Angular Basics: Angular Constructor Overview

Learn about Angular constructors and how they can help with dependency injection, setting default property values and running other initialization code.

Angular is a framework that has many different parts. To properly use it, you need to know the basics of how AngularJS works. This blog aims to help frontend developers gain a good understanding and competency with the use of Angular constructors. In this article, we will learn what Angular constructors are, when can we use them and how—with examples.

What Is a Constructor?

An Angular constructor is a function that is used to initialize an Angular application. The constructor is run when the application is first created, and it is responsible for setting up the Angular environment. The constructor can be used to inject dependencies, set the default values for properties, and run any other initialization code that is needed.

For example:

class CoffeeSwallower {
  name: string;

  constructor(name = "SpewCoffeeGalore") {
    this.name = name;
  }
}

Here, as you can see, it’s automatically called when we created a new instance of our class

Types of the Constructor

Default Constructor

A default constructor is a constructor that is automatically generated by the compiler if one is not explicitly provided by the programmer. A default constructor typically initializes member variables to their default values (e.g., 0 for ints, null for objects) and does not perform any other actions.

A default constructor is necessary for a class if any of the following are true:

• The class has any non-static member variables that require initialization
• The class has any base classes (i.e., is derived from another class)
• The class has any virtual member functions

If a class does not have a default constructor and one of the above conditions is true, the compiler will generate an error.

Parameterized Constructor

Parameterized constructors accept one or more parameters. Parameters are simply variables that are passed into the constructor when it is invoked. The parameterized constructor can be used to set the initial values of the instance variables of an object.

For example, a parameterized constructor for a class called Employee might accept an employee ID and a name. The constructor would then use these parameters to set the corresponding instance variables.

Copy Constructors

There are two types of copy constructors: shallow and deep.

The shallow copy constructor simply copies the values of the data members of the object being initialized to the data members of the new object.

The deep copy constructor also copies any pointers that the object being initialized has, and also copies the objects that those pointers point to.

Conversion Constructors

There are two types of conversion constructors: implicit and explicit.

An implicit conversion constructor is invoked without the user specifying that they want to convert an object.

In an explicit conversion, the constructor is invoked only when the user specifically requests a conversion.

In general, it is best to avoid conversion constructors because they can lead to unexpected results. For example, if an implicit conversion constructor is invoked when the user is expecting an explicit conversion, the results may not be what the user expects.

Move Constructors

There are two types of move constructors: lvalue move constructors and rvalue move constructors. Lvalue move constructors take an lvalue reference as their parameter, while rvalue move constructors take an rvalue reference.

Lvalue move constructors typically copy the contents of the object they are called on, while rvalue moves constructors typically move the contents of the object they are called on.

Uses of the Angular Constructor

An Angular constructor is a powerful tool that can be used to create, modify and manipulate AngularJS applications. Here are some of the use full uses of the Angular constructor:

• Bootstrap the application by instantiating the root scope and compiling the application’s templates.
• Angular constructor is used to initialize the Angular environment and to create the root Angular object.
• Create custom directives to manage dependencies for testing and debugging Angular applications.
• Also create custom services, filters, injectors and more. Each of these uses has its specific purpose and can be very helpful in certain situations.
• An Angular constructor can be used to inject dependencies into Angular controllers and to access the Angular global scope.

How to Initialize Angular Constructors

There are several ways to initialize constructors in Angular.

Method 1: Using ng-init Directive

One method is to use the ng-init directive. This directive can be used to initialize values in scope.

For example, the following code will initialize the name and city variables in scope:

<div ng-init="name='John'; city='New York'">
  {{name}} lives in {{city}}.
</div>

Method 2: Using Constructor Function

Another method to initialize constructors is to use the constructor function. This function is automatically called when an Angular component is created. The constructor can be used to initialize values in the component’s scope.

For example, the following code will initialize the name and city variables in the component’s scope:

constructor(scope) {
   scope.name = 'John';
   scope.city = 'New York';
}

Method 3: Initialization in ng-controller Directive

Finally, constructors can be initialized in the ng-controller directive. This directive can be used to create a new scope for the controller. The new scope will contain any values initialized in the constructor.

For example, the following code will initialize the name and city variables in the new scope:

<div ng-controller="MyController">
  {{name}} lives in {{city}}.
</div>

MyController.$inject = ['$scope'];

function MyController($scope) {
   $scope.name = 'John';
   $scope.city = 'New York';
}

What’s the Difference Between NgOnInit and Constructors?

A lot of times developers get confused between ngoninit and constructors. There are two ways to start up a component in Angular: using the constructor or ngOnInit.

Here are some of the key differences between the two:

• The constructor is called before the component is initialized, while ngoninit is a lifecycle hook that is called after the component is initialized.
• The constructor is used for setting global configuration variables that need to be set before the app starts running. NgOnInit is used for anything that needs to be done after the component has been initialized, such as fetching data from an API.
• The constructor is only used for initializing the component, while ngoninit allows you to access the component’s properties and make any necessary changes.

Final Words

Initializing the constructor is a critical part of programming. By carefully defining how objects should be created, the programmer can ensure that they are always in a valid state. This makes it easier to write reliable and maintainable code.