Typescript: Subscribed information mysteriously disappeared

Question

Typescript: Subscribed information mysteriously disappeared

[ Voting to avoid putting everything inside ngOnit because I need to reuse the API response and model array in multiple functions. Need a way to reuse without cluttering up ngOnInit.

I could simply call subscribe repeatedly in each function to solve the problem.

Tried other solutions on SO, but most suggest putting everything inside ngOnInit. I prefer to keep only necessary functions there.

Please assist in creating a function for reusing API responses or initialized models like this.menu = data;]

Attempting to display a menu from an API response.

Additionally, requiring the response to be used several times in different functions, facing null values outside of the subscribe block.

Code snippet:

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

// import { LoginModel } from "../../models/login/login-model";
import { MenuModel } from "../../models/menu/menu-model";
import { SubmenuModel } from "../../models/submenu/submenu-model";
// import { AccessModel } from "../../models/access/access-model";

import { MenuService } from "../../services/menu/menu.service";
import { SubmenuService } from "../../services/submenu/submenu.service";

@Component({
  selector: 'app-admin-access',
  templateUrl: './admin-access.component.html',
  styleUrls: ['./admin-access.component.css']
})
export class AdminAccessComponent implements OnInit {

  menu: MenuModel[] = null;
  submenu: SubmenuModel[] = null;

  constructor(private menuService: MenuService, private submenuSerive: SubmenuService) { }

  ngOnInit(): void {
    this.getMenu();
    this.printMenu();
  }

  getMenu() {
    this.menuService.GetAllMenu().subscribe((data: MenuModel[]) => {
      this.menu = data;
      console.log("first use : ");
      console.log(data);

      console.log("second use : ");
      console.log(this.menu);
    })
  }

  printMenu(){
    console.log("third use : ");
    console.log(this.menu);
  }
}

Output :

https://i.stack.imgur.com/r7tMp.png

All responses are null from the printMenu() function. Why is this happening? I subscribed and stored the value previously.

How can I permanently retain a value from an API response?

angular typescript asynchronous rxjs

Answer 1

Answer №1

(I am sharing this solution to help beginners who may struggle with similar issues in the future, so they don't have to waste time and effort trying to solve problems that others have already faced.

There are two key points I want to emphasize, especially for beginners [please refrain from commenting on them - everyone is entitled to express their feelings about their questions without judgment]:

1. pros:

Many people did not grasp what I was asking for. While some provided suggestions (which I appreciate), most didn't make an effort to understand or inquire further. So, beginners, be patient until you find someone who truly understands your question.

2. Docs

In short - the documentation can be a challenge for beginners.

My experience with Angular's documentation reveals that it may not be helpful unless you have a certain level of programming knowledge. Although I had never used `observable` or `ngOnInit` before in such a manner, someone guided me through it. I still have much to learn about `observable` and `ngOnInit`, but I now have a better understanding.

For beginners: If you find the documentation difficult to comprehend, consider turning to YouTube tutorials instead of relying solely on official docs.

Issue Explanation:

Remember my ngOnInit?

ngOnInit(): void {

    //line 1

    this.getMenu();

    //line 2

    this.printMenu();
    
}

`ngOnInit` does not execute line by line. If it did, I could have corrected the value earlier rather than ending up with a null value (as seen in the screenshot). `ngOnInit` executes all at once, not sequentially like in a regular function.

Solution

When I found the solution, I realized there was no coding error. I simply needed to call `printMenu()` outside of `ngOnInit` after initializing `getMenu()` within `ngOnInit`.

How do I call it outside? I need an event.

The easiest way to trigger an event is by creating a button with a click event.

Here is my .html:

<button (click)="this.printMenu()"> Let's test it </button>

.ts:

import { MenuService } from "../../services/menu/menu.service";
import { SubmenuService } from "../../services/submenu/submenu.service";

@Component({
  selector: 'app-admin-access',
  templateUrl: './admin-access.component.html',
  styleUrls: ['./admin-access.component.css']
})
export class AdminAccessComponent implements OnInit {

  menu: MenuModel[] = null;
  submenu: SubmenuModel[] = null;

  constructor(private menuService: MenuService, private submenuSerive: Sub
menuService) {
}

  ngOnInit(): void {
    this.getMenu();
  }

  getMenu() {
    this.menuService.GetAllMenu().subscribe((data: MenuModel[]) => {
      this.menu = data;

    })
  }

  printMenu(){
    console.log("third use : ");
    console.log(this.menu);
  }
}

With minimal changes, I removed `printmenu` from `ngonit` and called it using an event (such as a button click).

[I also added comments indicating "first use" and "second use" in my code]

https://i.stack.imgur.com/2RD02.png

Answer 2

(I am sharing this solution to help beginners who may struggle with similar issues in the future, so they don't have to waste time and effort trying to solve problems that others have already faced.

There are two key points I want to emphasize, especially for beginners [please refrain from commenting on them - everyone is entitled to express their feelings about their questions without judgment]:

1. pros:

Many people did not grasp what I was asking for. While some provided suggestions (which I appreciate), most didn't make an effort to understand or inquire further. So, beginners, be patient until you find someone who truly understands your question.

2. Docs

In short - the documentation can be a challenge for beginners.

My experience with Angular's documentation reveals that it may not be helpful unless you have a certain level of programming knowledge. Although I had never used `observable` or `ngOnInit` before in such a manner, someone guided me through it. I still have much to learn about `observable` and `ngOnInit`, but I now have a better understanding.

For beginners: If you find the documentation difficult to comprehend, consider turning to YouTube tutorials instead of relying solely on official docs.

Issue Explanation:

Remember my ngOnInit?

ngOnInit(): void {

    //line 1

    this.getMenu();

    //line 2

    this.printMenu();
    
}

`ngOnInit` does not execute line by line. If it did, I could have corrected the value earlier rather than ending up with a null value (as seen in the screenshot). `ngOnInit` executes all at once, not sequentially like in a regular function.

Solution

When I found the solution, I realized there was no coding error. I simply needed to call `printMenu()` outside of `ngOnInit` after initializing `getMenu()` within `ngOnInit`.

How do I call it outside? I need an event.

The easiest way to trigger an event is by creating a button with a click event.

Here is my .html:

<button (click)="this.printMenu()"> Let's test it </button>

.ts:

import { MenuService } from "../../services/menu/menu.service";
import { SubmenuService } from "../../services/submenu/submenu.service";

@Component({
  selector: 'app-admin-access',
  templateUrl: './admin-access.component.html',
  styleUrls: ['./admin-access.component.css']
})
export class AdminAccessComponent implements OnInit {

  menu: MenuModel[] = null;
  submenu: SubmenuModel[] = null;

  constructor(private menuService: MenuService, private submenuSerive: Sub
menuService) {
}

  ngOnInit(): void {
    this.getMenu();
  }

  getMenu() {
    this.menuService.GetAllMenu().subscribe((data: MenuModel[]) => {
      this.menu = data;

    })
  }

  printMenu(){
    console.log("third use : ");
    console.log(this.menu);
  }
}

With minimal changes, I removed `printmenu` from `ngonit` and called it using an event (such as a button click).

[I also added comments indicating "first use" and "second use" in my code]

https://i.stack.imgur.com/2RD02.png

Answer 3

Answer №2

Based on your response, it seems like there is still a misunderstanding about the reason why the variable remains undefined when you try to print it outside of the subscription. The assignment of this.menu happens asynchronously. To gain more insights into this issue, I recommend taking some time to go through the answer provided in the attached link: this.

The variable doesn't get assigned any value when printed within the printMenu() function. Therefore, your explanation is still incorrect as it assumes that the variable has been defined by the time you press the button, which may not always be the case. Remember, with asynchronous operations, you can never guarantee that a variable has already been defined.

To Replicate the Error

If you want to recreate this error intentionally, you can introduce an artificial delay using the RxJS delay operator. In my example, I have added a 10-second delay. So, if you click the button within the first 10 seconds after the application launches, the printMenu() function will still output null. In real-world situations, such delays might be caused by the processing time of the GetAllMenu() function over which the frontend has no control.

import { delay } from "rxjs/operators";

@Component({
  selector: 'app-admin-access',
  templateUrl: './admin-access.component.html',
  styleUrls: ['./admin-access.component.css']
})
export class AdminAccessComponent implements OnInit {
  menu: MenuModel[] = null;
  submenu: SubmenuModel[] = null;

  constructor(private menuService: MenuService, private submenuSerive: SubmenuService) { }

  ngOnInit(): void {
    this.getMenu();
  }

  getMenu() {
    this.menuService.GetAllMenu().pipe(
      delay(10000)             // <-- simulating a 10-second delay
    ).subscribe((data: MenuModel[]) => {
      this.menu = data;        
    });
  }

  printMenu() {
    console.log(this.menu);    // <-- will still show `null` if called within 10 seconds
  }
}

A better approach would be to make all subsequent statements asynchronous as well.

Solution: Leveraging RxJS `ReplaySubject`

ReplaySubject serves as a multi-cast observable that buffers and emits the last n emitted values immediately upon subscription. A buffer size of 1 should suffice for this scenario. You can push the source notification from GetAllMenu() into this observable where other components can subscribe to it.

import { ReplaySubject } from "rxjs";

@Component({
  selector: 'app-admin-access',
  templateUrl: './admin-access.component.html',
  styleUrls: ['./admin-access.component.css']
})
export class AdminAccessComponent implements OnInit {
  menu$: ReplaySubject<MenuModel[]> = new ReplaySubject<MenuModel[]>(1);
  submenu: SubmenuModel[] = null;

  constructor(private menuService: MenuService, private submenuSerive: SubmenuService) { }

  ngOnInit(): void {
    this.getMenu();
    this.printMenu();
  }

  getMenu() {
    this.menuService.GetAllMenu().subscribe((data: MenuModel[]) => {
      this.menu$.next(data);        // <-- pushing value to the `ReplaySubject` observable
      console.log("first use : ");
      console.log(data);
    });
  }

  printMenu(){
    this.menu$.subscribe((data: MenuModel[]) => {     // <-- subscribing to the `ReplaySubject` observable
      console.log("third use : ");
      console.log(data);
    });
  }
}

Answer 4

Based on your response, it seems like there is still a misunderstanding about the reason why the variable remains undefined when you try to print it outside of the subscription. The assignment of this.menu happens asynchronously. To gain more insights into this issue, I recommend taking some time to go through the answer provided in the attached link: this.

The variable doesn't get assigned any value when printed within the printMenu() function. Therefore, your explanation is still incorrect as it assumes that the variable has been defined by the time you press the button, which may not always be the case. Remember, with asynchronous operations, you can never guarantee that a variable has already been defined.

To Replicate the Error

If you want to recreate this error intentionally, you can introduce an artificial delay using the RxJS delay operator. In my example, I have added a 10-second delay. So, if you click the button within the first 10 seconds after the application launches, the printMenu() function will still output null. In real-world situations, such delays might be caused by the processing time of the GetAllMenu() function over which the frontend has no control.

import { delay } from "rxjs/operators";

@Component({
  selector: 'app-admin-access',
  templateUrl: './admin-access.component.html',
  styleUrls: ['./admin-access.component.css']
})
export class AdminAccessComponent implements OnInit {
  menu: MenuModel[] = null;
  submenu: SubmenuModel[] = null;

  constructor(private menuService: MenuService, private submenuSerive: SubmenuService) { }

  ngOnInit(): void {
    this.getMenu();
  }

  getMenu() {
    this.menuService.GetAllMenu().pipe(
      delay(10000)             // <-- simulating a 10-second delay
    ).subscribe((data: MenuModel[]) => {
      this.menu = data;        
    });
  }

  printMenu() {
    console.log(this.menu);    // <-- will still show `null` if called within 10 seconds
  }
}

A better approach would be to make all subsequent statements asynchronous as well.

Solution: Leveraging RxJS `ReplaySubject`

ReplaySubject serves as a multi-cast observable that buffers and emits the last n emitted values immediately upon subscription. A buffer size of 1 should suffice for this scenario. You can push the source notification from GetAllMenu() into this observable where other components can subscribe to it.

import { ReplaySubject } from "rxjs";

@Component({
  selector: 'app-admin-access',
  templateUrl: './admin-access.component.html',
  styleUrls: ['./admin-access.component.css']
})
export class AdminAccessComponent implements OnInit {
  menu$: ReplaySubject<MenuModel[]> = new ReplaySubject<MenuModel[]>(1);
  submenu: SubmenuModel[] = null;

  constructor(private menuService: MenuService, private submenuSerive: SubmenuService) { }

  ngOnInit(): void {
    this.getMenu();
    this.printMenu();
  }

  getMenu() {
    this.menuService.GetAllMenu().subscribe((data: MenuModel[]) => {
      this.menu$.next(data);        // <-- pushing value to the `ReplaySubject` observable
      console.log("first use : ");
      console.log(data);
    });
  }

  printMenu(){
    this.menu$.subscribe((data: MenuModel[]) => {     // <-- subscribing to the `ReplaySubject` observable
      console.log("third use : ");
      console.log(data);
    });
  }
}

Typescript: Subscribed information mysteriously disappeared

Answer №1

Answer №2

To Replicate the Error

Solution: Leveraging RxJS `ReplaySubject`

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Typescript: Subscribed information mysteriously disappeared

Answer №1

Answer №2

To Replicate the Error

Solution: Leveraging RxJS ReplaySubject

Similar questions

What are the different methods to display information in Angular?

Do I need to convert AngularJS to .ts for an Angular/AngularJS hybrid application?

The modal popup feature is dysfunctional within the hierarchical component structure of angular-bootstrap-md

Error: Unable to access the 'registerControl' property of the object due to a type mismatch

Having trouble figuring out how to display a tooltip using the show() method in @teamhive/ngx-tooltip?

Exploring the capabilities of the Angular 2 expression parser alongside the functionality of the

In Angular, use the ngFor directive to iterate through items in a collection and add a character to each item except

Error message: "ReferenceError occurred while trying to access the Data Service in

Implementing indexers in TypeScript to accommodate both string and numeric keys

Tips for effectively handling the data received from a webservice when logging into a system

An issue arises when using enums in TypeScript

The directive is failing to apply the active class to the host element

Ways to access a nested route in Angular 4

Exploring Angular Components with Jasmine and Karma while integrating third-party tools such as ExcelJS

The function RegisterOnChange is not a valid function

Testing an asynchronous function in JavaScript can lead to the error message: "Have you neglected to utilize await?"

The hidden pop-up window from a particular tool is currently not being displayed

Create an object using a combination of different promises

"Exploring the best way to open a new tab in Angular from a component

Obtain the coordinates of the pixel in an image on the canvas when a mouse

Solution: Leveraging RxJS `ReplaySubject`