Parameter decorators Used for parameters inside class constructors, e.g. @Inject, Optional
import{Component,Inject}from'@angular/core';
import{MyService}from'./my-service';
@Component({
selector: 'my-component',
template: '<div>Parameter decorator</div>'
})
exportclassMyComponent{
constructor(@Inject(MyService)myService){
console.log(myService);
// MyService
}
}
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Ques 17. What is angular CLI?
Angular CLI(Command Line Interface) is a command line interface to scaffold and build angular apps using nodejs style (commonJs) modules. You need to install using below npm command,
npm install @angular/cli@latest
Below are the list of few commands, which will come handy while creating angular projects
Creating New Project: ng new
Generating Components, Directives & Services: ng generate/g The different types of commands would be,
ng generate class my-new-class: add a class to your application
ng generate component my-new-component: add a component to your application
ng generate directive my-new-directive: add a directive to your application
ng generate enum my-new-enum: add an enum to your application
ng generate module my-new-module: add a module to your application
ng generate pipe my-new-pipe: add a pipe to your application
ng generate service my-new-service: add a service to your application
Running the Project: ng serve
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Ques 18. What is the difference between constructor and ngOnInit in Angular?
The Constructor is a default method of the class that is executed when the class is instantiated and ensures proper initialisation of fields in the class and its subclasses. Angular, or better Dependency Injector (DI), analyses the constructor parameters and when it creates a new instance by calling new MyClass() it tries to find providers that match the types of the constructor parameters, resolves them and passes them to the constructor. ngOnInit is a life cycle hook called by Angular to indicate that Angular is done creating the component. Mostly we use ngOnInit for all the initialization/declaration and avoid stuff to work in the constructor. The constructor should only be used to initialize class members but shouldn't do actual "work". So you should use constructor() to setup Dependency Injection and not much else. ngOnInit() is better place to "start" - it's where/when components' bindings are resolved.
exportclassAppimplementsOnInit{
constructor(privatemyService: MyService){
//called first time before the ngOnInit()
}
ngOnInit(){
//called after the constructor and called after the first ngOnChanges()//e.g. http call...
}
}
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Ques 19. What is a service?
A service is used when a common functionality needs to be provided to various modules. Services allow for greater separation of concerns for your application and better modularity by allowing you to extract common functionality out of components.
Let's create a repoService which can be used across components,
import{Injectable}from'@angular/core';
import{Http}from'@angular/http';
@Injectable({// The Injectable decorator is required for dependency injection to work
// providedIn option registers the service with a specific NgModule
providedIn: 'root',// This declares the service with the root app (AppModule)
The above service uses Http service as a dependency.
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Ques 20. What is dependency injection in Angular?
Dependency injection (DI), is an important application design pattern in which a class asks for dependencies from external sources rather than creating them itself. Angular comes with its own dependency injection framework for resolving dependencies( services or objects that a class needs to perform its function).So you can have your services depend on other services throughout your application.
