How to use expand operator to get all data from api call

How to use the expand operator to get all data from an API call

When working with TypeScript, you may often need to retrieve data from an API. In some cases, the API response may be paginated, meaning that you need to make multiple requests to fetch all the data. One way to handle this is by using the expand operator to recursively fetch all the data from the API. In this blog post, we will explore how to use the expand operator to achieve this.

Using the expand operator

The expand operator in TypeScript allows you to recursively expand an observable sequence. This means that you can use it to make multiple API requests until you have fetched all the data. Let’s see how this can be done.

First, you need to import the necessary modules:

import { of } from 'rxjs';
import { expand, concatMap } from 'rxjs/operators';

Next, you can define a function that makes the API call and returns an observable:

function fetchData(page: number) {
  return of(`https://api.example.com/data?page=${page}`);
}

Now, you can use the expand operator to recursively fetch the data:

fetchData(1)
  .pipe(
    expand((response: any) => {
      const nextPage = response.nextPage;
      if (nextPage) {
        return fetchData(nextPage);
      } else {
        return of();
      }
    }),
    concatMap((response: any) => response.data)
  )
  .subscribe((data: any) => {
    console.log(data);
  });

In the above code snippet, we start by making the initial API call using the fetchData function. Then, we use the expand operator to check if there is a nextPage property in the response. If there is, we make another API call using the fetchData function with the next page number. This process continues until there is no nextPage property in the response. Finally, we use the concatMap operator to concatenate all the data from the responses into a single observable stream.

By using the expand operator, you can easily fetch all the data from an API call, even if it is paginated. This approach ensures that you retrieve all the data without missing any pages.

Alternative solution

Another way to achieve the same result is by using the async/await syntax. This approach can make the code more readable and easier to understand. Here’s how you can do it:

async function fetchAllData() {
  let page = 1;
  let data = [];

  while (true) {
    const response = await fetchData(page);
    data = data.concat(response.data);

    if (!response.nextPage) {
      break;
    }

    page = response.nextPage;
  }

  return data;
}

fetchAllData().then((data: any) => {
  console.log(data);
});

In this alternative solution, we define an async function called fetchAllData. Inside the function, we use a while loop to continuously fetch data from the API until there is no nextPage property in the response. We then concatenate the data from each response into a single array. Finally, we return the complete data array.

By using the async/await syntax, you can write more readable and synchronous-looking code while still achieving the desired result.

That’s it! You now know how to use the expand operator to fetch all the data from an API call in TypeScript. Whether you choose to use the expand operator or the async/await syntax, you can efficiently retrieve all the data from paginated API responses.

Happy coding!