Retrieving object information in the constructor using Angular and Typescript

When attempting to access an object's data within a constructor, it results in an "undefined" object. Even though it functions properly in the ngOnInit() function, the data which is about to be reset is required every time the component is initiated.

   import { Component, OnInit, Input } from '@angular/core';
   @Input() data: any;


   constructor(dataService: DataService)
   {
     console.log(this.data); // returns undefined
   }


   ngOnInit()
   {
     console.log(this.data) // functions correctly here
   }

Answer №1

Have you taken a look at this documentation?

It mentioned:

Once a component/directive is created by calling its constructor, Angular proceeds to invoke the lifecycle hook methods in a specific sequence

It then proceeds to outline all the methods in their order of execution.

For a comprehensive understanding of lifecycle hooks, it's crucial to familiarize yourself with them when beginning to develop applications using Angular.

Answer №2

When working with component inputs in Angular, it's important to note that you cannot access the value of a component input inside its constructor. The only value available at that point is the initial value assigned to it.

Why is this the case?

Angular needs to create the component first by invoking its constructor before it can set the inputs. Therefore, attempting to use an input property in the constructor will result in it being undefined.

If you do need to use the input value, it's best to do so in the ngOnInit lifecycle hook. Alternatively, if you absolutely must access it in the constructor, consider retrieving the value using an injected service or a global shared object.

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