Best Practices for code in Constructor vs. ngOnInit in Angular
When working with Angular, understanding when to use the constructor and when to leverage the ngOnInit lifecycle hook is crucial. These two mechanisms serve different purposes and should be used appropriately for efficient and maintainable code.
The Constructor
The constructor is a fundamental method in a TypeScript class, and it plays a significant role in Angular components. Here are some key aspects of the constructor:
1. Initialisation and Dependency Injection
The constructor is primarily responsible for initialising class members and facilitating dependency injection. When you create a new instance of a class using new MyClass(), the constructor is executed. It's crucial to note that the Angular Dependency Injector (DI) analyses the constructor parameters to provide the necessary dependencies.
For instance, if your constructor looks like this:
constructor (private dataService: DataService) {}
Angular's DI system will automatically provide an instance of the DataService when you instantiate this component. This makes it a suitable place to set up essential services and perform class member initialisation.
2. Parameter Type Matching
In the constructor, it's vital to ensure that the parameters passed during component instantiation match the expected types defined in the constructor. For instance:
new
MyClass
(
arg1
:
number
,
arg2
:
string
,
argN
:
any
)
The parameters arg1, arg2, and argN must align with the types defined in the constructor of MyClass. This type safety ensures that you're using the class correctly.
ngOnInit
In contrast, ngOnInit is an Angular lifecycle hook used to indicate that Angular has finished creating the component. Here are some key points about ngOnInit:
1. initialisation After Component Creation
ngOnInit is the ideal place to put code that needs to execute as soon as the component is instantiated and the view is ready for interaction. It is not designed to take any parameters, and you need to implement it in your component class explicitly.
To use ngOnInit, you typically import it like this:
import
{
Component
,
OnInit
}
from
'@angular/core'
;
Then, you implement it in your component class:
export
class
MyComponent
implements
OnInit
{
ngOnInit
(
)
{
// Code to be executed after component initialisation
}
}
While implementing ngOnInit is not mandatory, it's considered good practice in Angular development because it makes your code more organised and predictable.
2. Separation of Concerns
One of the critical differences between the constructor and ngOnInit is the separation of concerns. The constructor should primarily handle initialisation and dependency injection, while ngOnInit is better suited for "actual work" that needs to be executed after the component is created.
For example, if you need to load data from a database to display in your HTML template, it's best to place this code in the ngOnInit method. This separation makes your code cleaner, more maintainable, and easier to understand.
Conclusion
In Angular development, it's essential to follow best practices for placing code in the constructor and ngOnInit to ensure your components are well-organised and maintainable. The constructor should focus on initialisation and dependency injection, while ngOnInit is the right place for code that should execute after the component is created. Remember that the less code in the constructor, the better. By adhering to these practices, you'll create more efficient and readable Angular applications.