Error in Angular8: Attempting to loop through an undefined property

Question

Error in Angular8: Attempting to loop through an undefined property

I have searched tirelessly for a solution, but none of them seem to work.

Every time I click on the edit button, it redirects me to edit-event page and shows me this error:

ERROR TypeError: Cannot read property 'categories' of undefined

  ngOnInit() {
    this.eventService
    .getEventById(this.route.snapshot.params.id)
    .subscribe((ev) => this.event = ev);

Once I subscribe, if I try console.log(this.event), it returns undefined. However, .subscribe(console.log(ev)); returns an object. I'm puzzled why this.event = ev is not working, considering I have used it successfully in another component with no issues.

The service being called is as follows:

getEventById(id) {
    return this.httpClient.get<EventsDto>(this.urlevent + '?id=' + id);
  }

In edit-event.component.html, {{event.name}} gets printed. This baffles me because earlier we saw that this.event was undefined using:

.subscribe((ev) => this.event = ev)

angular typescript asynchronous undefined

Answer 1

Answer №1

Summary: To ensure proper handling of asynchronous logic, organize your code by placing this.http.get() inside the button press function and perform any subsequent operations within its subscribe() method

When working with asynchronous code, it is crucial to wait for functions like this.httpClient.get to return data before attempting to use or display it.

Your .subscribe(console.log(ev)) correctly logs data, but the synchronous console.log(ev) does not. This is because the synchronous console.log(ev) runs immediately after calling this.httpClient.get(), when the variable ev is still undefined due to the asynchronous nature of the request. Placing console.log(ev) inside the subscribe() block ensures that it waits for the data to be returned before executing, ensuring that ev contains the requested data (or an error).

To address this issue effectively, consider moving the this.http.get() call inside the button click event and handle subsequent actions within the .subscribe() function. This approach guarantees that data is available when subsequent functions are executed.

.html

<button (click)="buttonClick()></button>

.ts

buttonClick() {
  this.eventService
    .getEventById(this.route.snapshot.params.id)
    .subscribe(ev => {
      this.event = ev
      // add your code here, e.g. createEventsDto(ev)
    });

Answer 2

Summary: To ensure proper handling of asynchronous logic, organize your code by placing this.http.get() inside the button press function and perform any subsequent operations within its subscribe() method

When working with asynchronous code, it is crucial to wait for functions like this.httpClient.get to return data before attempting to use or display it.

Your .subscribe(console.log(ev)) correctly logs data, but the synchronous console.log(ev) does not. This is because the synchronous console.log(ev) runs immediately after calling this.httpClient.get(), when the variable ev is still undefined due to the asynchronous nature of the request. Placing console.log(ev) inside the subscribe() block ensures that it waits for the data to be returned before executing, ensuring that ev contains the requested data (or an error).

To address this issue effectively, consider moving the this.http.get() call inside the button click event and handle subsequent actions within the .subscribe() function. This approach guarantees that data is available when subsequent functions are executed.

.html

<button (click)="buttonClick()></button>

.ts

buttonClick() {
  this.eventService
    .getEventById(this.route.snapshot.params.id)
    .subscribe(ev => {
      this.event = ev
      // add your code here, e.g. createEventsDto(ev)
    });

Error in Angular8: Attempting to loop through an undefined property

Answer №1

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