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Angular2 interview questions

Angular2 interview questions

Dear friends, below are the list of Angular2 interview questions that have been designed for Angular2 programmers who are preparing interviews on Angular2 interviews. Here, we have added some basic and advanced or both questions on Angular2 that help you to crack interview on Angular2.

Below are the list of Top 20 Angular2 interview questions and answers.

  1. What is angular2?
  2. List some advantages of Angular2 over Angular1?
  3. What are the new features of Angular2?
  4. How do you define the transition between two states in Angular?
  5. How to declare a component in Angular2?
  6. What is the difference between observable and promises?
  7. List the differences between Angular2 components vs. directives
  8. What is ECMAScript?
  9. What is Traceur Compiler?
  10. List the modern browsers supported by Angular2?
  11. When to use Ngoninit and constructor in angular2?
  12. How to cache an observable data?
  13. List out the differences between ActivatedRoute and RouterState, with reference to angular2.
  14. What would you have in a shared module in Angular2?
  15. What do you mean by a structural directive in Angular2?
  16. What do you understand by a template variable? How is it used?
  17. Explain the concept of lazy loading in Angular2.
  18. What is the difference between constructor and ngOnlnit in Angular js?
  19. What is the meaning of component life cycle in Angular2?
  20. What is the use of ngForTrackBy directive?

Q1. What is Angular2?

Angular2 is a completely revived component-based framework in which an application is a tree of loosely coupled components. It is a more advanced version of angularJS. It is more of an "all in one" framework so it also helps in creating a single website without getting trapped into different JS frameworks. An Angular2 is a modular framework in which our code is divided into individual procedures that offer a similar kind of functionality, hence improving the testing, up gradation and maintenance of the application. It has a lot of useful features such as- server-side rendering, cross-platform, and supports more languages than any other framework. It is a new typescript framework built around the concept of components which is paving the way for a better and spacious development. We can even make hybrid applications using Angular2 which gives us a sharp edge by providing us the flexibility to use the same standard codes for developing other applications.

Q2. List some advantages of Angular2 over Angular1.

Angular2 is a re-written version of Angular1 and not an update. The best way to compare Angular2 and Angular1 is by finding out the new features in Angular2. This way we will be able to find out the advantages of Angular2 over Angular1 precisely. So, some of the advantages of Angular2 are:-

Angular2 is a mobile oriented framework Whereas Angular1 was not developed with mobile base in mind.
Angular2 is a versatile framework, i.e. we have more choices for languages. We can use ES5, ES6, Typescript or Dart to write an Angular2 code Whereas an Angular1 code can written by using only ES5, ES6 and Dart. We don't have many choices of language in Angular1.
Nowadays, the controllers are replaced by components and Angular2 is completely component based. Whereas Angular1 was based on controllers whose scope is now over.

Angular2 directly uses the valid HTML DOM element properties and events which reduces the usage of many available built-in directives.

Q3. What are the new features of Angular2?

Angular2 is a platform that encompasses a wide range of capabilities. Some new features were added in Angular2 which includes:

  • Universal server rendering- It is the library which is used to make building universal apps a smooth experience. It is an important feature of Angular2.
  • A mobile toolkit- It provides all the mobile toolkit and techniques to build high-performance mobile applications. The web applications which are developed using the mobile toolkit can be loaded on any device with or without internet connection which is a great advantage.
  • A command line interface-it can generate components, routes, services, and pipes with the help of commands.
  • Data binding- data binding has been improved in Angular2. So, whatever DOM element property you need to bind, you just wrap it in square brackets. E.g.-
    <img[src]='product.image' />
  • Modular- various modules have been removed from angular’s core, which has resulted in better performance.
  • Modern- Angular2 has been targeted as modern browsers in which various hacks that make angular harder to develop have been removed.

Q4. How do you define transition between two states in angular?

Transitions between two states take place so that we can build simple animations between two states driven by a model attribute. Transition basically means navigating from the current state to a new state. In angular, transition is an animation-specific function which is used in angular’s animation DSL language. Transition declares the sequence of animation steps that will be executed when entered value is satisfied. A function is provided an argument for a transition and it will be executed each time a state change occurs. In this, if the function is true, then the animation will run else it won’t get executed.

These animation transitions are placed within the animation triggers. The transition depends upon what the animation was in the previous state and what it will become in the next state. In other words, if a transition is defined that matches the old/current state criteria then the associated animation will be triggered.

Syntax: –
 function transition (stateChangeExpr: string, steps: AnimationMetadata | AnimationMetadata []):AnimationTransitionMetadata;

Q5. How to declare a component in Angular2?

Components in Angular2 are simply directives that are always associated with a direct template. Angular2 components have an extremely well defined life-cycle. When working with angular components, we can make use of interfaces which allows us to implement functionality for different times in a components lifecycle. A component must belong to an NgModule in order for it to be usable by another component or application. Components can even control their runtime behaviour by implementing various Life-cycle hooks.

Declaration of component:-
@component ({selector: 'great', template: 'hello {{name}}!'})

Class greet{

Name: string = 'world';


Components always have a template and only one component can be instantiated per an element in a template. When a component is instantiated, angular creates a change detector, which is responsible for propagating the component’s building.

Q6. What is the difference between observable and promises?

The differences between observable and promises are:-

  1. Observable is a more powerful way of handling http asynchronous requests. Whereas,A promise handles a single event when an asynchronous operation completes or fails.
  2. An observable is like a stream which allows passing zero or more events where the callback is called for each event. Whereas,A promise eventually calls the success or failed callback even when you don’t need the notification or the result it provides anymore.
  3. Observable works with multiple values for a particular time. Whereas,Promises works with and even returns a single value at a time.
  4. Observables can be cancelled. Whereas,Promises cannot be cancelled.
  5. Observable supports map, filter, reduce and similar operators. Whereas,Promises have more readable codes with try/catch and async/await.
  6. In observable, one operator ‘retry’ can be used to retry whenever needed. Whereas, Promises cannot be retried. A promise should have access to the original function that returned the promise in order to have a retry capability.

Q7. List the differences between Angular2 components vs. directives.

Apart from components, directives are also used in Angular2 which allows us to attach behavior to elements in DOM. There are certain differences between the components and directives in Angular2. They are: –

  1. In Angular2, a component is a directive with a view whereas a directive is a decorator with no view. Components are the specific type of directive that allows us to utilize web component functionality throughout our application. Whereas, Directive is the mechanism by which we attach behavior to elements.
  2. A component is used to break up the application into smaller components. Whereas, Directive is used to design the re-usable components.
  3. Components can be used to define pipes. Whereas, We cannot define pipes using directives.
  4. Components can be present per DOM element. Whereas, Directive is used to add behavior to an existing DOM element.

Q8. What is ECMAScript?

ECMAScript is a standard for scripting languages. It is a subset of javascript. Languages such as ActionScript, JavaScript use ECMAScript as its core. ECMA stands for European Computer Manufacturer’s Association. Coders commonly use ECMAScript for client-side scripting on the World Wide Web. It is also used for server applications and services. It includes structured, dynamic, functional, and prototype-based features. The ECMAScript was developed by Brendan Eich of Netscape. The ECMAScript is standardized by the ECMA international standards organization in the ECMA-262 and ECMA-402 specifications. It is a programming language which is designed specifically for acting on an existing entity or system. It provides the rules, details, and guidelines that a scripting language must observe to be considered ECMAScript compliant.

Q9. What is Traceur Compiler?

Traceur is a compiler which takes ECMAScript and compiles it down to regular javascript that runs in your browser. Traceur can be used in several ways like- typing or pasting the ES6 code into the read-eval-print-loop page, or by including traceur in the web page and compiling ES6 code content on the fly, or many other ways. Even traceur is written in ES6, compiled to ES5. The main goal of a traceur compiler is to inform the designs of javascript features and allows us to write the code in a better manner. Nowadays, traceur compilers are broadly used in Angular2 platform. It also supports transpilling and type checking via type annotations.

Q10. List the modern browsers supported by Angular2.

Angular supports most of the recent browsers some of which are-

  • Google Chrome
  • Firefox
  • Edge
  • IE for versions 9-11
  • Safari
  • iOS 7.1
  • Android 4.1
  • IE Mobile

Q11. When to use Ngoninit and constructor in angular2?

Constructors are used for initializing class members and also for dependency injection. Ngonlnit is used for the initialization work. Both of these methods are called when the component is created. It is really important that we should know, when to and how to use them. These are used for providing the best structure for your component’s code. A constructor method is a pre-defined method in the constructor class which is only called when the class is instantiated. It is also used for properly initializing the fields. Constructor in angular2 is used to create a new instance of the class. Ngonlnit is the class we import when we implement the constructor in order to use it in a class. The method used in this case is ngOnlnit(). This method helps in initializing the directive or the component after the data bound properties are displayed and the directive or components input are set.

Q12. How to cache an observable data in Angular2?

Caching of an observable data is done with the help of "observable.cache". We can use caching in order to cache the response in the memory and then, on the next subscription, instead of requesting the remote server again. This operator is used at the end of the string. Caching is important for the performance, especially on bandwidth restricted devices and slow networks. You should have a good understanding of caching while working with promises but while translating it to observable, it is a bit difficult. Therefore, when interacting with observables, we typically set up a subscription on the consumer side and react to values coming through the pipe. We can easily add caching to the observables by adding publishReplay(1) and refCount.

Q13.List out the differences between ActivatedRoute and RouterState, with reference to angular2.

Here are some of the differences between ActivatedRoute and RouterState with reference to angular2: –

  1. ActivatedRoute consists of the information about a route associated with a component loaded in an outlet. Whereas, RouterState represents the state in which the writer actually is.
  2. We need ActivatedRouteSnapchat to traverse all the activated routes. Whereas, during a navigation, after redirects have been applied, the router creates a RouterStateSnapshot.
  3. ActivatedRouteSnapshot has old data. When route changes, ActivateRouteSnapshot has data from previous route. Whereas, the RouterState cares about application components, or, to be more specific, about their arrangements.

Q14. What would you have in a shared module in angular2?

Shared module is used to import the services in both eager and lazy loaded module. We all know that lazy loaded modules create their own branch on the dependency injection tree. Shared module consists of the services that are registered by the angular in the root app injector. For this, we need not import it in the lazy module because lazy loaded modules already have access to the services defined at the root. Components, pipes and directives are also defined in the shared module. Other modules that import the shared module can use it in their templates. This means that the modules can be imported normally in the lazy loaded module. The shared module contains the code that will be used across the applications and featured modules. It also consists of the common template components. “Dumb components” should also be present in the shared module. It typically consists of some common angular modules too. When you are importing the shared module, you will also need to import the module with its providers, because there is no app module in the test.

Q15. What do you mean by a structural directive in angular2?

Structural directives are used to manipulate DOM in angular. Structural directives are responsible for HTML layout. By adding, removing, or manipulating LMNs in angular, they shape or reshape the structure of DOM. This structural directive is applied to a host element with the help of other directives. The directives then do whatever it is supposed to do with that host element and its descendants. Structural directives can be easily recognized. It can also delay the instantiation of a component or an element. It can also be used for cosmetic effect or manually handling the timing of the loading of components. Structural directives are bound to a template. The two most common structural directives are “ngIf” and “ngFor”. The process occurring in a structural directive is dynamic.

Q16. What do you understand by a template variable? How is it used?

A template in angular2 is used to instantiate embedded views. A template variable can be accessed in two ways. Either by placing a directive on an element and have the template variable for this embedded view injected into the constructor of the directive using the template variable token, or you can query for the template variable from a component or a directive via query. A template variable in angular2 is a reference to a DOM element or directive within a template. Template variables are used to access the values of DOM element properties. It is declared with the help of “#” and “ref-“as prefix. For example: – #myVar and ref-myVar. Template variable names cannot be made duplicate as in this way, it might give unpredictable values. The scope of a reference variable is the entire template. It can be used anywhere inside a template. In angular2, a component needs to have a view and to define a view, a template variable is used. It allows us to express data and property binding, event binding and template concerns.

Q17. Explain the concept of lazy loading in angular2.

Lazy loading is a module which is used to decrease the start-up time. When lazy is used, then our system application does not need to load everything at once. It only needs to load what the user expects to see when the application first loads. The modules which are lazily loaded will only be loaded when the user navigates to their routes. Lazy loading improves the performance of our system applications. It keeps the initial payload small and these smaller payloads lead to faster download speeds. It helps in lowering the resource cost especially on mobile networks. If a user doesn’t visit a section of the application, they won’t ever download those resources. The concept of lazy loading in angular requires us to format the application in a certain way. All the assets that are to be lazy loaded should be added to its own module. Lazy loading is setup in the main routing file. Lazy loading overcomes the problem of slow loading of applications in their own way which hence improves the loading time of the application.

Lazy loading can be done only in four steps: –

  1. Update your route file
  2. Install angular-router-loader and add the loader to your webpack configuration file.
  3. Define the lazy routes
  4. Import the routes to the module.

Q18. What is the difference between constructor and ngOnlnit in Angular js?

The comprehensive comparison that taps into components initialization process is given below:-

  1. ngonInit is just a method in a class which structurally is not different to any other method in a class. Whereas, a constructor is a completely different thing. It will be called when an instance of a class is created.
  2. A class constructor in angular is used to inject dependencies, which is called constructor injection pattern. Whereas, when ngOnInit is called, it has finished creating a component DOM, injected all required dependencies through constructor and processed input bindings.
  3. A constructor is a default method of the class that is executed when the class is instantiated. Whereas, ngOnInit is a life cycle hook called by angular2 to indicate that angular is done creating the component.
  4. ngOnInit relies on the binding of the component. Whereas, it is not the case when a constructor is used.

Q19. What is the meaning of component life cycle in angular2?

The component life cycle hooks overview the life cycle sequence and the interfaces. Angular manages the life cycle of a component. Angular creates it, renders it. It can also create and render its children. It also checks when its data bound properties change. It can even destroy it before removing it from the DOM. The life cycle hook offered by angular provides the visibility into these key life moments and the ability to act when they occur. The components go through an entire set of processes or life cycle right from its initiation to the end of the application. There are a number of life cycle hooks which are listed below: –

There are a number of life cycle hooks which are listed below: –

  1. ngOnChanges
  2. ngOnInit
  3. ngDoCheck
  4. ngAfterContentInit
  5. ngAfterContentChecked
  6. ngAfterViewInit
  7. ngAfterViewChecked
  8. ngOnDestroy

Q20. What is the use of ngForTrackBy directive?

For iterating over a collection in angular2, the ngFor directive is used which instantiates a template once per item from the collection. If a data needs to be changed at some point in the collection, then a problem occurs because angular cannot keep a track of items in the collection and has no knowledge of the items which were added or deleted. This results in the deletion of all the DOM elements that are associated with the data and are again created. If the collection is big, then it becomes more complicated because a lot of DOM manipulation occurs which are expensive. So, to solve this problem, a trackBy function is used which takes the index and the current item as arguments and returns the unique identifier for this item.

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