Constructor vs ngOnInit in Angular

Introduction

In Angular, both the constructor and ngOnInit are lifecycle hooks that are commonly used in components. While they both serve important purposes, there are key differences between the two that developers should be aware of.

Constructor

The constructor is a special method that is called when a component is first created. It is responsible for initializing the component's properties and setting up any necessary dependencies. The constructor should only be used for tasks that are essential to the component's initialization, such as: * Setting default values for properties * Binding to input properties * Subscribing to events

ngOnInit

ngOnInit is a lifecycle hook that is called after the constructor has finished running. It is used to perform tasks that are not essential to the component's initialization, such as: * Fetching data from a remote server * Updating the component's UI * Subscribing to events that are not related to the component's initialization

Key Differences

The main difference between the constructor and ngOnInit is that the constructor is called before the component's properties are initialized, while ngOnInit is called after. This means that the constructor should only be used for tasks that do not depend on the component's properties, while ngOnInit can be used for tasks that do. Another difference between the constructor and ngOnInit is that the constructor is only called once, when the component is first created, while ngOnInit is called every time the component is initialized. This means that the constructor should only be used for tasks that need to be performed once, while ngOnInit can be used for tasks that need to be performed multiple times.

When to Use Each Method

The following table summarizes when to use the constructor and ngOnInit: | Task | Use Constructor | Use ngOnInit | |---|---|---| | Initialize component properties | Yes | No | | Set up dependencies | Yes | No | | Fetch data from a remote server | No | Yes | | Update the component's UI | No | Yes | | Subscribe to events | Yes
(if essential to initialization) | Yes
(if not essential to initialization) |