Ionic 4's http.get.subscribe method fails to retain the retrieved value

Question

Ionic 4's http.get.subscribe method fails to retain the retrieved value

I'm aware this might be a repeated question, but I haven't come across a straightforward answer yet, so here it goes.

Below is the code snippet in question:


fetchData() {
    let dataArray: Array<any> = [, , ,];
    this.prepareDataRequest().subscribe(data => {
        this.dataArray[0] = JSON.parse(JSON.stringify(data["results"]));
        this.dataArray[1] = Object.keys(JSON.parse(JSON.stringify(data["results"]))).length;
        console.log(this.dataArray[0]);
    },
    err => {
        this.dataArray[2] = `Error --> Status: ${err.status}, Message: ${err.statusText}`;
    });
    return this.dataArray;
}

prepareDataRequest() {
    const dataUrl = 'https://randomuser.me/api/?results=10&inc=gender,name,nat,email,phone,id';
    return this.http.get(dataUrl);
}

The console.log(this.dataArray[0]); line works correctly, however, the return statement appears empty. This issue seems to occur because return this.dataArray; is executed before fetching the items.

How can I resolve this? Is there a simple way to ensure everything is stored before returning? Or am I approaching this incorrectly (probably xD)?

I've experimented with async methods, but being relatively new to this, the results weren't satisfactory.

angular typescript ionic-framework http-get subscribe

Answer 1

Answer №1

You've approached this the wrong way; instead, consider returning an observable:

fetchData() {
  return this.prepareDataRequest().pipe(
    pluck('results'),
    catchError(err => `Error --> Status: ${err.status}, Message: ${err.statusText}`),
  );
}

someConsumerFunction() {
  fetchData().pipe(
    // perform optional processing here
  ).subscribe(
    results => console.log(results),
    err => console.log(err),
  );
}

If you prefer to wait for completion and utilize async along with a Promise:

async fetchData() {
  try {
    const data = await this.prepareDataRequest().pipe(
      first(),
    ).toPromise();

    return data.results;
  } catch (err) {
    return `Error --> Status: ${err.status}, Message: ${err.statusText}`;
  }
}

The first method is recommended as it provides more flexibility in processing data on the consumer's end.

Answer 2

You've approached this the wrong way; instead, consider returning an observable:

fetchData() {
  return this.prepareDataRequest().pipe(
    pluck('results'),
    catchError(err => `Error --> Status: ${err.status}, Message: ${err.statusText}`),
  );
}

someConsumerFunction() {
  fetchData().pipe(
    // perform optional processing here
  ).subscribe(
    results => console.log(results),
    err => console.log(err),
  );
}

If you prefer to wait for completion and utilize async along with a Promise:

async fetchData() {
  try {
    const data = await this.prepareDataRequest().pipe(
      first(),
    ).toPromise();

    return data.results;
  } catch (err) {
    return `Error --> Status: ${err.status}, Message: ${err.statusText}`;
  }
}

The first method is recommended as it provides more flexibility in processing data on the consumer's end.

Answer 3

Answer №2

When you return your result outside of the subscribe scope, it means the dataArray has not been initialized yet.

To fix this issue, make changes to your fetchData() method like this:

  fetchData() {
    let dataArray: Array<any> = [, , ,];
    this.prepareDataRequest()
      .subscribe(data => {
          this.dataArray[0] = JSON.parse(JSON.stringify(data["results"]));
          this.dataArray[1] = Object.keys(JSON.parse(JSON.stringify(data["results"]))).length;
          console.log(this.dataArray[0]);
          // Make sure to return your dataArray here
          return this.dataArray;

        },
        err => {
          this.dataArray[2] = `Error --> Status: ${err.status}, Message: ${err.statusText}`;
        });
  }

Additionally, there seems to be confusion regarding the declaration of a local variable called dataArray when you are only using the dataArray field. It may not be necessary in that context.

Answer 4

When you return your result outside of the subscribe scope, it means the dataArray has not been initialized yet.

To fix this issue, make changes to your fetchData() method like this:

  fetchData() {
    let dataArray: Array<any> = [, , ,];
    this.prepareDataRequest()
      .subscribe(data => {
          this.dataArray[0] = JSON.parse(JSON.stringify(data["results"]));
          this.dataArray[1] = Object.keys(JSON.parse(JSON.stringify(data["results"]))).length;
          console.log(this.dataArray[0]);
          // Make sure to return your dataArray here
          return this.dataArray;

        },
        err => {
          this.dataArray[2] = `Error --> Status: ${err.status}, Message: ${err.statusText}`;
        });
  }

Additionally, there seems to be confusion regarding the declaration of a local variable called dataArray when you are only using the dataArray field. It may not be necessary in that context.

Ionic 4's http.get.subscribe method fails to retain the retrieved value

Answer №1

Answer №2

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