What sets apart `this.user.id` from `this.user = {id: ....}`?

Question

What sets apart `this.user.id` from `this.user = {id: ....}`?

I am puzzled by the error thrown in the code below:

import { Component, OnInit } from '@angular/core';
import { ActivatedRoute } from '@angular/router';

@Component({
  selector: 'app-user',
  templateUrl: './user.component.html',
  styleUrls: ['./user.component.css']
})
export class UserComponent implements OnInit {
  user: {id: number, name: string};

  constructor(private route: ActivatedRoute) {}

  ngOnInit()
  {
    this.user.id = this.route.snapshot.params['id'];
    this.user.name = this.route.snapshot.params['name'];

    console.log(this.user.id);
    console.log(this.user.name);
  }
}

ERROR TypeError: this.user is undefined

However, when I modify the code as shown below, the error does not occur:

import { Component, OnInit } from '@angular/core';
import { ActivatedRoute } from '@angular/router';

@Component({
  selector: 'app-user',
  templateUrl: './user.component.html',
  styleUrls: ['./user.component.css']
})
export class UserComponent implements OnInit {
  user: {id: number, name: string};

  constructor(private route: ActivatedRoute) {}

  ngOnInit()
  {
    this.user = {
      id: this.route.snapshot.params['id'],
      name: this.route.snapshot.params['name']
    }

    console.log(this.user.id);
    console.log(this.user.name);
  }
}

I am struggling to comprehend why one version executes without errors while the other does not.

I wonder if my understanding of user being a JSON object is correct. If so, why am I unable to access it with this.user.id?

Any insights would be appreciated.

javascript angular typescript

Answer 1

Answer №1

// The declaration provided here is not initialization
// It simply outlines that the class contains a reference to a user object with specific id and name properties
user: {id: number, name: string};

The user object remains undefined until a value is assigned to it. In the second instance, this assignment occurs as follows:

// This code snippet creates an object with id and name properties
// and assigns it to the user class member, which is currently undefined
this.user = {
      id: this.route.snapshot.params['id'],
      name: this.route.snapshot.params['name']
    }

An issue arises in your initial example here:

// This part assumes that the user object already exists
// and attempts to modify the id property
this.user.id = this.route.snapshot.params['id'];

You are trying to set a value for a property of an object that does not exist yet.

Answer 2

// The declaration provided here is not initialization
// It simply outlines that the class contains a reference to a user object with specific id and name properties
user: {id: number, name: string};

The user object remains undefined until a value is assigned to it. In the second instance, this assignment occurs as follows:

// This code snippet creates an object with id and name properties
// and assigns it to the user class member, which is currently undefined
this.user = {
      id: this.route.snapshot.params['id'],
      name: this.route.snapshot.params['name']
    }

An issue arises in your initial example here:

// This part assumes that the user object already exists
// and attempts to modify the id property
this.user.id = this.route.snapshot.params['id'];

You are trying to set a value for a property of an object that does not exist yet.

Answer 3

Answer №2

const person: { id: number, name: string }; // Here we are simply declaring a variable.

person; // Essentially the same as above

// An error will occur here because we are attempting to
// access properties of the person object that do not yet exist.
this.person.id = 5;
this.person.name = 'John';

If we both declare and initialize the variable, this error will be avoided.

// Declare with object reference and initial values
person: {id: number, name: string} = {id: null, name: null}

// Or only initialize with default values
person = { id: null, name: null }

Now, if you try to access id or name and assign any value, it won't throw an error because the properties of the person object are already known.

this.person.id = 5;
this.person.name = 'John';

Answer 4

const person: { id: number, name: string }; // Here we are simply declaring a variable.

person; // Essentially the same as above

// An error will occur here because we are attempting to
// access properties of the person object that do not yet exist.
this.person.id = 5;
this.person.name = 'John';

If we both declare and initialize the variable, this error will be avoided.

// Declare with object reference and initial values
person: {id: number, name: string} = {id: null, name: null}

// Or only initialize with default values
person = { id: null, name: null }

Now, if you try to access id or name and assign any value, it won't throw an error because the properties of the person object are already known.

this.person.id = 5;
this.person.name = 'John';

Answer 5

Answer №3

The initial value is not set in the first case, causing an attempt to modify a property of an undefined object, resulting in an error being thrown.

Contrastingly, in the second situation, you intentionally assign a new object value to the user variable upon initialization.

Answer 6

The initial value is not set in the first case, causing an attempt to modify a property of an undefined object, resulting in an error being thrown.

Contrastingly, in the second situation, you intentionally assign a new object value to the user variable upon initialization.

What sets apart `this.user.id` from `this.user = {id: ....}`?

Answer №1

Answer №2

Answer №3

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