How to Set Array Values in React Native MobX: Solving the Common Error undefined is not an object

Learn how to successfully set array values in React Native using MobX. Discover common errors, their causes, and solutions to enhance your app development skills!
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Understanding Array Values with React Native and MobX

When developing applications with React Native, integrating state management solutions like MobX can significantly enhance our app's performance and maintainability. However, using MobX can sometimes lead to complications, especially for newcomers. One common challenge developers face is setting array values correctly, particularly when dealing with asynchronous operations. In this guide, we'll explore a specific problem with setting values in a MobX store and how to resolve it effectively.

The Problem: Setting Array Values in MobX

Let’s say you have a MobX store where you want to save a list of pets (let’s call this array petList) that's being retrieved from a bookmarkApi. An error occurs when you try to set this array, and you receive a message that reads:

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

This error can be frustrating, especially if you’re not sure where it's coming from. It usually occurs when you lose the context of this inside your asynchronous callback functions. So how do we tackle this?

Solution: Using Arrow Functions for Context Binding

The key to solving this problem is to ensure that you maintain the context of this when invoking functions within your MobX store. In JavaScript, regular function calls (like the one used in the then block of a Promise) create their own this, leading to confusion. Using arrow functions is a straightforward fix since they don’t create their own this context; instead, they inherit it from the surrounding scope.

Step-by-Step Implementation

Let’s look closer at the bookMarkList method inside the Favorites class and identify how to implement the solution.

Here is the original code where the issue occurs:

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

1. Update the Callback Function

Change the regular function to an arrow function. Here’s the updated code:

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

2. Full Improved Code

Here’s how your Favorites class looks after making the change:

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

Conclusion

In React Native with MobX, setting array values can sometimes be tricky due to the intricacies of JavaScript's handling of this. By simply switching to arrow functions for your asynchronous callbacks, you can prevent the context issues that often lead to errors. With this knowledge, you can confidently manage your MobX states without running into the undefined errors.

Now, dig into your MobX store and ensure that you're using the correct function types for your asynchronous operations, and watch your array values set properly! Happy coding!