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Learn how to efficiently make multiple `HTTP requests` in TypeScript using a for loop with practical examples and optimized solutions.
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This video is based on the question https://stackoverflow.com/q/64721885/ asked by the user 'Max R' ( https://stackoverflow.com/u/13391505/ ) and on the answer https://stackoverflow.com/a/64724135/ provided by the user 'user3366195' ( https://stackoverflow.com/u/3366195/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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How to Make Multiple HTTP Requests Using For Loop in TypeScript

In modern web applications, it is common to send multiple HTTP requests based on user actions, like on form submission. A typical scenario would be when a user selects multiple dates and submits the form, requiring an HTTP request for each selected date. However, implementing this correctly, especially in TypeScript, can sometimes be tricky. This guide will guide you through how to effectively manage multiple HTTP requests using a for loop in TypeScript, ensuring no data is lost during the process.

The Problem: Loss of Selected Dates

When working with multiple selected dates, a common issue is that only the last selected date is assigned to the booking object. This happens because the loop executes quickly, and by the time the requests are sent, the value of booking.bookDate holds only the last date.

The Original Code Example

Here’s the initial code you might be working with:

[[See Video to Reveal this Text or Code Snippet]]

In this example, the loop executes for each date in selectedDate, but each request sends the same booking object, ending with only the last selected date.

The Solution: Using mergeMap() and toArray()

To tackle this issue effectively, we can utilize RxJS operators like mergeMap() and toArray(). This approach not only solves the problem but also improves performance. The following code snippet demonstrates how to modify the existing submit method to properly handle multiple HTTP requests:

Updated Code Example

[[See Video to Reveal this Text or Code Snippet]]

Explanation of the Code

from(this.selectedDate): This creates an observable from the selected dates array, allowing us to process each date individually.

mergeMap(date => {...}): This operator takes each date and allows us to transform it into an observable. For each date, we create a new booking and send an HTTP request using the bookingService.create(...) method.

toArray(): After processing all the dates, we use toArray() to collect the results. This allows us to handle all responses once they finish, making the code cleaner and more efficient.

subscribe(...): This is where you handle the responses from all requests, letting the user know if the bookings were successful.

Why Use This Approach?

Using mergeMap():

Helps manage multiple asynchronous operations.

Allows for better control over API call responses.

Using toArray():

Ensures all requests are resolved before proceeding, thus maintaining the integrity of the booking process.

Incorporating these RxJS features not only resolves the immediate problem but also adheres to best practices for handling asynchronous operations in TypeScript.

Conclusion

Managing multiple HTTP requests efficiently is critical in TypeScript application development. By employing techniques like mergeMap() and toArray(), you set yourself up for success, ensuring all requested operations are processed without losing important data. Implement this pattern in your applications, and watch as it dramatically improves the responsiveness and reliability of your API interactions.

By following this guide, you now have a solid understanding of handling multiple HTTP requests with a for loop in TypeScript, allowing for cleaner, more maintainable code.