When attempting to retrieve and process a JSON from the internet, I encounter "undefined" errors despite the fact that the data appears correctly in the log

I'm having trouble processing the JSON data received from a server. When I attempt to use .forEach on it, I receive an error stating that the data is undefined even though the console.log shows the correct values.

What could be causing this issue? Is there a missing async/await somewhere in my code? Could it be that I am calling the data processing function too early? If so, how can I resolve it?

Here are some relevant snippets from the component.ts:

all: any;
  constructor(private feedService: FeedService) { }

  ngOnInit(): void {
    this.fetchPosts();
    console.log(this.all);
  }

  ngAfterContentInit() {
    this.feedService.getTags(this.all.posts[0]);
  }

  async fetchPosts() {
    (await this.feedService.getJSON(this.url)).subscribe((data) => {
      console.log(data);
      this.all = data;
      console.log(this.all);
    });

  }

Relevant parts of the service:

constructor(private http: HttpClient) {
  }

  public async getJSON(url: string) {
    return this.http.get<any>(url);
  }

  public async getTags(postData: any) {
    let tags = [];
    await postData['tags'].array.forEach(tag => {
      tags.push(tag); 
    });
    return tags;
  }

Additionally, here is a screenshot of the console output: https://i.sstatic.net/v57R8.png

Answer №1

Try using this.all?.posts[0]. It seems the issue may be that the type of this.all is not specified. If this.all is null or undefined, attempting to access this.all.posts will result in trying to read undefined.posts, which is invalid.

Answer №2

While attempting to retrieve the value of "this.all.posts[0]" within the "ngAfterContentInit" function, the "this.all" variable is still null and undefined. There are several ways to address this issue. One workaround is to invoke the "this.feedService.getTags(this.all.posts[0])" function only after setting the "this.all = data;"

Answer №3

this.getTagsFromFeedService is being executed with this.postsCollection[0] before the full this.postsCollection object is defined.

The result from the fetchPostsData method is eventually stored in this.postsCollection after the initial call. Due to the asynchronous nature of server responses, it's crucial to not assume that the data will be available by any specific time, including when the afterContentInit lifecycle hook triggers.

To address this issue, you should wait until the subscription within fetchPostsData completes and returns the necessary information.


  postsCollection: any;
  constructor(private feedService: FeedService) { }

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

  async fetchPostsData() {
    this.feedService.retrieveData(this.url).subscribe((data) => {
      this.postsCollection = data;
      // Adding a check for undefined posts would be prudent at this point.
      this.getTagsFromFeedService(this.postsCollection[0]);
    });
  }

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