How to Fix Undefined Values When Fetching MySQL Results in Express.js with Promises

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The Problem: Undefined Values from MySQL Queries

You may have encountered a situation where, after executing a MySQL query, your results turn out to be an empty array or an undefined value. This often happens because the promise that handles the query results isn’t structured correctly, leading to unexpected behavior in your code.

Example Code

Here’s an example that highlights the issue:

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In this code snippet, you would expect data to hold the results of the SQL query. However, due to the way the Promise is handled in the test function (not returning the Promise), data ends up being undefined or an empty array.

The Solution: Properly Return the Promise

To resolve this issue, it is crucial that you return the Promise from the test function. By doing this, you allow the awaited result to be correctly assigned to the variable data. Here's how you can adjust the code:

Updated Code

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

Key Modifications

Return the Promise: By adding the return keyword before the Promise object in the test function, you ensure that the promise is returned for further processing.

Error Handling: It's a good practice to handle potential errors during database queries. This is accomplished by using the reject function if an error occurs.

Conclusion

Handling asynchronous data fetching in JavaScript can sometimes lead to confusion, especially when dealing with promises. In this example, simply returning the promise from the test function allowed us to properly retrieve the MySQL query results. Always remember to handle errors in your queries to maintain a robust application.

By following these guidelines, you can avoid the pitfalls of undefined values and ensure your application's data persists where you need it. Happy coding!