instead of create.

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The class sorry.

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The object instance by hand.

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We will leave that for Nest.js.

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And that's it for dependency injection.

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And now let me show you clearly the idea.

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Now, for example, I have a controller.

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You can see I have a controller class.

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And I have a service.

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Right.

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In here we will use a syntax constructor(private service)

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To show it right here.

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Right.

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And in order to use the service.

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In order to use the controller, we must initialize the service, for example.

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Controller.

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New controller.

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Here is not work, right?

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We will create a service.

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And we will pass the service into here.

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Right.

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It very similar with this problem right now.

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You can see, this is do something like here.

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And we must pass the.

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The object of the service object creation of the service into the parameter of the controller right

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here.

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And how we can pass that.

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We don't have any ability to pass that, right?

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And so that reason we will use a dependency injection.

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Right now it means we will remove everything right here.

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We don't need this anymore.

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We just need to run the controller and the dependency injection will do the rest for me.

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Okay, that's the idea.

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Hopefully this makes sense.

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And let me show you how we can use the dependency injection inside the NestJS.

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Okay.

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Click on the provider here.

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Now we will scroll down, in order to make it.

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Into the dependency injection we must use the decorator injectable from the NestJS.

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Common

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Common

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Now let me go to the test service

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Sorry.

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In injectable in the nestjs/common right here.

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And that's the first step.

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Now let's see it worked or not.

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If I refresh, I will run the project again.

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Good.

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Click

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Wait a bit

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And now it's still not working.

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The reason is we have an.

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We haven't completed this step work.

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This is just the first step.

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Okay And now let's see the second step.

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If I scroll down to here.

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Go into here.

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Now you can see inside the provider registration, we must pass the service into the provider of the modules.

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Now let me go to the app module and inside the provider right here, I will pass the test service.

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Now you can see this application successfully.

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Start.

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If I refresh it worked right.

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And let me refactor the repository, in the service over here.

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Instead of using this, I will comment this and I will here to private test repository.

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And if I remove.

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Okay now you will see it will crash application.

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And in the fix that issue, we used the similar techniques, firstly mark it @Injectable.

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Injectable

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And the secondly. register it inside a module, the provider of the module.

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Test repository.

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Now application if I refresh it working right.

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And now until now we already Understand what is the provider, right?

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The provider is just the register of the dependency injection.

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Okay.

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In order to achieve the dependency injection inside the.

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NestJS framework, we use the provider.

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Okay.

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And firsly, let me summarize.

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Firstly we must use the decorator of the service or the repository okay.

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With @Injectable.

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Right.

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Injectable.

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And secondly we may add it inside the provider.

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And only we complete two steps.

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We tell the NestJS managing

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The test service

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And test repo for me and as the controller, it will auto go to the dependency injection and look at

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the test service and the test service.

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It will be auto go to the test repo inside the dependency injection okay.

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And auto write it inside the controller.

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It's already done by the framework.

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We don't need to worry about this step.

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And that's the idea about providers.

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Hopefully this makes sense and I will see you in the next one.