Resolving TypeError: Cannot read property 'then' of undefined in JavaScript Promises

Learn how to fix the common `TypeError` related to promises in JavaScript by ensuring you return the promise from your functions correctly.
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Fixing the TypeError: Cannot read property 'then' of undefined in JavaScript

Working with promises in JavaScript can sometimes lead to confusing errors, especially when dealing with asynchronous operations. One common error message developers run into is the pesky TypeError: Cannot read property 'then' of undefined. In this guide, we’ll go through what causes this error and how to effectively solve it.

Understanding the Problem

Imagine that you’re building a reservation system, similar to Airbnb, where users can click a "Make Reservation" button. This process involves sending an HTTP request to the server to add a reservation to the database. However, while implementing this functionality, you encounter a frustrating error:

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

This error is thrown when you attempt to call .then() on a variable that is undefined. In simpler terms, you are trying to use the promise method on something that isn’t a promise at all.

Where the Error Occurs

Let’s take a closer look at the code that generates this error:

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

The line that throws the error is this:

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

The problem lies within the addReservation function, which is supposed to return a promise but ends up returning undefined.

Solution to the Problem

To fix this issue, we need to ensure that the addReservation function returns a promise. Here’s how we can achieve this:

Step 1: Modify the addReservation Function

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

Step 2: Add Error Handling

Additionally, ensure that you are not swallowing the error when it occurs. Instead of simply logging it, you should re-throw the error so that it can be handled appropriately by the function that called addReservation.

Final Code Structure

After applying these changes, your addReservation function should look like this:

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

Conclusion

By ensuring that your functions correctly return promises, you can avoid the frustrating TypeError and create smoother asynchronous code. In summary:

Always return the promise in your functions to allow further .then() and .catch() chaining.

Handle errors carefully, letting them propagate to the upper layers of your application where they can be managed.

With these strategies, you'll be well on your way to mastering promises in JavaScript and handling errors gracefully.