AngularJS Interview Questions and Answers

Contents

Angular Interview questions and answers

What is AngularJS ?

Explain Directives in Angular?

What are controllers and need of ng-controller and ng-model in Angular?

What are expressions in Angular?

How can we initialize Angular application data?

Explain $scope in Angular?

What is “$rootScope” and how is it related with “$scope”?

Explain the concept of digest cycle, watchers and dirty checking?

What can be the performance implications of watchers and digest cycle ?

How can we measureno: of watchers & time spent on digest cycle?

How can we decrease digest cycle time ?

Can we force the digest cycle to run manually?

Do I need Jquery for Angular?

How is the data binding in Angular ?

Explain compile and link phase?

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How do we make HTTP get and post calls in Angular?

How do we pass data using HTTP POST in Angular ?

What is dependency injection and how does it work in Angular?

How does DI benefit in Angular?

What are services in Angular?

Are Service object instances global or local?

What is a Factory in Angular?

What is the difference between Factory and Service?

How are validations implemented in Angular?

How to check error validation for a specific field?

What does SPA (Single page application) mean?

How can we implement SPA with Angular?

How to implement routing in Angular?

How to implement SPA using angular-UI route?

Can we load HTML content rather than a full page ?

How can we create controllers and pass parameters in Angular UI route?

How to implement nested views using Angular UI route?

How can we create a custom directive in Angular?

What kind of naming conventions is used for custom directives?

What are the different custom directive types in AngularJS?

What if I want custom directives to be applied on element as well as attributes ?

Can I set an Angular directive template to a HTML web page?

Explain $q service, deferred and promises?

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Angular Interview questions and answers

AngularJS is one of those hot topics which interviewer’s ask for Web programming. In this article we will run through some important Interview questions around AngularJS and how we should be go about answering the same.

In case you are new to Angular you can Learn AngularJS in 8 hours 

What is AngularJS ?

“AngularJS is a JavaScript framework which simplifies binding JavaScript objects with HTML UI elements.”

Let us try to understand the above definition with simple sample code.

Below is a simple “Customer” function with “CustomerName” property. We have also created an object called as “Cust” which is of “Customer” class type.

function Customer() 
{
this.CustomerName = "AngularInterview";
}
var Cust = new Customer();

Now let us say the above customer object we want to bind to a HTML text box called as “TxtCustomerName”. In other words when we change something in the HTML text box the customer object should get updated and when something is changed internally in the customer object the UI should get updated.

"

TxtCustomerName" onchange="UitoObject()"/>

So in order to achieve this communication between UI to object developers end up writing functions as shown below. “UitoObject” function takes data from UI and sets it to the object while the other function “ObjecttoUI” takes data from the object and sets it to UI.

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function UitoObject() 
{
Cust.CustomerName = $("#TxtCustomerName").val();
}
function ObjecttoUi() 
{
$("#TxtCustomerName").val(Cust.CustomerName);
}

So if we analyze the above code visually it looks something as shown below. Your both functions are nothing but binding code logic which transfers data from UI to object and vice versa.

Now the same above code can be written in Angular as shown below. The javascript class is attached to a HTML parent div tag using “ng-controller” directive and the properties are binded directly to the text box using “ng-model” declarative.

So now whatever you type in the textbox updates the “Customer” object and when the “Customer” object gets updated it also updates the UI.

<div ng-controller="Customer">
<input type=text id="txtCustomerName"  ng-model="CustomerName"/>
</div>

In short if you now analyze the above code visually you end up with something as shown in the below figure.You have the VIEW which is in HTML, your MODEL objects which are javascript functions and the binding code in Angular.

Now that binding code have different vocabularies.

• Some developers called it “ViewModel” because it connects the “Model” and the “View” .
• Some call it “Presenter” because this logic is nothing but presentation logic.
• Some term it has “Controller” because it controls how the view and the model will communicate.

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To avoid this vocabulary confusion Angular team has termed this code as “Whatever”. It’s that “Whatever” code which binds the UI and the Model. That’s why you will hear lot of developers saying Angular implements “MVW” architecture.

Explain Directives in Angular?

Directives are attributes decorated on the HTML elements. All directives start with the word “ng”. As the name says directive it directs Angular what to do.

For example below is a simple “ng-model” directive which tells angular that the HTML textbox “txtCustomerName” has to be binded with the “CustomerName” property.

"

txtCustomerName" ng-model="CustomerName"/>

Some of the most commonly used directives are ng-app,ng-controller and ng-repeat.

What are controllers and need of ng-controller and ng-model in Angular?

“Controllers” are simple javascript function which provides data and logic to HTML UI. As the name says controller they control how data flows from the server to HTML UI.

For example below is simple “Customer” controller which provides data via “CustomerName” and “CustomerCode” property and Add/ Update logic to save the data to database.

Note: - Do not worry too much about the $scope , we will discuss the same in the next question.

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function Customer($scope)
{
        $scope.CustomerName = "Shiv";
        $scope.CustomerCode = "1001";
        $scope.Add = function () {
        }
        $scope.Update = function () {
        }
}

“ng-controller” is a directive.Controllers are attached to the HTML UI by using the “ng-controller” directive tag and the properties of the controller are attached by using “ng-model” directive. For example below is a simple HTML UI which is attached to the “Customer” controller via the “ng-controller” directive and the properties are binded using “ng-model” directive.

<div ng-controller="Customer">
<input type=text id="CustomerName"  ng-model="CustomerName"/><br />
<input type=text id="CustomerCode"  ng-model="CustomerCode"/>
</div>

What are expressions in Angular?

Angular expressionsare unit of code which resolves to value. This code is written inside curly braces “{“.

Below are some examples of angular expressions:-

The below expression adds two constant values.

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{{1+1}}

The below expression multiplies quantity and cost to get the total value.

The value total cost is {{ quantity * cost }}

The below expression displays a controller scoped variable.

<div ng-controller="CustomerVM">
The value of Customer code is {{CustomerCode}} 
</div>

The value of Customer code is {{CustomerCode}}

How can we initialize Angular application data?

We can use “ng-init” directive to achieve the same. You can see in the below example we have used “ng-init” directive to initialize the “pi” value.

<body ng-app="myApp" ng-init="pi=3.14">
The value of pi is {{pi}}
</body>

Explain $scope in Angular?

“$scope” is an object instance of a controller. “$scope” object instance get’s created when “ng-controller” directive is encountered.

For example in the below code snippet we have two controllers “Function1” and “Function2”. In both the controllers we have a “ControllerName” variable.

function Function1($scope)
{
$scope.ControllerName = "Function1";        
}
function Function2($scope)
{
$scope.ControllerName = "Function2";
}

Now to attach the above controllers to HTML UI we need to use “ng-controller” directive. For instance you can see in the below code snippet how “ng-controller” directive attaches “function1” with “div1” tag and “function2” with “div2” tag.

<div id="div1" ng-controller="Function1">
Instance of {{ControllerName}} created 
</div>
<div id="div2" ng-controller="Function2">
Instance of {{ControllerName}} created 
</div>

So this is what happens internally. Once the HTML DOM is created Angular parser starts running on the DOM and following are the sequence of events:-

• The parser first finds “ng-controller” directive which is pointing to “Function1”. He creates a new instance of “$scope” object and connects to the “div1” UI.
• The parser then starts moving ahead and encounters one more “ng-controller” directive which is pointing to “Function2”. He creates a new instance of “$scope” object and connects to the “div2” UI.

Now once the instances are created, below is a graphical representation of the same. So the “DIV1” HTML UI is binded with “function1” $scope instance and the “DIV2” HTML UI is binded with “function2” $scope instance. In other words now anything changes in the $scope object the UI will be updated and any change in the UI will update the respective $scope object.

What is “$rootScope” and how is it related with “$scope”?

“$rootScope” is a parent object of all “$scope” angular objects created in a web page.

Let us understand how Angular does the same internally. Below is a simple Angular code which has multiple “DIV” tags and every tag is attached to a controller. So let us understand step by step how angular will parse this and how the “$rootScope” and “$scope” hierarchy is created.

The Browser first loads the above HTML page and creates a DOM (Document object model) and Angular runs over the DOM.Below are the steps how Angular creates the rootscope and scope objects.

• Step 1:- Angular parser first encounters the “ng-app” directive and creates a “$rootScope” object in memory.
• Step 2:- Angular parser moves ahead and finds the expression {{SomeValue}}. It creates a variable
• Step 3:- Parser then finds the first “DIV” tag with “ng-controller” directive which is pointing to “Function1” controller. Looking at the “ng-controller” directive it creates a “$scope” object instance for “Function1” controller. This object it then attaches to “$rootScope” object.
• Step 4:- Step 3 is then repeated by the parser every time it finds a “ng-controller” directive tag. Step 5 and Step 6 is the repetition of Step 3.

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If you want to test the above fundamentals you can run the below sample Angular code. In the below sample code we have created controllers “Function1” and “Function2”. We have two counter variables one at the root scope level and other at the local controller level.