How to Run Multiple Functions Sequentially Within forEach in TypeScript/JavaScript

Learn how to ensure your functions run sequentially using `await` and `for...of` loops in TypeScript/JavaScript to improve data accuracy.
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Running Multiple Functions Sequentially Within forEach in TypeScript/JavaScript

When working with asynchronous functions in JavaScript or TypeScript, we often encounter situations where we need to handle multiple operations sequentially. A common challenge is found in the forEach loop, where operations may not execute in the correct order, leading to inaccurate results. This guide addresses a common problem: how to ensure that multiple functions run sequentially within a forEach loop, and shares practical solutions to achieve this.

The Problem

Imagine you have a collection of programs stored in an array, and for each program, you want to run a series of asynchronous functions to gather specific data points. You may encounter a situation like this:

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

This approach often leads to issues related to execution order. Specifically, when dealing with promises, the functions may not wait for one another to complete before starting the next. As a result, the data being populated might be incorrect or not reflective of the expected sequence.

Understanding the Errors

The forEach loop initiates all the async operations simultaneously instead of waiting for each one to complete.

Symptoms of the Problem:

Data gets populated but ends up being inaccurate.

The subsequent program starts processing before the previous one has completed.

The Solution

To solve this problem, we can use a traditional for...of loop instead of forEach. A for...of loop allows us to use the await keyword effectively, ensuring that one operation completes before moving on to the next. Here’s how you can refactor the code:

Step-by-Step Refactoring

Use a for...of Loop: Replace the forEach call with a for...of loop.

Here’s the improved code:

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

Key Changes Explained:

Sequential Execution: The for...of loop forces each iteration to finish before continuing to the next, thereby ensuring that data is processed correctly for each program.

Conclusion

If you're facing issues with asynchronous function executions within a forEach loop in JavaScript or TypeScript, consider switching to a for...of loop combined with await. This technique not only resolves execution order issues but also enhances the accuracy of the collected data. By reworking how we handle promises, we can create more effective and reliable applications.

By mastering these concepts, you’ll improve your skills in asynchronous programming and better manage complex data flows in your applications.