Unlocking the Power of Angular 12: Leveraging the Subscribe Method to Access Multiple REST APIs

Question

Unlocking the Power of Angular 12: Leveraging the Subscribe Method to Access Multiple REST APIs

We have a task where we need to make multiple REST API calls from the ngOnInit() method, one after the other. After making the first call, we need to pass the response to the second API call, and similarly for the third call, we need to get the value from the second call.

However, when we try calling them like this, we always end up with an undefined value.

this.globalVar1 : any;
this.globalVar2 : any;
this.globalVar3 : any;

async ngOnInit() {

this.apiService.getFirstAPICall(request).subscribe(info => {
            this.globalVar1 = info; //the value is coming from the API service call here
}
console.log(this.globalVar1); //this logs as undefined 

//Now we are trying to call the second API **here we need the first variable 
//but since it's undefined, we are encountering an error

this.apiService.getSecondAPICall(request.globalVar1).subscribe(info => {
            this.globalVar2 = info;
}
console.log(this.globalVar2);  //this also shows undefined

angular typescript visual-studio-code

Answer 1

Answer №1

When dealing with asynchronous code, it's important to remember that requests can be executed simultaneously, leading to situations where the second request is made before the results of the first are received.

To ensure sequential execution in your code, one approach is to utilize the await keyword. This instructs the program to wait for the current line of code to complete before moving on to the next line.

An example implementation could look like this:

this.data1: any;
this.data2: any;

async fetchData() {
  this.data1 = await this.apiService.getDataFromFirstAPI(request);
  console.log(this.data1);

  this.data2 = await this.apiService.getDataFromSecondAPI(this.data1);
  console.log(this.data2);
}

Alternatively, you could also address the issue by making the second request within the subscription of the first request.

Answer 2

When dealing with asynchronous code, it's important to remember that requests can be executed simultaneously, leading to situations where the second request is made before the results of the first are received.

To ensure sequential execution in your code, one approach is to utilize the await keyword. This instructs the program to wait for the current line of code to complete before moving on to the next line.

An example implementation could look like this:

this.data1: any;
this.data2: any;

async fetchData() {
  this.data1 = await this.apiService.getDataFromFirstAPI(request);
  console.log(this.data1);

  this.data2 = await this.apiService.getDataFromSecondAPI(this.data1);
  console.log(this.data2);
}

Alternatively, you could also address the issue by making the second request within the subscription of the first request.

Answer 3

Answer №2

When using the subscribe method, it is important to remember that it operates asynchronously. This means that it will not wait for its execution to complete before moving on to the next line of code. To work around this, you can convert your .subscribe() into a promise by using .toPromise().

By converting it into a promise, you can then use the await keyword to pause the execution until the promise resolves and assign the result to a variable that can be passed to the next function.

Here is an example:

async ngOnInit() {
    const res1 = await this.apiService.getData(); // Assuming getData() returns a Promise<YourType>
    const res2 = await this.apiService.getData2(res1);
    // continue with additional operations...
}

Answer 4

When using the subscribe method, it is important to remember that it operates asynchronously. This means that it will not wait for its execution to complete before moving on to the next line of code. To work around this, you can convert your .subscribe() into a promise by using .toPromise().

By converting it into a promise, you can then use the await keyword to pause the execution until the promise resolves and assign the result to a variable that can be passed to the next function.

Here is an example:

async ngOnInit() {
    const res1 = await this.apiService.getData(); // Assuming getData() returns a Promise<YourType>
    const res2 = await this.apiService.getData2(res1);
    // continue with additional operations...
}

Answer 5

Answer №3

One effective solution is to utilize the rxjs switchMap operator. This allows you to seamlessly pass data from the initial call to the inner observable (the second call). By doing so, there is no need for global variables and no requirement to convert to a promise since httpClient already returns an Observable.

import { switchMap } from 'rxjs/operators';
...

this.apiService.getFirstAPICall(request).pipe(
  map(response1 => // manipulate your data here)
  switchMap((editedResponse1) => this.getSecondAPICall(editedResponse1) )
).subscribe()

Answer 6

One effective solution is to utilize the rxjs switchMap operator. This allows you to seamlessly pass data from the initial call to the inner observable (the second call). By doing so, there is no need for global variables and no requirement to convert to a promise since httpClient already returns an Observable.

import { switchMap } from 'rxjs/operators';
...

this.apiService.getFirstAPICall(request).pipe(
  map(response1 => // manipulate your data here)
  switchMap((editedResponse1) => this.getSecondAPICall(editedResponse1) )
).subscribe()

Unlocking the Power of Angular 12: Leveraging the Subscribe Method to Access Multiple REST APIs

Answer №1

Answer №2

Answer №3

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