What is the difference between ngOnInit() and the constructor() of a component?

In Angular, both the constructor() and ngOnInit() methods are used in components, but they serve different purposes and are called at different times during the component's lifecycle.

constructor():

1. The constructor is a fundamental feature of JavaScript (and TypeScript) classes, and it is called when an instance of the class is created.
2. The constructor is used mainly for dependency injection to provide instances of required services or to set up the initial state of the component.
3. It is not recommended to perform complex logic or DOM-related operations in the constructor, as the component's input properties have not been initialized yet, and Angular's change detection mechanism has not started.

ngOnInit():

1. ngOnInit() is an Angular lifecycle hook method that gets called once the component is initialized and its input properties are set.
2. This method is the ideal place to perform initialization tasks, such as fetching data from a server, setting up event listeners, or initializing component state based on input properties.
3. ngOnInit() is called after the constructor, which means that the component's dependencies have already been injected, and the input properties have been set, making it safer to perform complex operations or access component properties.

In summary, the constructor() is used mainly for dependency injection and setting up the initial state, while ngOnInit() is the recommended place for component initialization tasks that depend on input properties or require complex logic. Understanding the difference between these two methods is essential for creating well-structured and maintainable Angular components.